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anatomy and physiology notes

What is the Difference between Arteries and Veins?

Arteries and veins are the parts of the circulatory system which carry blood between the heart, lungs, and all other areas of the body. While they both carry blood, they do not have much else in common. Arteries and veins are made of somewhat different tissue, each performing certain functions in a specialized way. The first and most important difference between the two is that all arteries carry blood away from the heart, and all veins carry blood to the heart from outlying areas. Most arteries carry oxygenated blood, and most veins carry de-oxygenated blood.

Arterial tissue is designed and specialized in a way to make it particularly suited to the quick and efficient delivery of blood, which carries the oxygen essential for the functioning of every bodily cell. The outer layer of an artery is made of connective tissue, which covers the muscular middle layer. These muscles contract between heartbeats in such a reliable way that when we take our pulse, we are not actually feeling our heartbeat per se, but arterial muscle contraction instead.

Beyond the arterial muscle is the innermost layer, made of smooth endothelial cells. These cells are specialized to provide a smooth pathway for blood to flow through. This area of cells is also what can become damaged and compromised over a person's lifetime, leading to two common causes of death, namely heart attack and stroke.

Veins have a different structure and function from arteries. Veins are very flexible, and collapse when they are not filled with blood. They carry de-oxygenated blood, usually rich in carbon dioxide, toward the lungs to receive oxygen. After blood is oxygenated, other veins carry it to the heart, where it is pumped into arteries. The layers of vein tissue are similar in some ways to those of arteries, although the muscle does not contract like arterial muscle does.

While the location of arteries is very similar from person to person, this is not so much the case with veins, which have greater variability. Veins, unlike arteries, are used as access points to the bloodstream in the medical field, such as when a person receives medicine or fluids directly into the bloodstream, or when blood is drawn. Because veins do not contract as arteries do, there are valves present in veins which keep blood flow going in one direction only. Without these valves, gravity would quickly cause blood too pool in the extremities, causing injury or at the very least impairing the system's efficiency.

How does the Nervous System Work?

The human nervous system is possibly the single most complex object in the entire cosmos, or at least the most complex object in our section of the galaxy. This is because it includes the brain, with ten billion neurons and many times more interneural connections. The human brain is a more dense source of complexity than anything we have yet seen, including the bodies of all animals and any variety of inanimate phenomena or objects.

However, the human nervous system is more than just the brain. All animals have a nervous system, but only vertebrates have a complex nervous system that include the central nervous system (CNS) and peripheral nervous system (PNS) as components. The central nervous system consists of the nerves and neurons found in the spine and brain, while the peripheral nervous system is everything else.

In older animals, the nervous system was mainly a sensor network connected directly to the skeletomuscular system, allowing external and internal causes to give rise to organism-centric effects called behaviors. In more complex organisms, the nervous system functions as an independent entity, processing inputs extensively before returning carefully chosen outputs. In all animals, including humans, the vast majority of this is unconscious, automatically executed by neural programs that have been hardwired by millions of years of evolutionary design.

The central nervous system is the most complex system and the most difficult to understand or reverse-engineer in any species, although efforts in this direction have had some success. For the purposes of this article, the central nervous system can be thought of as the brain and spine, which includes the brain stem. Further subdivisions are the province of cognitive science and neuroanatomy.

The peripheral nervous system has several subdivisions. The first level consists of the somatic nervous system and the autonomic nervous system, which refer to the nerves just under the skin and the nerves everywhere else, respectively. Despite their names, both mostly execute automatically, but the autonomic nervous system is so named because it is responsible for the body’s maintenance functions, which have a reputation for being opaque to conscious control. The nerves we use to consciously control our bodies are part of the somatic nervous system, but these function automatically even in the event of a coma.

The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. A third division, the enteric nervous system, is also occasionally referred to. The sympathetic nervous system responds to stress, danger, and the like, and is responsible for the release of adrenaline, among other things. The parasympathetic nervous system is dominant during rest and helps us go to sleep and digest food. These two nervous systems tend to balance each other, creating a degree of harmony in the body. The enteric nervous system is responsible for some nerves around the intestines, and is known to function properly even when disconnected from the other two systems.

What is Lymph?

Lymph is a clear to yellowish watery fluid which is found throughout the body. It circulates through body tissues picking up fats, bacteria, and other unwanted materials, filtering these substances out through the lymphatic system. It is sometimes possible to see your own lymph; cuts sometimes weep clear lymph rather than blood, for example. The circulation of lymph through the body is an important part of immune system health.

This clear fluid contains white blood cells, known as lymphocytes, along with a small concentration of red blood cells and proteins. Because white blood cells are smaller than red blood cells, they can pass easily through membranes which red blood cells cannot penetrate. The lymph circulates freely through the body, bathing cells in needed nutrients and oxygen while it collects harmful materials for disposal. You could think of lymph as the milkman of the body, dropping off fresh supplies and picking up discarded bottles for processing elsewhere.

As lymph circulates, it is pulled into the lymphatic system, an extensive network of vessels and capillaries which is linked to lymph nodes, small nodules which act as filters to trap unwanted substances in the lymph. Lymph nodes also produce more white blood cells, refreshing the lymph before it is pumped out of the lymphatic system and back into the body. Lymph may not be as showy as blood, but it is related to an equally complex and ornate system of vessels.

Lymph also explains why things like intramuscular shots at the doctor's office work. When the doctor injects a substance into your muscle tissue, the substance is picked up by the lymph and then slowly filtered into the bloodstream. You may also have noticed that when you wear tight clothing or your circulation is otherwise impeded, fluids build up, causing edema, which can be both painful and dangerous. Edema happens when lymph cannot circulate to pull these fluids out.

The lymphatic system can sometimes be used as a diagnostic tool to help doctors understand disease. Lymph nodes can be biopsied, for example, to collect evidence about bacterial agents and toxins in a patient's body. Some types of bodywork are also designed to promote the healthy circulation of lymph to encourage drainage and healthy tissues. Lymphatic massage and other types of bodywork require special training, as a massage therapist can inadvertently cause lymphedema, a collection of fluid on a limb which can become dangerous and extremely painful.

What are Platelets?

Platelets, along with red cells and plasma, form a major proportion of both human and animal blood. Microscopically, they look like little thorned or spiky ovals, and they can only be viewed microscopically, as the average size is about four hundred thousandths of an inch (1 to 3.5 um). Platelets are actually fragments of the cells in bone marrow, called megakaryocytes. Stimulated by the hormone thrombopoietin, platelets break off the megakaryocytes and enter the blood stream, where they circulate for about 10 days before ending their short lives in the spleen. In the healthy body, thrombopoietin will help to maintain the count of platelets at a normal level, which is approximately 4.2-6.1 million of these tiny cells in two hundred thousandths of a teaspoon (1ul) of blood.

Most are familiar with the blood's ability to coagulate should one receive a cut or bruise. Specifically, platelets provide the necessary hormones and proteins for coagulation. Collagen is released when the lining of a blood vessel is damaged. The platelet recognizes collagen and begins to work on coagulating the blood by forming a kind of stopper, so further damage to the blood vessel is prevented.

A higher than normal count of platelets, known as thrombocytosis, can cause serious health risks. Too much clotting of the blood can lead to formation of blood clots that can cause stroke. Conversely, lower than normal counts can lead to extensive bleeding.

However, in some cases, inducing a lower platelet count is desirable, for instance if a person has susceptibility to strokes or has had extensive heart repair. Platelet counts can be lowered by a daily intake of aspirin or other clot reducing drugs. Additionally, when a patient has an intravenous drip (IV), heparin is used to keep the IV from clotting so fluids can be either taken from or added to the body.

While disease or a genetic disorder can cause a lower number of platelets, other times, they are depleted because of a specific treatment or surgery. Burn victims, organ transplant patients, marrow transplant patients, those undergoing chemotherapy, and those who have undergone heart surgery often require not only blood transfusions but platelet transfusions as well.

Almost anyone who is able to donate blood, and is not taking aspirin or other anti-coagulants, is also eligible for platelet donation, called platepheresis or apheresis. In this case, blood is drawn and placed in a centrifuge, where the platelets are separated from the other blood products. The rest of the blood is returned to the body, instead of being collected as it would be in a regular blood donation. The procedure takes from about 90 minutes to two hours.

Once collected, platelets only have a shelf life of about five days, and one donation provides only a sixth of a platelet transfusion unit. Given that bone marrow transplant patients often require up to 120 units of platelets, it is a foregone conclusion that new platelet donations are required daily. Information about platelet donation is available from local blood banks.

What is Systolic Blood Pressure?

Blood pressure is a measurement of how much force the blood exerts on the walls of the blood vessels. There are many different events occurring within the body as the heart pumps blood, known collectively as the cardiac cycle, and so blood pressure is measured at different points throughout this cycle. Systolic blood pressure measures the maximum pressure in the arteries during the cardiac cycle, which occurs when the heart contracts, or beats, to pump blood.

Systolic blood pressure is measured during routine visits to your physician. It is recorded in mmHg, or millimeters of mercury. Most doctors use the ausculatory method of blood pressure measurement. The reading is taken with the patient seated with his or her arm slightly bent and at the same level as the heart.

A cuff is wrapped around the arm an inch above the elbow, and a stethoscope placed on the large brachial artery in the arm. The cuff is inflated to about 30 mmHg higher than the patient's last recorded systolic blood pressure, at which point blood flow is negligible. The cuff is then slowly deflated, and the health care provider records the level at which the patient's pulse can first be heard through the stethoscope. This is the systolic blood pressure.

When the pulse is no longer audible, a second number is recorded: the diastolic pressure, or the lowest amount of pressure in the arteries, occurring while the heart is at rest between beats. These two numbers are recorded as your blood pressure -- 110/70, for example. The systolic blood pressure reading is the first number, and the diastolic pressure the second. Some doctors also use the oscillometric method of measurement, which is similar but uses an electronic pressure sensor to record readings instead of a stethoscope.

Blood pressure is affected by medication, cardiovascular or urological disorders, neurological conditions, and psychological factors such as stress or anger. Even diet and posture can play a role. Because there are so many variables, healthy blood pressure readings can fall anywhere inside a large range. A healthy adult will have systolic blood pressure between 90 and 135 mmHg. Diastolic pressure ranges from 50 to 90 mmHg.

If a patient's systolic blood pressure reading is consistently higher than 120 mmHg, they should consult their physician immediately, particularly if they are middle-aged or older. High systolic blood pressure readings are the most accurate means of detecting hypertension, or high blood pressure, in middle-aged and older adults. In many cases, only systolic blood pressure is high, a condition known as isolated systolic hypertension or ISH. Lowering systolic blood pressure to healthy levels can help prevent congestive heart failure and stroke.

What is Hemoglobin?

Hemoglobin is a protein-based component of red blood cells which is primarily responsible for transferring oxygen from the lungs to the rest of the body. Hemoglobin is actually the reason red blood cells appear red, although oxygen-rich blood is noticeably brighter than the depleted blood returning to the heart and lungs. Fresh hemoglobin is produced in the bone marrow as needed.

The creation of hemoglobin is controlled by a complicated genetic code. Because unborn babies obtain their oxygenated blood from their mothers and not their own lungs, two separate substances called hemoglobin alpha and hemoglobin gamma combine with several nitrogen atoms and one iron atom. This allows the fetus to receive oxygen-rich blood without respiration. Once the infant is born, however, the body replaces hemoglobin gamma with a new variant called hemoglobin beta. The combination of these two substances continues for a lifetime.

