The Endocrine System - Workings: how the endocrine system functions





The Endocrine System Workings How The Endocrine System Functions 2517
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The main functions of the endocrine system and its hormone messengers are to maintain homeostasis (a stable internal environment in the body) and to promote permanent structural changes. Maintaining homeostasis is a response to a change in the body, such as low sugar or calcium levels in the blood. Permanent structural changes, occurring over a period of time, are those associated with growth and development.

Hormones bring about their effect on the body's cells mainly by altering the cells' metabolic activity—increasing or decreasing the rate at which they work. The effect is often rapid, such as increased or decreased heart rate. A few hormones, after binding to their target cells, cause those cells to produce proteins, which lead to long-term effects such as growth or sexual maturity.

Many professional and amateur athletes around the world take anabolic steroids in the hopes of enhancing their performance. Anabolic steroids are synthetic (man-made) drugs derived from the male hormone testosterone. The full name of the drug is androgenic (promoting masculine characteristics) anabolic (building) steroid (class of drug). Common names for the drug include 'roids, sauce, and juice.

It is estimated that 10 to 20 percent of male high school athletes, up to 30 percent of college and professional athletes, and up to 80 percent of bodybuilders use anabolic steroids to increase skeletal muscle and lean body mass. The drugs are taken either orally or injected.

Anabolic steroids do increase body weight and muscle mass. They also may improve muscular strength and endurance. These are the few benefits.

The drawbacks are many and serious. The major side effects include liver tumors, jaundice (yellowing of the skin), fluid retention, high blood pressure, severe acne, and trembling. In men, steroids can additionally cause shrunken testes, reduced sperm production, sterility, baldness, and the development of breasts. In women, they can also cause the growth of facial hair, menstrual irregularity, smaller breasts, and a deeper voice. In adolescents, the drugs can permanently stop bones from growing, resulting in shortened height for life.

Anabolic steroids not only affect the body but the mind as well. Users suffer from aggression, irritability, delusions, paranoid jealousy, and impaired judgment.

Taking anabolic steroids for nonmedical reasons is illegal under federal law. Hard training is still the most effective and safe way to improve muscle strength and overall athletic performance.

Hormones travel in the bloodstream or in the interstitial fluid (fluid between cells). Some hormones are long-distance travelers, floating throughout the body in search of their target cells. Others travel shorter distances, having been secreted near theirs.

Negative feedback

Hormones are secreted by endocrine glands in response to a stimulus. That stimulus may be either changing blood levels of certain nutrients or other hormones. When a gland senses a change in the composition of blood or tissue fluid (low blood sugar, for example) and releases its hormones, that action is known as a direct response. When a gland releases its hormones because it has been stimulated by other hormones released by other glands, that action is known as an indirect response.

A feedback system tightly controls the on/off workings of endocrine glands. This system can be compared to a furnace thermostat on a wall in a house. When the temperature in a house falls below the temperature set on the thermostat, the thermostat is triggered and signals the furnace to turn on and begin heating. After the furnace has heated the air in the house to a temperature higher than that set on the thermostat, the thermostat signals the furnace to turn off.

Endocrine glands react to changes in the blood and body in much the same way. When nutrients or chemicals in body fluids are abnormal (either high or low), endocrine glands secrete their hormones. After those levels return to normal (reaching a state of homeostasis), the glands stop secreting their hormones. This control of hormone secretion, where information is fed back to the gland to stop its hormone production, is called negative feedback.

Actions of the hypothalamus

Receiving nerve signals from other parts of the brain, the hypothalamus functions as a monitoring and control station for many body activities. It thus plays an important role in the actions of other endocrine glands, especially the pituitary. Therefore, its role is best considered under the discussions of the actions of those various other glands.

Actions of the pituitary gland

The "master" pituitary gland is small in size, but large in its actions. The eight hormones secreted by its two lobes have a direct effect on the actions of other endocrine glands, controlling growth and fluid balance in the body. The anterior pituitary secretes six hormones: growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and three gonadotropic hormones. The posterior pituitary secretes antidiuretic hormone and oxytocin.

GROWTH HORMONE. Growth hormone (GH) or human growth hormone stimulates overall body growth by spurring target cells to grow in size and divide. GH increases the rate at which those cells take in and utilize proteins (cell structure is made up largely of proteins). GH also causes fats to be broken down and used by the cells for energy. Its greatest effects are on the development of muscles and bones, especially in children. The release of GH is controlled by two regulatory hormones from the hypothalamus: growth hormone releasing hormone (GHRH) and growth hormone inhibiting hormone (GHIH). GHRH stimulates the pituitary to release GH during exercise, when blood sugar levels are low, when amino acid levels in the blood are high, or when the body in under stress. When the body is returned to a state of homeostasis or when blood sugar levels are high, the hypothalamus secretes GHIH, and the pituitary stops releasing GH.