Essentially, hemoglobin develops a hunger for oxygen molecules. When the blood is carried into the lungs, hemoglobin proteins containing iron atoms attract whatever oxygen is available. This oxygenated blood then travels throughout the entire bloodstream, releasing oxygen into the muscles and organs. The spent red blood cells are transferred to the gastrointestinal system for disposal and new red blood cells with hemoglobin take their place in the bloodstream.

This ongoing system of hemoglobin proteins obtaining oxygen from the lungs and delivering it to the cells is based on ideal conditions, however. Sometimes the alpha or beta proteins produced by the genetic code are not perfectly formed, as in the case of sickle cell anemia. One of the components is shaped like a sickle, causing an imperfect bond to form.

Anemia means that the red blood cells lack sufficient levels of iron. Without an iron atom, the damaged hemoglobin pigment cannot attract oxygen in the lungs very effectively, if at all. The result can be a slow wasting process leading to complete body dysfunction.

Hemoglobin can also be compromised by blood conditions such as diabetes or cancer. Many standard blood tests included a general check of hemoglobin levels. The amount of glucose in the bloodstream may vary from hour to hour, but an examination of hemoglobin often provides a more accurate reading for diabetics.

Another difficulty with hemoglobin is its affinity for gases other than oxygen. Hemoglobin is 200 times more attracted to carbon monoxide than oxygen, for example. This means that someone breathing in carbon monoxide from automobile exhaust could be replacing the oxygen in their lungs with a poison. If enough hemoglobin is exposed to carbon monoxide, the result could be the same as asphyxiation. Cigarette smokers who regularly breath in carbon monoxide could compromise as much as 20% of their total lung oxygen supply.

This attraction to other gases can actually be beneficial under controlled circumstances. Hemoglobin is also attracted to gases used during anesthesia proceedings before surgery. The nitrous oxide or other breathable anesthetic is carried into the brain through the hemoglobin, which allows the surgical team to control the patient's level of consciousness. As oxygen is reintroduced into the patient's lungs, the hemoglobin refreshes itself and the other gases become waste products.

What Is the Respiratory System?

The respiratory system is a group of organs that supply the body with oxygen. The system consists of the nose, mouth, throat, lungs, and diaphragm. These organs work together to convert the air that is breathed in into oxygen for the blood. The body’s cells require oxygen in order to function, so if the respiratory system does not work properly, it can cause serious health complications or even be fatal.

The process of receiving oxygen from the respiratory system begins when a person inhales air from the outside through his or her nose or mouth. Once the outside air makes it way into the body, it travels into the throat. The first area of the throat the oxygen passes through is the larynx, also known as the voice box, which is responsible for speech. Oxygen then goes through the trachea, also known as the windpipe, which is a thin cylinder that connects the bottom of the larynx down into the chest.

The chest cavity is the main location of the majority of the respiratory system organs. The bottom of the trachea divides into two cylinders known as bronchi. Oxygen travels through the bronchi into the alveoli, which are millions of small pockets of air. These little pockets hold inhaled air and deliver it into the bloodstream. They also take unneeded products from the bloodstream, such as carbon dioxide, so it can be prepared to exit the body.

The respiratory system is also the process of ridding the body of carbon dioxide, a chemical produced by cells that cannot be used for energy. When the alveoli bring down the carbon dioxide from the bloodstream, it goes back up to exit through the same organs that brought the oxygen in. The diaphragm is a muscle group located in the chest and both constricts and loosens during the breathing process. It controls the simultaneous entrance of fresh oxygen and exit of carbon dioxide. During the diaphragm constriction, fresh oxygen enters the body, and carbon dioxide exists during the loosening of the diaphragm.

When a part of the respiratory system fails to function correctly, it can make breathing difficult. A common respiratory condition is bronchitis, in which the bronchial tubes become irritated and end up making too much mucus. That extra mucus makes a person with bronchitis cough profusely in an attempt to clear the mucus out of the bronchi. A more serious, lifelong respiratory condition is asthma, in which the lungs constrict in the presence of dust, smoke, or any other substances that are inhaled. People with asthma may not be able to breathe on their own when their lungs constrict and will have to use an inhaler, a device that helps loosen the lungs with medication.

What is the Cardiovascular System?

The cardiovascular system, also known as the circulatory system, is a system of the body comprised of the heart, the blood, and the blood vessels. This system is responsible for transporting blood. As the cardiovascular system moves blood throughout the body, cells receive oxygen and nutrients. Carbon dioxide and other wastes are removed from the body as well. The word cardiovascular stems from the Greek word kardia which means "heart" and the Latin word vasculum which means "small vessel."

In this complex system, the heart acts as a pump, forcing the blood to move through the body and relaxing so that more blood can enter its chambers. The majority of the blood is comprised of plasma, a watery fluid filled with protein. Less than half of the blood is made up of platelets and red and white blood cells. The platelets help blood to clot if a person suffers a cut or hemorrhages.

It's important to maintain a healthy cardiovascular system since the blood and blood vessels are crucial to good health. The cardiovascular system is the workhorse of the body, continuously moving to push blood to the cells. If this important system ceases its work, the body dies.

The heart contracts more than 100,000 times daily as it pushes blood through the blood vessels. As it contracts, it forces blood into the bloodstream. The blood transports nutrients from the digestive system and oxygen from the lungs to the body's cells. Then the blood carries waste products that are removed by the kidneys and carbon dioxide that is expelled by the lungs.

The heart is a muscle about the size of a fist and is divided into four chambers. These chambers are the right atrium, the left atrium, the right ventricle, and the left ventricle. During the circulatory process, blood enters the heart's right atrium. As the heart contracts, blood moves through a valve from the right atrium into the right ventricle. The blood then flows through another heart valve into the lungs.

This is where the blood picks up oxygen. At this point, the blood flows to the heart's left atrium and through a valve into the left ventricle, from where it then flows through a valve into the aorta. Upon leaving the aorta, the blood travels to the remainder of the body, carrying much needed nutrients and oxygen to the body's cells.

When problems arise within the cardiovascular system, a person suffers from a cardiovascular disease. More than 60 kinds of cardiovascular diseases can cause serious health problems. Common diseases include stroke or heart disease. Some conditions such as congenital heart disease are present when a person is born; other cardiovascular diseases develop gradually as a person grows into adulthood.

What is the Central Nervous System?

The central nervous system (CNS) is one of the two parts of the nervous system. The other is the peripheral nervous system which includes nerves in the organs, muscles, arms, and legs. The central nervous system consists of the brain and the spinal cord. The central nervous system is the "control center" for the entire body and regulates how the body will function.

The average brain weighs three pounds (1.3 kg) and contains 100 billion nerve cells, or neurons. The skull encloses the brain in bone. The brain contains three main areas: the cerebrum, the cerebellum, and the medulla oblongata. The cerebrum is the conscious part of the brain, while the cerebellum and the medulla oblongata form the part of the brain that controls unconscious behavior from the central nervous system.

The medulla oblongata, along with the pons, regulates heartbeat, blood pressure, breathing, and reflexes such as swallowing and coughing. The medulla oblongata is in the part of the brain known as the hindbrain and is nearest to the spinal cord. The cerebellum, connected to the back of the brainstem, is also part of the hindbrain in the central nervous system. The cerebellum coordinates fine motor movement and regulates balance and posture.

The cerebrum is in the forebrain and relates to the central nervous system functioning of reasoning, intelligence, learning, and memory. The cerebrum also regulates sensory and motor controls. The cerebrum is the biggest part of the brain and is divided into the left and right hemispheres. The corpus callosum separates the cerebral hemispheres and is made up of nerve fibers.

The cerebral cortex is the covering on the brain's two hemispheres. Much of the sensory and motor control functioning of the central nervous system is located in the four divisions of the cerebral cortex lobes. The four divisions are the occipital, temporal, parietal, and frontal lobes.

The occipital lobe relates to the functioning of the eye and its visual messages. The temporal lobe relates to the ear and its processing of sounds. The parietal lobe relates to sensory messages such as taste, touch, pain, pressure, and temperature sensations such as hot or cold. The frontal lobe relates to thought, speech, and motor skills. All four lobes function together as part of the central nervous system.

The spinal cord section of the central nervous system connects the brain to the body. The spinal cord is enclosed by the backbone, or vertebral column. Nerve cells work two ways between the brain and the spinal cord in the central nervous system: they carry messages to the brain from the rest of the body and they carry messages from the brain to the rest of the body.

What is the Difference Between Arteries, Veins, Blood Vessels and Capillaries?

Arteries, veins, and capillaries are in fact all forms of blood vessels, just with different shapes and roles in the body. Blood vessels are an integral part of the circulatory system, which transfers oxygen and important components of life around the body and removes waste. Each of the three major types of blood vessels play their own role in this complex system, helping to keep a human body functioning at full strength and health.

The arteries are those blood vessels that carry blood away from the heart. This means that, with only two exceptions, arteries are carrying highly oxygenated blood to transport oxygen to the tissue of the body. Arteries are the higher-pressure part of the circulatory system, as they are getting blood from the heart. The pressure in the arteries differs between when the heart contracts and when it expands, the systolic and diastolic pressure, respectively. It is this pressure shift that can be felt as a pulse.

The largest artery in the body is the aorta, in the heart. The aorta receives blood from the heart’s left ventricle, then branches off into smaller and smaller arteries, eventually turning into arterioles, which supply the capillaries with blood. Pulmonary arteries are another special type of artery, which carry deoxygenated blood from the heart to the lungs, when they can be replenished, disposing of their carbon monoxide and gathering oxygen.

Veins are those blood vessels that carry blood back to the heart, with a few minor exceptions. For the most part, veins are carrying deoxygenated blood back to the heart, although this is not the case in either pulmonary or umbilical veins, where they carry oxygenated blood. Veins are basically tubes that just collapse when not filled with blood. Within veins are flaps that keep the blood flowing towards the heart, rather than being pulled down and pooling by the effects of gravity.

The blood carried by veins, in addition to having little oxygen, is also filled with carbon dioxide and various forms of cellular waste. Blood moves through the veins back to the heart, where it enters in the right ventricle, where it is then pumped into the lungs by the pulmonary artery, and then back through the heart via the left atrium.

Both veins and arteries are most easily defined not by the oxygen content of the blood, which is generally high for arteries and low for veins, but not always, but rather by the direction of blood flow. Arteries are always moving blood away from the heart, while veins are always moving blood towards the heart. Capillaries, on the other hand, act as intermediaries, connecting arterioles and venules.

Capillaries serve the function in the circulatory system of helping to facilitate the exchange of various things between blood and tissue. When the arteries bring blood to an area of tissue, they pump the blood into the capillaries, which can then essentially drop off the oxygen, water, and nutrients. The tissue can then dispose of its cellular waste and carbon dioxide, which the capillaries then pump back into the veins to be returned to the heart and lungs.

What Is Proprioception?

Proprioception — from Latin proprius, meaning "one's own," and perception — is one of the human senses. There are between nine and 21 in all, depending on which sense researcher you ask. Rather than sensing external reality, proprioception is the sense of the orientation of one's limbs in space. This is distinct from the sense of balance, which derives from the fluids in the inner ear, and is called equilibrioception. Proprioception is what police officers test when they pull someone over and suspect drunkenness. Without proprioception, we'd need to consciously watch our feet to make sure that we stay upright while walking.

Proprioception doesn't come from any specific organ, but from the nervous system as a whole. Its input comes from sensory receptors distinct from tactile receptors — nerves from inside the body rather than on the surface. Proprioceptive ability can be trained, as can any motor activity.

Without proprioception, drivers would be unable to keep their eyes on the road while driving, as they would need to pay attention to the position of their arms and legs while working the pedals and steering wheel. And I would not be able to type this article without staring at the keys. If you happen to be snacking while reading this article, you would be unable to put food into your mouth without taking breaks to judge the position and orientation of your hands.

Learning any new motor skill involves training our proprioceptive sense. Anything that involves moving our arms or legs in a precise way without looking at them invokes it — baseball, basketball, painting, you name it. Proprioception is often overlooked as one of the senses because it is so automatic that our conscious mind barely notices it. It is one of the oldest senses, probably even more evolutionarily ancient than smell.

Among other reasons, proprioception is known to be a distinct sense because there are cases in which the proprioceptive ability is absent in a patient. This means that proprioception uses dedicated brainware. Proprioception-disabled patients can only walk by paying attention to where they put their legs. Thankfully, this condition is extremely rare

What Is Homeostasis?

Homeostasis is a point of balance or internal equilibrium. All living organisms strive for homeostasis, using a variety of techniques which range from the release of hormones to physical reactions like sweating or panting. In a simple example of homeostasis, the human body uses several processes to regulate its temperature, trying to stay within an optimal range for healthy functioning. Spikes or declines in body temperature reflect an inability to maintain homeostasis, and a corresponding problem.

One way to think about homeostasis is to imagine a set of scales. If coins are poured into one side of the scale, the scales slip out of balance. If weights are piled onto the other side, the scales will eventually balance. If too many weights are added, the scales will become unbalanced again. The body is like a set of scales, working constantly to achieve a state of balance. Unlike scales, the body is extremely complex, requiring numerous tiny adjustments every second, and new input is constantly putting the body off-balance.

There is one process in homeostasis known as negative feedback. Negative feedback reflects the body's desire to return to a normal state, signaling that a problem is occurring and regulating the resulting processes to ensure that the body reaches homeostasis rather than going too far in the wrong direction. In positive feedback, the body encourages the rapid increase of an activity to deal with an emerging situation, as for example when white blood cell production increases to cope with an infection.

A number of things can interfere with the body's desire to achieve homeostasis, causing a variety of medical conditions. Hormone imbalances, infections, dehydration, gout, and hypoglycemia are all examples of conditions which are related to homeostasis. Homeostasis can also be disrupted by introducing toxins into the bloodstream, including toxins which are meant to have a medical benefit, such as chemotherapy. Blood pressure, temperature, blood sugar, hormone levels, and enzyme levels are all homeostatic processes in the human body, and problems with any of these processes can indicate the presence of an underlying medical condition.

People are generally healthier when their bodies are in a state of homeostasis, and their bodies send numerous messages to promote homeostasis. For example, when low blood sugar appears to be developing, people tend to feel hungry, because their brains tell them to eat. Specific cravings can emerge in response to nutritional deficiencies, with the brain essentially creating a shopping list which will help the body reach a state of balance. People may also feel driven to drink water, exercise, or engage in other activities.

How does the Digestive System Work?

The human digestive system is a sequence of organs that use mechanical and chemical means to take in food, break it down, extract nutrients and energy, and eject waste products in the form of urine and feces. The digestive system evolved incrementally over the course of hundreds of millions of years and is the only natural way for humans to obtain energy for movement and thinking. It is capable of handling a variety of food sources, both animal and vegetable, but tends to handle food best when it is cooked. Because cooked food has been around for so long, humanity as a species is slightly “spoiled” in its favor, and many people get sick if they consume food that has not received adequate cooking.

The mouth is the entrance to the human digestive system. Teeth gnash the food, breaking it down mechanically, while the three salivary glands release saliva containing the enzyme amylase, which breaks down starch and fat chemically. Saliva makes food easier to swallow by moistening it, as well as preventing the erosion of tooth enamel by modulating pH.

After entering the body at the back of the throat, food travels down the esophagus, being transported not by gravity but by muscular contractions. This is why it is possible to eat while hanging upside down. The interior of the esophagus is very moist, which helps to further break down food and prevent damage to the rest of the digestive system.

After moving through the esophagus portion of the digestive system, food and drink reaches the stomach, where it is further broken down into manageable pieces. Because the nutrients in food are ultimately meant to be consumed by cells, they must be broken into very small parcels for delivery. The primary agent of digestion in the stomach is gastric juices, which are produced in large amounts and can be very acidic. A secondary agent is muscular contractions within the stomach.

After the stomach, the broken down food moves into the small intestine, the portion of the digestive system where most of the nutrient extraction takes place. As the food moves through the small intestine, it is mixed with bile, which is produced by the liver, as well as pancreatic juices, which perhaps unsurprisingly come from the pancreas. These two liquids help further the digestive process, breaking down the nutrients in food to the point where it can be absorbed by the blood. The inner intestine is home to the famous villi, tiny living extrusions which gather nutrients on a fine scale.

The final components of the digestive system are the large intestine or colon, the anus, and the urinary tract, which separate the liquid matter from the solid matter and send them to their respective exit ports. Of course, the human digestive system is not 100% efficient, and there are many nutrients left over in this “waste”, which will be consumed happily by bacteria or sent through a waste processing plant.

What are Hormones?

Hormones carry messages from glands to cells to maintain chemical levels in the bloodstream that achieve homeostasis. "Hormone" comes from a word that means, "to spur on." This reflects how the presence of hormones acts as a catalyst for other chemical changes at the cellular level necessary for growth, development, and energy.

As members of the endocrine system, glands manufacture hormones. Hormones circulate freely in the bloodstream, waiting to be recognized by a target cell, their intended destination. The target cell has a receptor that can only be activated by a specific type of hormone. Once activated, the cell knows to start a certain function within its walls. Genes might get activated, or energy production resumed. As special categories, autocrine hormones act on the cells of the secreting gland, while paracrine hormones act on nearby, but unrelated, cells.

There are two types of hormones known as steroids and peptides. In general, steroids are sex hormones related to sexual maturation and fertility. Steroids are made from cholesterol either by the placenta when we're in the womb, or by our adrenal gland or gonads (testes or ovaries) after birth. Cortisol, an example of a steroid hormone, breaks down damaged tissue so it can be replaced. Steroids determine physical development from puberty on to old age, as well as fertility cycles. If we are not synthesizing the correct steroidal hormones, we can sometimes supplement them pharmaceutically as with estrogen and progesterone.

Peptides regulate other functions such as sleep and sugar concentration. They are made from long strings of amino acids, so sometimes they are referred to as "protein" hormones. Growth hormone, for example, helps us burn fat and build up muscles. Another peptide hormone, insulin, starts the process to convert sugar into cellular energy.

Hormones so perfectly and efficiently manage homeostasis due to negative feedback cycles. Our goal is to keep the concentration of a certain chemical, such as testosterone, at a constant level for a certain period of time, the way that a thermostat works. Using negative feedback, a change in conditions causes a response that returns the conditions to their original state. When a room's temperature drops, the thermostat responds by turning the heat on. The room returns to the ideal temperature, and the heater turns off, keeping the conditions relatively constant.

What is the Difference Between Red and White Blood Cells?

Red blood cells and white blood cells are, in essence, completely different. While both are necessary for the body's proper functioning, they each have singular roles. Red blood cells carry oxygen, while white cells do not, for example. Red blood cells in humans do not have nuclei, while white cells do.

Red blood cells, also called erythrocytes, are responsible for the characteristic color of our blood. They are responsible for picking up carbon dioxide from our blood and for transporting oxygen. The essential component of red blood cells is hemoglobin, which can hold oxygen so the cells can then transport around the body. This process is what gives the body energy, which explains why people who suffer from anemia — low count red blood cells — often feel tired and sleepy. A high count of red blood cells is rare, but it can happen. Causes include kidney disease, dehydration, anabolic steroid use, and pulmonary fibrosis. People suffering from a high count of red blood cells usually have impaired circulation, and are at a high risk for heart disease.

While blood cells or leukocytes, on the other hand, are primarily responsible for fighting foreign organisms that enter the body. This includes everything from bacterial and parasitic infections to allergic response. T-cells, a form of white blood cells, are the ones that stop functioning properly in the presence of an HIV infection. An overproduction of white blood cells can lead to leukemia. On the other hand, certain medications, such as Clozapine®, used in psychiatry, can reduce the number of white cells significantly.

There are approximately 5 million red blood cells in every cubic millimeter of blood; there are only 3,000 - 7,000 white blood cells in the same amount of blood. Red blood cells have an average lifespan of 120 days, while white cells live a maximum of four days.

Red blood cells have a circular shape that resembles a shallow bowl, but they can change shape without breaking to squeeze through smaller spaces if necessary. White blood cells have different shapes, depending on their function. While they can multiply easily, they don't change shape.

What is Cartilage?

Cartilage is a type of connective tissue in the body. It is made of cells called chondrocytes embedded in a matrix, strengthened with fibers of collagen and sometimes elastin, depending on the type of cartilage. There are three different types: hyaline cartilage, elastic cartilage, and fibrocartilage. Cartilage serves to provide structure and support to the body's other tissues without being as hard or rigid as bone. It can also provide a cushioning effect in joints.

Cartilage is avascular, meaning that it is not supplied by blood vessels; instead, nutrients diffuse through the matrix. Cartilage is usually flexible, again depending on the type. Some of the bodily structures that include cartilage are the ears, nose, ribcage, and intervertebral discs.

Hyaline cartilage makes up the majority of the body's cartilage. It lines the bones in joints, helping them to articulate smoothly. Hyaline cartilage contains mostly type II collagen fibers.

Elastic cartilage is more flexible than the other types of cartilage because of the elastin fibers it contains. This type of cartilage is found in the outer ear, the larynx, and the Eustachian tubes, for example. It provides the perfect balance of structure and flexibility and helps keep tubular structures open.

Fibrocartilage is the strongest and most rigid type of cartilage. It contains more collagen than hyaline cartilage, including more type I collagen, which is tougher than type II. Fibrocartilage makes up the intervertebral discs, connects tendons and ligaments to bones, and appears in other high-stress areas. Damaged hyaline cartilage is often replaced with fibrocartilage, which unfortunately does not bear weight as well due to its rigidity.

There are a few disorders associated with cartilage. Chondrodystrophies are a group of disorders in which the cartilage is ossified, or transformed into bone. Arthritis is characterized by the degradation of cartilage in the joints, leading to limited movement and pain. Achondroplasia is a cartilage disorder resulting in dwarfism. Benign tumors called chondroma can also arise in the cartilage.

What are Ligaments?

Ligaments are the fibrous, slightly stretchy connective tissues that hold one bone to another in the body, forming a joint. Ligaments control the range of motion of a joint, preventing your elbow from bending backwards, for example, and stabilizing the joint so that the bones move in the proper alignment.

Ligaments are composed of strands of collagen fibers. While ligaments are slightly stretchy, that they are arranged in crossing patterns prevents the joint itself to become loose.

Stretching exercises increase the length and flexibility of the muscles, allowing the joint to move farther than before. The ligaments themselves are not stretched, as they provide the support for the joint. This stretching of the muscles is what allows martial artists to perform seemingly impossible kicks and contortionists to stretch their bodies into fantastic positions.

If ligaments are stretched, either by injury, excess strain on a joint, or by improper stretching techniques, the joint will become weaker, as the elongated ligaments are unable to properly support it. People who are said to be "double jointed" simply have extra long ligaments that allow their joints to stretch beyond the normal range.

Because ligaments are so important in the stabilization of joints, they are also highly susceptible to injury. The anterior cruciate ligament located behind the knee, often referred to as the ACL, is commonly damaged in rough sports.

Because connective tissue such as ligaments must withstand a great deal of stress in day to day activities and have a relatively low blood supply, injuries can take a very long time to heal, and sometimes require surgery. Many professional athletes have had multiple surgeries to ligaments over the course of their careers.

Severe injury to ligaments can often require physical therapy. Even with surgery and physical therapy, injured ligaments tend to be less flexible, and more prone to repeat injury, so patients should be careful when engaging in strenuous activities that can put excess pressure on the injured ligaments.

What Is the Musculoskeletal System?

All of the bones, cartilage, muscles, joints, tendons and ligaments in a person's body compose what is known as the musculoskeletal system. The bones provide the body with a framework, giving it shape and support; they also serve as protection for internal organs such as the lungs and liver. Muscles are fibers that help to make deliberate movement of a body part or involuntary movement within an internal organ possible. Some people view the musculoskeletal system as two body systems in one or two systems that work very closely together, with one being the muscular system and the other being the skeletal system.

The bones of the musculoskeletal system are categorized according to their appearance or shape — short, long, flat and irregular. For example, the humerus, or bone of the upper arm, and the femur, or thigh bone, are long. The vertebrae, which protect the spinal cord, are irregularly shaped.

Bones store salts and metabolic materials and serve as a site for the body's production of erythrocytes, or red blood cells. Although many people think of them as hard structures that don't really present life-threatening situationsw hen fractured or broken, bones are living tissues that, if fractured, can cause the loss of enough blood to bring on hypoperfusion, also known as shock. A broken femur often causes a blood loss of two pints (1,000 cc), and a pelvic fracture can cause a person to lose as much as four pints (4,000 cc) of blood.

There are three categories of muscles in the musculoskeletal system — voluntary, involuntary and cardiac. Voluntary muscles are fibers that allow for conscious movement of body parts; when a person walks, voluntary muscles are at work. Involuntary muscles are located within certain internal organs such as the esophagus, the tube that leads to the stomach, in which muscles contract to help move food downward. Cardiac muscles are found within the heart, an organ whose movement is a constant pumping action.

Joints, as well as voluntary muscle, are involved in a person's ability to move a body part deliberately. They are the places where bones articulate, such as the elbow or knee. Cartilage is a strong and tough tissue that covers joint ends, and it helps to form some parts of the body, such as the outer ear. Tendons are pieces of tissue that connect muscles to bones, and ligaments connect one bone end to another. This connective system of the musculoskeletal system not only allows for the power of movement but also permits a person to have a specific range of motion.

What is the Endocrine System?

The endocrine system is a collection of glands and organs that produce and regulate hormones in the bloodstream to control many functions of the body. This system overlaps with the nervous and exocrine system, and its responsibilities include metabolism, growth, and sexual development. Most animals with advanced physiology, such as vertebrates and crustaceans, house an endocrine system.

The major glands of the endocrine system are the pituitary, hypothalmus, and pineal located in the brain, the thyroid and parathyroids in the neck, the thymus, adrenals, and pancreas in the abdomen, and the gonads, either ovaries or testes, in the lower abdomen. To a lesser degree, organs such as the heart, lungs, and stomach are involved in hormone management. These glands must control everything from when we fall asleep to when we reach our adult height.

Hormones made in our glands work like messages. Just like in life, the proper message must reach its intended destination to be effective. So certain hormones are designed to only end up at certain cells, called target cells. Once the hormone reaches the right cell, it links to a receptor spot, which informs the cell what to do next. It might start making energy out of sugar or trigger ovulation. But the hormone won't interfere with a non-target cell, and the target cell won't react to any chemical other than its special hormone. Scientists don't completely understand the multiplicitous roles of all hormones, but they do know how good they are at maintaining homeostasis, a healthy balanced state.

The hormones produced by these important glands are far too numerous and complicated to list. For example, the pituitary gland is often called the "master gland" because it controls the functioning of other members of the endocrine system. The pineal gland makes melatonin, which decides that we sleep when its dark and wake when its light because we are diurnal animals. The pancreas produces insulin, which decides how much sugar to keep circulating in our blood.

We can think of the endocrine system as one that determines the cycles of our body. For instance, we have a reproductive cycle, a sleep cycle, and a nutrition cycle. We don't need to eat at perfectly spaced intervals to have a constant supply of energy. Nor do we have the same fertility rates throughout our lives. Without the endocrine system, we would have no long-term development like bone growth, nor any short-term cycles like hunger.

How Does the Circulatory System Work?

The circulatory system brings the body's cells what they need to survive - oxygen and nutrients. Only the most primitive animals lack a circulatory system.

The center of the circulatory, or cardiovascular system, is the heart, a powerful pump organ designed to beat many millions of times over the lifetime of an organism. The heart circulates blood throughout the veins and arteries. Arteries carry oxygen-rich blood away from the heart, providing it to tissues, then returning the depleted red bloods cells back to the heart through the veins for reoxygenation.

All the body's stationary cells are surrounded by interstitial fluid, also known as extracellular fluid, which is designed to draw oxygen and nutrients from red blood cells passing by. Red blood cells float in a medium called plasma which is similar to interstitial fluid, and makes up most of the volume of the blood, the primary fluid of the circulatory system.

The largest artery in the human body is the aorta, running through the neck and immediately proximate to the heart. The heart oxygenates red blood cells in its ventricles, or compartments, regulated by valves. The lungs receive fresh oxygen from the air outside, then relaying it to the heart. Complex multicellular organisms such as human beings need air with a fair amount of oxygen (15-25%) in it to survive. Plants, and many microbes, can survive in oxygen-free environments - unlike animals, they require carbon dioxide for respiration.

If the operation of the heart is interrupted, the organism is likely to quickly die, after brain damage begins to set in. This sometimes happens during major heart attacks. By using artificial heart stimulation systems, modern medicine is able to keep the circulatory systems of such victims alive long enough for surgery.

Anthropods and molluscs lack typical circulatory systems - in their bodies, there is no distinction between blood and the interstitial fluid - a material taking both properties simply bathes the organs in the necessary oxygen.

What is the Muscular System?

The muscular system is an extensive network of muscle and nervous tissue which is spread throughout the body. It is controlled by the central nervous system, which sends out an assortment of signals to keep the body running smoothly. There are over 650 muscles active in the human body, and the muscular system can comprise up to 40% of someone's weight. This complex interconnected system is essential for human life; without it, people cannot move and perform an assortment of bodily processes which are essential to keep the body in working order.

There are three different kinds of muscle: voluntary, involuntary, and cardiac. Cardiac muscles, as you might imagine, are located in the heart, and they are a form of involuntary muscle. These muscles keep the heart beating, ensuring the blood is pumped through the body. They are controlled by the autonomic nervous system.

Involuntary muscles line the internal organs of the body, contracting and relaxing to push a variety of substances through the body. These muscles are also controlled by the autonomic nervous system, which sends an assortment of signals to keep them working smoothly. Involuntary muscles are also known as “smooth muscles,” and they control things like your stomach, digestive tract, reproductive tract, breathing, and so forth. When signals to these muscles are interrupted, it can be catastrophic.

Voluntary muscles are the muscles you use to do things like throwing balls, walking, lifting objects, and so forth. They are also called skeletal muscles, and they are controlled by signals from your brain which trigger them into a contraction, generating the desired movement. They often work in pairs to accomplish the desired goal. These muscles are vulnerable to stress, as you may be aware if you have ever “pulled” a muscle.

The amount of coordination and communication involved to keep the muscular system running is rather impressive. In addition to keeping the functions of the body in order, the muscular system also provides the support which allows your body to stand up, and it connects the skeletal system. Many people like to exercise to tone and strengthen the muscular system, expanding their range of motion and allowing themselves to perform a wide variety of tasks, from competing in triathlons to dancing.

What Is a Motor Unit?

A motor unit consists of one alpha motor neuron together with all the muscle fibers it stimulates. Since the human body contains, on average, 250,000,000 muscle cells and approximately 420,000 motor neurons, a motor unit will generally consist of a single motor neuron paired with many muscle fibers. In strength training, the early strength gains seen by novices are often not gains in size or number of muscle fibers, but activation of motor units that had been previously dormant.

The motor neuron is a specialized type of nervous cell that runs between the central nervous system and the muscles. Neurons typically consist of a cell body — the axon — and the dendrites. If a neuron were to be seen as a tree, the axon would be analogous to the trunk and the dendrites to the branches. Neurons found within the brain normally have relatively short axons, but neurons that are part of a motor unit — because they must connect to the muscles of the body — have elongated axons that run through the spinal cord, and out to the associated muscle fibers. Each muscle fiber is connected to a particular dendrite, and it is through the dendrites that messages are relayed between the central nervous system and the muscle fiber.

Muscle fibers are elongated cells, specialized to carry out the functions of the specific muscles of which they are a part. This is true of the cardiac muscles of the heart, the smooth muscles that make up the lining of many internal organs, and skeletal muscles. Only skeletal muscles, however, are under conscious control. The size and shape of the muscle fiber is dependent upon its function, with the smooth muscle cells being flattened and tile-like; skeletal muscle cells, long and rope-like; and cardiac muscle cells having some properties of the other two.

A single muscle usually consists of a number of motor units working together, known as the motor pool. When the central nervous system requires that a muscle contract, an electrical signal is sent along the motor neuron, stimulating the muscle fibers to contract. Normally, each contraction is followed by a brief period of relaxation of the muscle fibers, and this pattern repeats in a wave-like pattern, known as a twitch. Skeletal muscle fibers can be divided into slow twitch and fast twitch fibers, depending on the length of time required for contraction and relaxation to occur. Slow twitch fibers are associated with endurance, while fast twitch muscle fibers are associated with power.

Individuals may have a preponderance of one type of muscle fiber or the other, or a combination of the two. All the muscle fibers within a motor unit will be of a single type, meaning either fast twitch or slow twitch. This may include up to 1,000 muscle fibers, as in the large quadriceps muscles of the thigh, or fewer than ten, as seen in motor units requiring a high degree of precision, such as the muscles that control eye movement.

Upon contraction, the smallest motor unit, that is, the one associated with the fewest muscle fibers, is the normally the first activated. As the contraction progresses, larger motor units are brought into play. Efficient muscle contraction depends on the motor units within a muscle working effectively together. Regular physical training makes this kind of coordination easier.

Occasionally, a motor unit will receive a series of rapid contractile stimulations in such quick succession that the motor pool has no time to enter the relaxation phase of each twitch. When this occurs, it can build up to a state of maximal contraction, known as tetanic contraction. Significantly stronger than a natural twitch, tetanic contraction can result from a number of causes, such as illness or an adverse drug reaction. One of the more well-known reasons for this phenomenon is associated with tetanus infections.

What is Nervous Tissue?

Nervous tissue has two main functions: sensing stimuli and sending impulses to different parts of the body as a response. Nervous tissue is what makes up the body’s nervous system, which is split into the central nervous system and the peripheral nervous system. Central nervous tissue can be found in the brain and spinal chord. The peripheral nervous system is made up of all nervous tissue outside of these areas. Peripheral nervous tissue gathers signals from all parts of the body and sends them to the central nervous system. Nervous tissue is responsible for many of the body’s activities and processes, including memory, reasoning and emotions. Signals from nervous tissue also cause muscle contractions.

Nervous tissue is formed by neurons and glial cells. Humans have billions of neurons, which vary in size, in their bodies. Neurons can be broken down into the cell body, which contains each neuron’s nucleus and mitochondria, and nerve processes. Nerve processes are made of cytoplasm and resemble thin fingers. They extend outward from the neuron and are responsible for transmitting signals both to the neuron and away from it. There are two types of nerve processes: axons and dendrites. Axons carry messages away from the neuron and dendrites transmit signals to the neuron. Axons and dendrites come together to form nerves.

Glial cells, called neuroglia when located in the central nervous tissue, are often found in bunches around neurons in both the central and peripheral nervous systems. These nervous tissue cells are smaller than neurons. Glial cells have a special function when surrounding axons, though they do not transmit neurological signals. These glial cells, called Schwann cells, provide the neurons of the nervous tissue with support, nutrition, and protecting neurons against bacteria. They hold the neurons together. Other types of glial cells include microglia and oligodendrocytes. Microglia help repair damage to the neurons. Oligodendrocytes support the axons.

What Are the Functions of Blood?

The functions of blood are numerous but can essentially be broken down into two subsets. Blood transports things to the various tissues of the body, and it removes things from the tissues of the body. It may be easiest to think of blood as a never-ending series of light rail cars or trains that travel through an immense transportation network. In this sense, the "cars" are constantly either delivering needed elements or carrying away things produced by the body.

As a delivery system, there are numerous functions of blood. Key among these is to deliver oxygen to all tissues, since no area of the body survives without a regular supply of oxygen. This is not the only thing on the "light rail system" that gets delivered, however. Other things that travel in blood include hormones, nutrients, temperature regulating elements, and various forms of nourishment, such as minerals and vitamins.

Certain types of blood cells go into action only if the body requires it, and this is one of the major functions of blood. If bacteria enter the body, white blood cells arrive to fight it. An injury requires cells and materials to help clot blood. These cells are not apart from the rest of blood and are constantly traveling with it. Looking at a cut bleeding, people can’t see the cells as different, but under a microscope, it's easy to see there are many elements that make up blood. By all of these elements traveling together, they are ready to work in the ways needed when required, and these additional "as needed" functions could be called special deliveries, based on individual bodily circumstances.

It's thus clear that one of the functions of blood is to work as a delivery system, but the transportation network has another important job. It also needs to be able to clear away things the body produces or doesn't need. Various types of waste are carried away by blood and may ultimately be excreted from the body or transformed through chemical processes. Blood must also get rid of excess carbon dioxide so that blood levels retain a normal pH balance.

When the functions of blood are suspected as being insufficient or abnormal, one thing doctors may do is perform a complete blood count (CBC) test or other medical tests to determine the efficacy of the cars and the network. Doctors could for instance look at veins and arteries to see if narrowing is creating problems with the "cars" traveling. Alternately, they could look at the blood itself to check it for insufficiencies. Since so much of what bodies are able to do is fully dependent on this vital transportation system, problems with it can be become hugely important and require immediate repair or medical treatment.

What Is a Movable Joint?

A movable joint is the most common type of joint in the human body. Freely movable joints are found wherever movement is important. Movable joints are more commonly known as synovial joints because each contains synovial fluid — a liquid that helps to decrease friction when the joint moves. Synovial fluid is found inside the synovial capsule of the joint. There are many types of movable joint such as hinge joints and saddle joints.

Examples of movable joints include the elbow and shoulder joints. The elbow is a hinge joint that allows movement in a single plane. On the other hand, the shoulder joint is a ball and socket joint that allows a much wider range of movement.

In order for the joint to move correctly and without pain, a layer of fibrocartilage is found within all movable joints. This layer works in tandem with the synovial fluid to make sure the joint glides smoothly. Fibrocartilage gets its name from the fact that it closely resembles a type of fibrous tissue. There is also a layer of hyaline cartilage in the joint that again allows for smoother movement.

Each kind of movable joint throughout the human body provides a different type of movement. For example, pivot joints, such as those found in the neck, allow a person to move his or her head sideways. Ellipsoidal joints allow for movement with a small amount of rotation. Aside from synovial fluid and cartilage, a movable joint is able to have such a wide range of motion whilst still being stable because of the support provided by ligaments and muscles.

Due to its complex nature, a joint can be more prone to injury than other parts of the body. For example, the knee is regularly injured. Often, a problem with one joint can cause a different issue further up the kinetic chain. This is why people with knee injuries are often told to concentrate on increasing the flexibility and mobility of their ankle and hip joints. Injuries to movable joints are treated using many different techniques ranging from stretching exercises to surgery.

Movable joints should not be confused with slightly movable joints. Synovial joints have a large range of mobility whilst slightly movable joints can only move a limited amount. An example of a slightly movable joint is the small joints that are located in the middle of vertebrae in the spinal column.

What is a Ball and Socket Joint?

Many different types of joints exist in the human body, but the kind which allows the greatest range of motion is the ball and socket joint. These joints are present where one bone ends with a spherical knob that lies in a circular depression in the other bone. This arrangement theoretically allows for 360 degrees of rotation -- in other words, a full circle. Each shoulder has a ball and socket joint where the upper arm meets the shoulder blade. The hips also have a ball and socket joint on either side, where the femur meets the pelvic bone.

Both the shoulder and hip, in addition to having a ball and socket joint, are known as synovial joints. These are the most common type of joints in the human body. In a synovial joint, the two bones are not connected in the same way as other joints, but have additional tissue around the moving bones to provide lubrication and nourishment. On the end of each bone, where it touches the other, is a layer of cartilage which allows the bones to move past each other with a minimum of friction. Surrounding the cartilage is a fluid-like substance called synovial fluid. This aids in lubricating the joint, as well as helping the cells of the cartilage to operate efficiently.

The cartilage and synovial fluid are contained within the synovial membrane, which in turn is contained in a fibrous structure called the joint capsule. Beyond the joint capsule are the ligaments which hold the bones in place, and the muscles and tendons that move the bones. This is the basic structure of every ball and socket joint.

There are slight variations in some cases. The shoulder joint, for instance, contains a small sac called the bursa, filled with synovial fluid, which helps the many tendons, bones, and muscles in the shoulder to glide past each other without friction. It also acts as a cushion between the joint itself and nearby bones.

Due to the complex nature of ball and socket joints, they are usually the ones that are most subject to disease and wear. Surgical replacement of the hips and shoulders is not uncommon if the joints become worn enough that they cause severe pain when used. Other diseases and problems characterized by inflammation and/or degeneration, such as arthritis, can exact a particularly heavy toll on ball and socket joints, because of how much we depend on them for movement.

What Are Myofibrils?

Formerly known as sarcostyles, myofibrils are long, bundled tubes of cytoskeleton that run the length of striated muscle fibers. Like all cytoskeletons, myofibrils function in cellular support, movement, and intra-cellular transport. To facilitate this purpose, they are made up of long chains of regular, repeating units known as sarcomeres. These units house the contractile apparatus of the cell. Two microfilaments, primarily composed of actin and myosin, interact within the sarcomeres to produce cellular contraction — enabling movement of the cell, the muscle, and the entire organism.

The two microfilaments that make up the myofibril are generally referred to as thick and thin filaments. Thick filaments are composed mostly of myosin protein and reside near the center of the sarcomere. Thin filaments are made up of three proteins, most notably actin, and sit at the outer edges of the sarcomere. The border between sarcomeres is known as the Z line, a dark band of material that acts as a base for the thin filaments.

Muscle cells themselves are analogous to other cells in a number of ways, however, their increased size and high degree of specialization results in many of their attributes being given names that are particular to muscle cells. This usually involves the use of the prefix 'sarco-.' The cytoplasm of a muscle cell, therefore, becomes the sarcoplasm; the endoplasmic reticulum is known as the sarcoplasmic reticulum; and the cellular membrane is often termed the sarcolemma.

Myofibrils reside within the sarcoplasm and typically occupy most of the space within the muscle cell. Running parallel with the myofibrils are infoldings of sarcolemma known as transverse tubules, or T tubules. These internal channels primarily provide a pathway for neurons. Following the same pathways as other structures within the cell, a specialized organelle known as sarcoplasmic reticulum runs alongside the T tubules. The sarcoplasmic reticulum acts as a storage system for calcium ions.

When a T tubule carries an electrical signal, known as an action potential, into the muscle fiber, the sarcoplasmic reticulum responds by releasing calcium ions into the sarcoplasm. Once they are moving freely through the sarcoplasm, the calcium ions are able to bind to specialized structures on the actin and myosin proteins within the myofibrils. In doing so, they pull the thin filaments toward the center of the sarcomere, effectively shortening the whole unit. This process is known as the sliding filament model of muscular contraction.

What Is the Appendicular Skeleton?

The appendicular skeleton is the portion of the skeleton that includes the limbs, the pelvis, and the pectoral girdle. For adult humans, this includes approximately 126 of the roughly 206 bones that make up the skeleton. Its counterpart, the axial skeleton, is comprised of the skull, the spinal column, the sternum and the ribs. The primary function of the appendicular skeleton is locomotion and support, whereas the primary function of the axial skeleton is protection of internal organs. In general, all bones of the human skeleton also assist in mineral storage and blood cell production.

Two girdles, the pectoral girdle and the pelvis, serve as anchors to attach the appendicular skeleton to the axial skeleton. The pectoral girdle, consisting of the collarbone and shoulder blades, connects the upper limbs to the sternum. Each shoulder blade sits at rest over the ribs of the back, and the collarbone attaches in the front of the body with the sternum. The upper arm bone fits into a cuff of muscles that sits between the shoulder blade and collarbone. As this is the only point of attachment of the upper limbs to the axial skeleton, the shoulder joint is allowed a wide range of motion compared to other joints, but it also holds increased potential for injury, so care must be taken by athletes to avoid dislocation of the shoulder.

Lower limbs attach at the hips where the femur – the largest bone in the human body – fits into the pelvis. A comparatively sturdy structure, the pelvis is actually comprised of several distinct bones joined by five different cartilaginous joints. The largest and most prominent areas of the pelvis are the hips. Each hip is made up of three fused bones: the pubic bone, the ilium, and the ischium. At the back of the body, the hips come together at the sacrum, which connects the lower appendicular skeleton to the axial skeleton by fusing with the tailbone.

Both the upper and lower limbs of the appendicular skeleton attach to the girdles using ball and socket joints, in which the rounded end of a bone fits into a cup-shaped socket of muscles. This arrangement allows for a maximum degree of flexibility, allowing the limbs to move freely in a rotating motion. By comparison, the hinge joints of the knees and elbows allow a relatively restricted motion along a single plane.

Perhaps the most complex structures of the human skeleton, the hands and feet together make up more than half of the bones in the human body, with 27 bones in each hand and 26 in each foot. The wrists and ankles are a particular type of joint known as condyloid. These joints allow movement along two planes but with less freedom of rotation than the ball and socket joints. The delicate structure and flexibility of the hands and feet are remarkable in that they allow humans to do many of the things that are considered to be uniquely human, such as writing, playing music, and walking upright.

What Is the Anatomical Position?

The anatomical position is a frame of reference which is used when people describe anatomy and movement. Because organisms often change position and orientation, using a standardized position when describing their features is important, as it allows people to clearly understand the anatomical location of features of interest, the relationship between various anatomical features, and the overall structure of an organism.

In an example of why anatomical position is important, when a human is standing upright, the top of the skull is considered “up,” and the soles of the feet are “down.” But, when the same human lies on his or her back, suddenly the positions of “up” and “down” change, with the front of the body facing up, and the back of the body facing down. The situation becomes even more complicated with organisms like fish, making it very difficult to describe anatomy in absolutes; if, for example, a structure is said to be “at the top” of an organism, the location of that structure might be unclear because the “top” varies, depending on position.

Creating an absolute anatomical position eliminates this confusion. In humans, the anatomical position is a standing human facing forward, with the feet together and the arms relaxed at the sides, with the palms out and the thumbs facing away from the body. Many people may have noticed that anatomical illustrations and guidebooks usually show the body in this position for convenience, and to get people familiar with thinking of the anatomical position as a frame of reference when thinking about the human body.

Once the anatomical position is defined, people can start to create additional definitions which can be used in discussions of anatomy, such as anatomical planes, arbitrary divisions of the body at various cross sections. Additionally, people can develop anatomical terms of location to refer to specific areas of a body in anatomical position. Terms like “dorsal” and “ventral,” for example, can be used to describe to describe the back and belly of an organism in anatomical position, with no confusion about where the dorsal and ventral sides are.

Anatomical position can be used in a variety of settings. In keys which help people identify organisms ranging from fish to mushrooms, for example, questions or prompts which involve anatomical features are usually framed with anatomical position in mind. Health care practitioners and bodyworkers also usually use anatomical terms of location when they discuss their patients and take notes which they or other practitioners may need to reference.

What Is Pulmonary Physiology?

Pulmonary physiology is the study of lung function. The lungs are critical to the overall well being of an organism, creating a location for gas exchange so that carbon dioxide can be expressed from the body while oxygen is absorbed so that it can diffuse to the tissue through the blood. Medical and veterinary schools usually cover pulmonary physiology in their curricula, and this field is of particular interest to anesthesiologists, pulmonary specialists, and cardiologists, among several others.

Lung function is complex, and involves several interrelated systems. In addition to including the study of the lungs, pulmonary physiology is also interested in heart function, and in the circulatory system, as these physical systems are all involved in the diffusion of oxygen and carbon dioxide. This field of study includes the physical structure of these systems, as well as the chemical reactions and processes which allow them to work.

Pulmonary physiologists are interested in all aspects of lung function, including the involuntary signals sent from the brain to tell the body to breathe, heart abnormalities which can interfere with blood oxygenation, environmental factors which can depress lung function, and the sensitive clusters of cells which can alert the brain to dangerous concentrations of carbon dioxide in the blood.

In medical practice, an understanding of pulmonary physiology is very important for pulmonary specialists who deal with diseases of the lungs and respiratory disorders. Using a variety of methods, a pulmonary specialist can assess pulmonary function and knowledge of pulmonary physiology is important for analyzing a patient's performance on tests. This information can be used to develop a treatment plan to help a patient address a respiratory condition.

The function of the lungs is also of critical concern to anesthesiologists, who introduce carefully formulated mixtures of gases to the lungs during general anesthesia. These gases keep the patient anesthetized and ensure that he or she is getting enough oxygen to remain stable and healthy. After anesthesia, follow up care is needed to monitor the patient's lung function to confirm that the lungs have not been impaired during the surgery.

Physiotherapists are also interested in pulmonary physiology, applying their knowledge to help patients with respiratory disorders. A physiotherapist can assess lung function, help patients develop monitoring regimens to track their lung function, and promote the development of healthier, stronger lungs. They can also use their knowledge to educate patients about topics of interest and to explain how a course of treatment is designed to act upon the lungs.

Actin:-

Actin is a protein found inside the cells of all living things whose cells contain a membrane-bound nucleus. The actin protein is a component of two kinds of cell filaments: microfilaments and thin filaments. Microfilaments contribute to the cytoskeleton — a structure inside of cell membranes that helps the cell maintain its shape. Thin filaments, which are found in muscle cells, are involved in muscle contraction.

The way in which actin filaments facilitate muscle contraction can be explained by using the sliding filament theory. Within each muscle cell, actin protein chains form passive thin filaments that work in conjunction with thick filaments of myosin — a motor or movement protein that produces the force of muscle contraction. To do so, the myosin filaments slide back and forth along the actin filaments within a unit inside the muscle cell, called the sarcomere. Each muscle cell can contain hundreds of thousands of sarcomeres — a band-like structure that expands and contracts as a unit as the actin and myosin filaments slide past each other. It is the bands of sarcomeres that give muscles their striated appearance.

Under the sliding filament model, myosin filaments are alternated with actin filaments in horizontal lines, much like the red and white stripes on the American flag. The myosin proteins slide along the actin, releasing calcium ions that allow the head of each myosin protein to bind to a site on the actin filament. Once the myosin binds to the actin along these sites — much like a crew of rowers in a scull pulling their oars simultaneously — the myosin pulls the two filaments past each other, resulting in an overall shortening of the sarcomere. This collective shortening is made possible by the hydrolysis of adenosine triphosphate (ATP) — the body's main energy source for many cellular functions — and results in the contraction of the muscle cell.

Once the actin and myosin filaments bind and the stroke occurs — pulling the actin filaments toward the center of the sarcomere — the myosin heads detach from the actin and the ATP is recharged in these filaments, then causing the next stroke of the filaments. The protein tropomyosin could cover the actin filaments and block the binding sites; this process would prevent myosin from binding to the actin and result in the relaxation of the muscles. This mechanism of myosin and actin filaments binding and sliding is also how cytokinesis, or cell division, takes place, with the sliding filament action resulting in the pinching off of one cell into two during mitosis.

What Is Aerobic Metabolism?

Aerobic metabolism uses oxygen to removing energy from glucose and stores it in a biological molecule called adenosine triphosphate (ATP). ATP is the human body's source of energy, and breaking apart ATP molecules releases energy that is used for a variety of biological processes, including movement of molecules across membranes. Aerobic metabolism is also called aerobic respiration, cellular respiration, and aerobic cellular respiration. Anaerobic metabolism is another form of metabolism, but occurs without oxygen but the human body is not built to maintain anaerobic respiration for a long time, and doing so causes great stress.

The first stage of aerobic metabolism is called glycolysis. Glycolysis happens in the cytoplasm of the cell. Complex sugars are broken down into glucose by a variety of enzymes, and this glucose is then broken down further into two molecules of pyruvic acid, otherwise known as pyruvate. The energy released by this breakdown is stored in two molecules of ATP. Glycolysis is unique in that it is the only stage of metabolism to occur in the cytoplasm, and the other two stages occur inside the mitochondria.

In the second stage of aerobic metabolism, called the citric acid cycle, the two molecules of pyruvate are used to create energy-rich reducing molecules that are used later on in the respiration process. Some of these molecules can be converted directly to ATP if necessary, although this doesn't always happen. Water and carbon dioxide are produced as waste products from this cycle, which is the reason human beings breathe in oxygen and breathe out carbon dioxide. The citric acid cycle, like glycolysis, yields 2 ATP.

The finalstage of aerobic metabolism is called the electron transport chain and occurs on the inner membrane of the mitochondria. In this step, the energy-rich molecules derived from the citric acid cycle are used to sustain a gradient of positive charge, called a chemiosmotic gradient, that is used to generate many molecules of ATP. This step generates the most ATP out of the aerobic metabolism process, creating an average of about 32 ATP molecules. After the electron transport chain has generated ATP, the energy rich molecules are free to be reused by the citric acid cycle.

Aerobic metabolism generates approximately 36 molecules of ATP. Anaerobic respiration generates only about ten percent of that amount. The use of oxygen is most important at the end of the electron transport chain, as it aids the chemiosmotic gradient. The existence of oxygen-dependent metabolism is why mitochondria are commonly known as the body's powerhouse.

What Is Lipid Metabolism?

Lipid metabolism is the process by which fatty acids are digested, broken down for energy, or stored in the human body for later energy use. These fatty acids are a component of triglycerides, which make up the bulk of the fat humans eat in foods like vegetable oils and animal products. Triglycerides can be found in the blood vessels as well as stored for future energy needs in the cells of adipose tissue, better known as body fat, and in liver cells. Though the body’s main source of energy is carbohydrates, when this source is exhausted, the fatty acids in triglycerides will then be broken down as a backup energy source. Examples of times the body draws energy from lipid metabolism are during exercise, when the supply of glycogen, or the stored form of the carbohydrate glucose, is used up, or when there is insufficient carbohydrate in the diet to meet the body’s energy needs.

Triglycerides, also known as lipids or fats, are well suited for their role as a form of stored energy as each gram supplies 9 calories (37 kilojoules), whereas carbohydrates supply only 4 calories (17 kilojoules) per gram. As calories are units of energy, fats are considered to be an energy-dense nutrient. Triglycerides are made up of three fatty acid chains bonded to a hydrogen-containing compound called a glycerol, fatty acids that can be liberated during lipid metabolism when the body requires these calories for energy.

The first step in lipid metabolism is the consumption and digestion of triglycerides, which are found both in plant foods like olives, nuts, and avocados, and animal foods like meats, eggs, and dairy products. These fats travel through the digestive tract to the intestine where they are unable to be absorbed in triglyceride form. Instead, they are divided via an enzyme called lipase into fatty acids and, most often, a monoglyceride, which is a single fatty acid chain attached to a glycerol. These divided triglycerides then can be absorbed through the intestines and reassembled into their original form before being transported by chylomicrons, a type of a substance similar to cholesterol known as a lipoprotein, into the lymph system.

From the lymph system the triglycerides get into the bloodstream, where the process of lipid metabolism can be completed in one of three ways, as they are either transported to the liver, to muscle cells, or to fat cells, where they are either stored or used for energy. If they end up in liver cells, they are converted into a type of “bad” cholesterol known as very-low-density lipoprotein (VLDL) and released into the blood stream, where they work to transport other lipids. Triglycerides sent to muscle cells can be oxidized in the mitochondria of those cells for energy, whereas those sent to fat cells will be stored until they are needed for energy at a later time. This results in an increase in the size of the fat cells, visible on a person as an increase in body fat.

What Is Oxidative Metabolism?

Oxidative metabolism is the catabolic first half of metabolism in which the cell breaks down molecules into energy, or adenosine triphosphate (ATP). The second half of metabolism involves the use of that cellular energy to build molecules such as tissues and organs, and it is referred to as anabolism. Aerobic cellular respiration, a process requiring the use of oxygen, is the most efficient form of ATP production. ATP can also be produced anaerobically, without the presence of oxygen.

Oxidative metabolism begins with the breakdown of organic nutrients such as carbohydrates, sugars, proteins, vitamins and fats. Glucose, a simple sugar, is the most common nutrient to be broken down in a process known as glycolysis, or glucose metabolism. Glucose metabolism produces two pyruvate molecules that enter the mitochondria of the cell and are initiated into the Krebs cycle. The mitochondrion is an organelle that supplies cellular energy to the rest of the cell.

The Krebs cycle, referred to as the citric acid cycle as well as the tricarboxylic acid (TCA) cycle, describes the oxidative part of oxidative metabolism. Oxidation is the reduction of electrons and the release of energy. This cycle begins with one pyruvate molecule that, after a series of chemical reactions, is input into the cycle as oxaloacetic acid. The cycle begins and ends with oxaloacetic acid, which undergoes a series of enzyme-initiated chemical reactions during the cycle to produce energy.

In the citric acid cycle, oxidation of the carbon atoms results in the production of carbon dioxide and energy. There are two pyruvate molecules input into the mitochondria from one glucose metabolism reaction, so the TCA cycle involves two cycle turns for completion. Each turn produces one ATP, and so at the completion, two ATP are produced. Oxidative metabolism is an efficient process in that it produces numerous byproducts, known as reaction intermediates, that are almost immediately used for anabolism after catabolism is complete.

Oxidative metabolism is affected by diseases such as type 1 diabetes. Type 1 diabetes prevents glucose from entering the cell, and if it is left untreated, there will be no glucose available for normal production of energy via glycolysis. The body will then resort to the breakdown of fatty acids to fuel itself. The breakdown of fatty acids results in an acidic byproduct known as ketone bodies. If let untreated, the quantity of ketone bodies acidifies the potenz hydrogen (pH) of the blood and leads to the life-threatening condition ketoacidosis.

Why is Metabolism Important?

Metabolism is important because it is literally the powerhouse of the body, providing energy to keep the body going. In fact, many science and biology dictionaries describe metabolism as a process which is necessary to sustain life. Without metabolism, living organisms will die, and errors in metabolic processes can cause health problems such as diabetes, in which the body fails to metabolize blood sugar properly.

Living organisms are in a constant state of flux. To do anything, from firing a neuron to alert the brain that smoke is in the air to generating an extra burst of power to pull ahead in a foot race, the body needs energy. This energy is provided through metabolism, in which the body breaks down the substances ingested, and rebuilds them into useful substances, including raw energy and components which can be used to transport energy from place to place.

Likening metabolism to a powerhouse is very accurate, because this process involves the generation, storage, and transmission of power, and like an electrical grid, the body is very vulnerable to metabolic imbalances. For example, if someone metabolizes food too quickly, he or she tends to remain very thin, because the body cannot store energy in fats and muscle. Conversely, people who metabolize slowly may not be able to access the energy they need, because their bodies may not have generated it yet.

Some people have genetic conditions which cause problems with their metabolisms. These inborn errors of metabolism can include things like lacking enzymes which are necessary to break down food, and they often require medical intervention to be corrected. Metabolic problems can also be acquired, as in the case of someone who develops diabetes late in life, or in the case of someone with an eating disorder who causes permanent damage to the metabolism through consistent starvation.

One of the most common reasons to explore the metabolism is because someone is trying to build up strength for athletics, or to lose weight. Understanding how the metabolism works is critical for both of these tasks, as people can engage in activities which will support the metabolism to accomplish the desired task, or they can undermine their metabolic processes, making it harder.

Everyone's "powerhouse" is slightly different, which is one reason why there can be a lot of physical diversity between people who have similar diet and exercise habits. Finding one's own metabolic rates can be valuable for maintaining general health, as one can make lifestyle adjustments to cater to the specifics of the body.

What Are the Different Types of Metabolism Pathways?

Metabolism pathways include the basic chemical reactions that provide cells with the energy to remain alive and repair themselves. Cellular respiration is the central metabolic activity, and it operates through three different pathways — glycolysis, the Krebs cycle, and oxidative phosphorylation — that make energy-rich molecules that fuel cells. Different metabolism pathways are specialized for manufacturing proteins, nucleic acids, and other essential molecules. Toxic byproducts of metabolism are disposed of in the urea cycle pathway.

There are two classes of metabolism reactions: anabolism, which uses energy to build proteins and other cellular components, and catabolism, which generates energy by breaking down food into energy-rich compounds. Most metabolic pathways fall into one of these categories. Pathways are catalyzed by enzymes that regulate each individual chemical reaction, usually producing many intermediate compounds and chain reactions in the process. At many points in the evolution of life, the same molecules have been used to meet different metabolic needs, so the same enzymes direct metabolism in many different organisms.

Cellular respiration converts food to a high-energy molecule, adenosine triphosphate (ATP), that cells use for energy. It comprises three metabolic pathways. The first of these, glycolysis, is the splitting of glucose, a six-carbon sugar, into two molecules of a three-carbon sugar and into acetyl coenzyme A. Glycolysis generates two molecules of ATP along with other energy-rich molecules. In anaerobic metabolism, such as occurs in some bacteria and yeast, glycolysis is called fermentation and is a single step-form of cellular respiration.

In animals, glycolysis is just the first step of cellular respiration. Its second metabolism pathway is the citric acid cycle, also known as the Krebs or TCA cycle. This begins when the acetyl coenzyme A from glycolysis is converted into several energy-carrying chemicals, and into two ATP molecules. The last of the metabolism pathways in cellular respiration is oxidative phosphorylation, which requires oxygen and a series of electron-carrying molecules to get started. This pathway uses the transfer of electrons from the energy-rich chemicals to oxygen to power the production of ATP.

Beta-oxidation is a metabolic pathway that converts fatty acids into ATP. The metabolism of fats involves both catabolism to form ATP and anabolism to produce phospholipids. Proteins are broken up into their constituent amino acids in many different pathways, beginning with digestion and continuing with processing at the cellular level. Two other common metabolism pathways are the urea cycle, which removes the toxins formed by nitrogen metabolism from the body, and glycogenesis, which converts glucose into starch for long-term storage.

What Is Skeletal Muscle Contraction?

A skeletal muscle contraction is the mechanism by which muscles of the movable joints of the body produce movement at those joints. Skeletal muscle is differentiated from cardiac muscle, which pumps the heart, and smooth muscle, which is a component of several internal organs and produces movements like pushing food along the digestive tract, in that it connects at both of its ends to bone. As such, when it contracts — that is, when its fibers shorten and lengthen — it pulls on the two bones, causing motion at the joint it crosses. Skeletal muscle contraction, which involves a chemical reaction at the level of protein components contained within every muscle cell, is what makes movement of the skeleton possible.

There are a few different types of contractions that skeletal muscle can produce. A contraction in which the muscle fibers shorten, as seen when the rib cage is drawn closer to the pelvis during an abdominal crunch, is known as a concentric contraction. When the muscle fibers lengthen, as in the lowering phase of a crunch, an eccentric contraction is taking place. A skeletal muscle contraction involving both the concentric and eccentric phase of a movement is known as an isotonic contraction. An isometric contraction, on the other hand, is one in which the muscle does not change in length while contracting, as in holding a squat position without moving.

Skeletal muscle is made up of bundles of muscle fibers, which in turn are bundles of muscle cells. Muscle cells are long, narrow, and cylindrical in shape, and made up of units called sarcomeres that are responsible for skeletal muscle contraction. The model that explains what occurs in the sarcomere as a muscle contracts is known as sliding filament theory. It can be used to explain all types of muscle contraction, which differ only as a result of whether the force applied to the muscle is less than, greater than, or equal to the force produced by the muscle cells.

Within each sarcomere, a unit occurring in the hundreds of thousands in each muscle cell, are proteins organized into long filaments called actin and myosin. The actin proteins are passive, meaning that they form chains that receive the active myosin proteins. Arranged in alternating lines, the myosin slides back and forth past the actin, and in the process it emits calcium ions that cause each myosin protein to bind to a corresponding site on each actin protein.

During skeletal muscle contraction, the myosin filaments grab onto the actin and pull past it. This happens simultaneously in the cell’s many sarcomeres, which are arranged in bands. This “stroke,” as it is commonly known, causes a collective shortening of the muscle, which then returns to its resting length as the myosin releases itself from the actin.

Tropomyosine

Tropomyosin is a protein that is involved in the contraction of skeletal muscle. It is, in fact, the compound responsible for preventing muscles from contracting when they are at rest. This protein acts as a block during the chemical process that produces muscle contraction by wrapping itself around chains of another protein found in muscle cells known as actin. A third protein called myosin must be able to bind to sites along this actin protein in order for muscles to contract. It is this binding of the two proteins that tropomyosin obstructs.

To understand the role of tropomyosin in preventing muscle contraction, one should first understand the mechanism, known as sliding filament theory, that causes muscles to contract. Within muscle cells, which make up muscle fibers arranged in bundles, actin and myosin are arranged in alternating filaments. Myosin is the motor or movement protein that generates the force behind muscle contraction by sliding back and forth along the actin filaments within a structure inside the muscle cell known the sarcomere — one of thousands — that can expand and contract as a unit. During muscle contraction, the myosin proteins slide past the actin, discharging calcium ions that cause each myosin protein to bind to an adjacent site on the actin filament. When this happens, the myosin pulls itself past its neighboring actin, causing a collective shortening of the sarcomere that produces a contraction of the muscle cel

If the muscle is in a resting state and no muscle contraction is required, tropomyosin wraps itself around the actin filaments, blocking the binding sites and thereby preventing the myosin from binding to the actin so that no muscle contraction may occur. A single tropomyosin molecule blocks seven binding sites on the actin molecule. It does so with the assistance of a protein complex called troponin, which is actually three proteins, each of which plays a different role in blocking or initiating muscle contraction. One, troponin T, joins with tropomyosin to block the sites of myosin attachment. Another, troponin I, attaches to the actin itself to hold these two in place across the binding sites.

A third type of troponin, troponin C, helps the process of contraction to begin all over again by attaching to calcium ions. It is the release of these calcium ions from channels inside the muscle cell that stimulates contraction. When they are released, they bind to troponin C, which moves the tropomyosin-troponin T out of the way so that the myosin once again has access to the binding sites on actin, and contraction of the sarcomere can begin again.

What Is Muscle Physiology?

Muscle physiology is the study of muscle function. A muscle is a bundle of fibers that contract to produce heat, posture, and motion, either of internal organs or of the organism itself. Muscle physiology studies the physical, mechanical, and biochemical aspects of muscles in development, fiber structure, muscle structure, contraction, and strength-building.

The body has three types of muscle: cardiac, smooth, and skeletal. Skeletal muscle is a voluntary muscle, or a muscle that can be consciously controlled, characterized by even striations, or stripes. Skeletal muscle attaches to bones to effect movement of the skeleton for purposes such as posture and locomotion. Smooth muscle is an involuntary muscle, marked by a lack of striations, that effects movement in the internal organs. Cardiac muscle is an involuntary, unevenly striated muscle that composes the heart and causes its contractions, or the beating of the heart.

Understanding the muscle physiology of skeletal muscle requires a basic grasp of its structure. Skeletal muscles typically attach to bones via tendons and often appear in antagonistic pairs, so that when one muscle contracts, the other lengthens. The muscle itself is made up of a bundle, or fascicle, of long, cylindrical cells called muscle fibers. Each fiber contains many string-like structures called myofilaments that sit within the sarcoplasm, a fluid similar to cytoplasm which is held in by the fiber’s sarcolemma, or membrane. The myofilaments contain contractile structures called myofibrils, whose elements repeat geometrically to create functional units called sarcomeres.

Each sarcomere contains overlapping thick filaments, composed of myosin molecules, and thin filaments, composed of actin, troponin, and tropomyosin molecules. Sliding filament theory of contraction proposes that, during contraction, the myosin binds to the molecules of thin filament to pull the thin filaments over or under the thick filament. The sarcomere becomes shorter as a whole, though no element of the fiber is actually shrinking in size. The binding of molecules responsible for this contraction is stimulated by a release of calcium ions from the sarcoplasm. The calcium is released in response to an electrical impulse called an action potential sent from a neuron to a muscle through a neuromuscular synapse.

Smooth muscle physiology differs from skeletal muscle physiology because smooth muscles do not have sarcomeres, explaining the lack of striations in smooth muscle. Instead, smooth muscle contracts as a single unit, with electrical impulses being communicated from cell to cell through gap junctions. These electrical impulses are communicated by neurons stemming from the autonomic nervous system. Some smooth muscle may contract spontaneously, without stimulus from a neuron, due to the presence of pacemaker cells, which can create their own electrical impulses. Like skeletal muscle, contractions occur from the binding and sliding of thick filaments with thin filaments in response to a release of calcium within the muscle fiber.

Cardiac muscle physiology is similar to skeletal muscle physiology in several ways. Cardiac muscle contracts in response to elevated levels of calcium and is also striated; indicating that it also uses sarcomeres as its contractile unit. Like smooth muscle and unlike skeletal muscle, cardiac muscle does not need to be innervated at every fiber because it can communicate electrical signals from cell to cell. This communication is achieved through intercalated discs, a feature unique to cardiac muscle.

What Is a Sarcomere?

A sarcomere is the basic functional unit of striated muscle. In the human body, each muscle is made up of multiple bundles of muscle fibers, or cells. The muscle fibers, in turn, are comprised of numerous finer strands called myofibrils. When viewed under electron microscopy, it can be seen that each myofibril is primarily composed of two kinds of filaments — termed thick and thin — organized into regular, repeating sub-units. These sub-units are the sarcomeres, and it is their patterned arrangement that gives striated muscle its characteristic banded appearance.

In between sarcomeres lies the Z line, or Z disc. When stained and viewed microscopically, the Z line appears as a dark, distinct border. The Z lines of adjacent myofibrils generally line up, appearing as a series of parallel dark lines across the muscle cell. Arising from the Z lines, numerous thin filaments reach toward the center of the sarcomere, where they overlap slightly with the free-floating thick filaments. These filaments together represent the active structures of the sarcomeres.

Thick filaments are made up of hundreds of molecules of the protein myosin. Myosin molecules are characterized by long, fibrous, tail regions that run along the axis of the filament, and globular head regions that project outwards along the axis. Individual myosin molecules within a filament are oriented in opposite directions, resulting in the head regions lining up along each end of the molecule, with the tails gathered together in the middle. Overall, the filament has the shape of an elongated dumbbell, with bumpy heads projecting at the ends, and a smooth region in the center. The interior region of the sarcomere, corresponding to the length of the thick filaments, is called the A band.

Thin filaments are approximately half the diameter of thick filaments, and contain primarily the protein actin. Roughly spherical, actin molecules arrange in double strands like a beaded necklace, with each strand twisting around the other to form a helix. Thin filaments project inward from the Z lines at each end of the sarcomere, partially overlapping with the bumpy regions of thick filaments.

At rest, The very center of the sarcomere typically will only contain the smooth, middle region of the thick filaments. This region is termed the H zone. Similarly, the outer edges of the sarcomere will generally only include thin filaments when the muscle is at rest, forming a narrow strip around the Z line known as the I band. In its entirety, this arrangement enables the contraction of the sarcomere, the myofibril, and the entire muscle.

Muscles contract according to the sliding filament model of muscular contraction. Upon contraction, the distance between Z lines becomes shorter, but the filaments themselves do not change length. Instead, the two types of filaments slide longitudinally against each other, increasing the degree of overlap and thereby shortening the length of the sarcomere. When this occurs, the I bands shorten, the H zone disappears, and the length of the A bands remains constant.

The sliding filament model is made possible by the interaction of the actin and myosin proteins within the filaments. Using energy from the energy-transfer nucleotide adenosine triphosphate (ATP,) myosin molecules form and release bonds with the actin molecules of neighboring filaments, effectively pulling them toward the center of the sarcomere. Under optimal conditions, the process typically continues at a rate of five bonds per molecule per second. Bond formation is generally regulated by the concentration of calcium ions within the cell, and can also be affected by the amount of available glycogen and creatine phosphate.

What Is an Isotonic Contraction?

Isotonic contraction is a form of muscular exertion principally characterized by a change in both muscle length and joint angle. Also known as dynamic contraction, isotonic exercises typically involve the rhythmic, repetitive motion of large muscle groups. This is the type of muscular exertion that is most often used during strength training and cardiovascular exercise, resulting in net gains in muscular size, strength, and endurance.

Other forms of muscular contraction include isometric contraction and auxotonic contraction. Isometric contraction, in which there is no change in muscle length and no visible movement of the joints, occurs when muscular force is exerted against an immovable object. Isometric training is sometimes used by athletes to overcome specific weaknesses in the dynamic range of motion of a particular muscle group, or to prevent muscle atrophy when a limb is immobilized. Auxotonic contraction, in which the resistance increases as the force is applied, is most commonly seen in cardiac muscle.

During isotonic contraction there is a distinct physiological response that is not seen during isometric contraction. As the working muscles consume oxygen, the heart rate increases and blood is shunted toward areas of demand. Along with a boost in heart rate, the heart's stroke volume — the amount of blood pumped with each heartbeat — also becomes elevated. As the isotonic contraction continues, there is a progressive rise in systolic blood pressure combined with a stable, or slightly decreased, diastolic blood pressure.

In this way, isotonic contraction imposes an increased volume load on the heart muscle. The heart adapts to the increased load by building up strength and endurance. This adaptation is known as the cardiac training effect, and occurs most often in response to the demands of dynamic exercise. Isometric exercise, by contrast, typically results in a rise in both systolic and diastolic blood pressure, accompanied by a moderate increase in cardiac output without significant increase in blood flow to working muscles.

Isotonic contraction can be further subdivided into eccentric and concentric contraction. Concentric contraction occurs when muscular force is greater than the force of resistance, and the muscle shortens. The shortening of the muscle results in a net decrease in the angle of the working joint. In resistance training, this is generally the phase of motion that moves against gravity — for example, the portion of a bicep curl when the elbow is flexed and the barbell is moved upward.

Eccentric contraction occurs when the force of resistance exceeds the force exerted by the muscle. In this case, there is typically an overall lengthening of the muscle, and an increase in the angle of the joint. A muscle's weight bearing limit is up to 40% greater during eccentric contraction than concentric contraction. Both forms of isotonic contraction are effective for building muscular strength, but there are other adaptations that are particular to eccentric exercise.

Extreme athletes, such as bodybuilders and ultra-marathoners, tend to engage in more eccentric exercises than the general population. As a result, these athletes appear to have a greater than average amount of connective tissue around the muscles. This is thought to be an adaptation to protect the muscles from the high levels of force associated with this form of exercise. Conversely, exercise programs that reduce or eliminate the eccentric phase of contraction have been associated with stress injuries and limited gains in muscular strength.

What Is Energy Metabolism?

Energy metabolism is generally defined as the entirety of an organism's chemical processes. These chemical processes typically take the form of complex metabolic pathways within the cell, generally categorized as being either catabolic or anabolic. In humans, the study of how energy flows and is processed in the body is termed bioenergetics, and is principally concerned with how macromolecules such as fats, proteins, and carbohydrates break down to provide usable energy for growth, repair, and physical activity.

Anabolic pathways use chemical energy in the form of adenosine triphosphate (ATP) to power cellular work. The building of macromolecules out of smaller components, such as the synthesis of proteins from amino acids, and the use of ATP to power muscular contraction are examples of anabolic pathways. To power anabolic processes, ATP donates a single phosphate molecule, releasing stored energy in the process. Once a working cell's supply of ATP is depleted, more must be generated by catabolic energy metabolism for cellular work to continue.

Catabolic pathways are those that break down large molecules into their constituent parts, releasing energy in the process. The human body is able to synthesize and store its own ATP through both anaerobic and aerobic energy metabolism. Anaerobic metabolism takes place in the absence of oxygen, and is associated with short, intense bursts of energy. Aerobic metabolism is the breakdown of macromolecules in the presence of oxygen, and is associated with lower intensity exercise, as well as the daily work of the cell.

Anaerobic energy metabolism occurs in two forms, the ATP-creatine phosphate system and fast glycolysis. The ATP-creatine phosphate system uses stored creatine phosphate molecules to regenerate ATP that has been depleted and degraded to its low-energy form, adenosine diphosphate (ADP). The creatine phosphate donates a high-energy phosphate molecule to the ADP, thereby replacing spent ATP and re-energizing the cell. Muscle cells typically contain enough free-floating ATP and creatine phosphate to power approximately ten seconds of intense activity, after which the cell must switch to the fast glycolysis process.

Fast glycolysis synthesizes ATP from glucose in the blood and glycogen in the muscle, with lactic acid produced as a byproduct. This form of energy metabolism is associated with brief, intense bursts of activity &mash; such as power lifting or sprinting — when the cardio-respiratory system does not have time to deliver adequate oxygen to the working cells. As fast glycolysis progresses, lactic acid accumulates on the muscle, causing a condition known as lactic acidosis or, more informally, muscle burn. Fast glycolysis produces the majority of ATP that is used from ten seconds to two minutes of exercise, after which time the cardio-respiratory system has had opportunity to deliver oxygen to the working muscles and aerobic metabolism begins.

Aerobic metabolism takes place in one of two ways, fast glycolysis or fatty acid oxidation. Fast glycolysis, like slow glycolysis, breaks down glucose and glycogen to produce ATP. Since it does so in the presence of oxygen, however, the process is a complete chemical reaction. While fast gycolysis produces two molecules of ATP for every glucose molecule metabolized, slow gycolysis is able to produce 38 ATP molecules from the same amount of fuel. As there is no lactic acid accumulation during the reaction, fast glycolysis has no associated muscle burn or fatigue.

Finally, the slowest and most efficient form of energy metabolism is fatty acid oxidation. This is the process used to power activities such as digestion and cellular repair and growth, as well as long-duration exercise activities, such as marathon running or swimming. Rather than using glucose or glycogen as fuel, this process burns fatty acids that are stored in the body, and is capable of producing as many as 100 ATP molecules per unit of fatty acids. While this is a highly efficient, high-energy process, it requires large amounts of oxygen and only occurs after 30 to 45 minutes of low-intensity activity.

What Is the Sliding Filament Theory?

Sliding filament theory is a model used to explain the mechanism by which muscles contract. The contraction of skeletal muscle, which is what makes movement possible, occurs in three ways. Concentric muscle contraction involves the shortening of muscle fibers, as in the lifting phase of a bicep curl, while eccentric muscle contraction is made possible by the lengthening of muscle fibers, as in the lowering phase of a bicep curl. Isometric contraction is another possibility, during which the muscle does not change in length while sustaining a contraction, as in stopping the weight midway through a bicep curl and holding the elbow at 90 degrees. Sliding filament theory describes the process that makes these changes in muscle length, and therefore muscle contraction, possible.

Two kinds of proteins found in muscle cells, actin and myosin, work together to produce these contractions, as they are arranged in filaments that slide past each other, giving sliding filament theory its name. Within each muscle cell, actin protein chains form passive thin filaments that work in conjunction with thick filaments of myosin, a motor or movement protein that produces the force of muscle contraction. To do this, the myosin filaments slide back and forth along the actin filaments within a unit inside the muscle cell called the sarcomere. Each muscle cell can contain hundreds of thousands of sarcomeres, a band-like structure that expands and contracts as a unit as the actin and myosin filaments slide past each other. It is the bands of sarcomeres that give muscles their striated appearance.

Under sliding filament theory, myosin filaments are alternated with actin filaments in horizontal lines, much like the red and white stripes on the American flag. The myosin proteins slide along the actin, releasing calcium ions that allow the head of each myosin protein to bind to a site on the actin filament. Once the myosin binds to the actin along these sites, much like a crew of rowers in a scull pulling their oars simultaneously, the myosin pulls the two filaments past each other, resulting in an overall shortening of the sarcomere. This collective shortening is made possible by the hydrolysis of adenosine triphosphate (ATP), the body's main energy source for many cellular functions, and results in the contraction of the muscle cell.

Once the actin and myosin filaments bind and the “stroke” occurs, pulling the actin filaments toward the center of the sarcomere, the myosin heads detach from actin, and the ATP is recharged in these filaments again, causing the next stroke of the filaments. If no muscle contraction is needed and the muscle is at rest, a protein called tropomyosin wraps itself around the actin filaments, blocking the binding sites and thereby preventing the myosin from binding to the actin so that no muscle contraction may occur. Sliding filament theory also explains how cytokinesis, or cell division, takes place, with the sliding filament mechanism causing one cell to pinch off into two during mitosis.