THYROID-STIMULATING HORMONE. Thyroid-stimulating hormone (TSH), as its name implies, influences the growth and activity of the thyroid. TSH prompts the thyroid to release thyroxine, which stimulates the cells in the body to increase their metabolism (energy production) and intake of oxygen. A releasing hormone from the hypothalamus signals the pituitary to secrete TSH. This occurs when the body's metabolic rate decreases.

ADRENOCORTICOTROPIC HORMONE. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete cortisol and other hormones. During any stressful situation such as injury, low blood sugar levels, and exercise, the hypothalamus secretes a releasing hormone that triggers the pituitary to release ACTH.

It is known that some people can automatically awake in the morning without an alarm clock. German researchers sought an explanation for this phenomenon, and in early 1999 they announced their results. The researchers discovered that the actions of two hormones, adrenocorticotropic hormone (ACTH) and cortisol, were the reason.

In stressful situations, the hypothalamus secretes a releasing hormone that triggers the anterior pituitary to release ACTH. ACTH then travels to the adrenal cortex, stimulating it to release cortisol. In short, cortisol stimulates most body cells to increase their energy production, which heightens the body's ability to react quickly to a stressful or emergency situation.

The researchers found that during the latter stages of sleep, these hormones were released, causing the body to awaken. Most scientists agree that sleep is a state of unconsciousness. From their findings, however, the researchers concluded that even during sleep, the mind maintains some voluntary control. When an individual goes to bed knowing he or she has to get up earlier than normal because of a stressful event (an exam at school or a big presentation at work), the mind "remembers" and so awakens the body in anticipation of that event.

GONADOTROPIC HORMONES. As their name suggests, the gonadotropic hormones affect the gonads or reproductive organs. Releasing hormones from the hypothalamus regulate the secretion of all three gonadotropic hormones: prolactin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). In females, prolactin stimulates the development of mammary glands in breasts and their secretion of milk. FSH stimulates the development of follicles in the ovaries of females. Ovarian follicles are tiny, saclike structures within which ova or eggs develop. FSH also stimulates the secretion of estrogen by the follicle cells. In males, FSH begins the productions of sperm in the testes. LH stimulates ovulation (the release of an egg from an ovary) and the release of estrogens and progesterone from the ovaries in females. In males, LH stimulates the testes to produce testosterone.

ANTIDIURETIC HORMONE. Antidiuretic hormone (ADH) is produced by the hypothalamus and stored in the posterior pituitary. ADH causes the kidneys to reabsorb water from the urine that is being formed. That water is then transported into the bloodstream, maintaining blood pressure. When too much water is lost from the body, such as through sweating, diarrhea, or any type of dehydration, the hypothalamus detects an increased amount of "salt" in the blood. It then triggers the posterior pituitary to release ADH. The kidneys decrease the production of urine and blood pressure increases. Alcohol and certain drugs, however, inhibit the secretion of ADH. Large amounts of urine are consequently excreted from the body and blood pressure decreases. If that fluid is not replaced, an individual may feel dizzy due to low blood pressure.

OXYTOCIN. Oxytocin is also produced by the hypothalamus and stored in the posterior pituitary. The hormone plays an important role in childbirth. When a woman goes into labor, the uterus begins to stretch and nerve impulses are sent to the hypothalamus. The hypothalamus then stimulates the posterior pituitary to release oxytocin, which travels to the uterus. Once there, it triggers strong contractions of the uterine muscles, helping to bring about delivery of the baby. After birth, oxytocin promotes the release of milk from the mammary glands. When a baby suckles a mother's nipple, nerve impulses are sent to the mother's hypothalamus, which then signals the release of oxytocin. The hormone stimulates the contraction of the muscle cells around the mammary ducts, causing the ejection of milk through the nipple.

Actions of the pineal gland

Scientists believe that melatonin, secreted by the pineal gland, establishes the body's sleep and waking patterns. Keeping the body in sync with the cycles of day (light) and night (dark), the pineal gland functions as the body's biological clock. Scientists know that, in general, the secretion of melatonin is spurred by darkness. Known as the sleep "trigger," melatonin is secreted cyclically in response to the fall of darkness at the end of each day. In the morning, when light enters the eyes, that visual information is relayed from the eyes to the hypothalamus. The pineal gland is then stimulated to decrease melatonin production during daylight hours. Scientists also theorize that the hormone plays a role in the timing of puberty and sexual development, preventing it from occurring during childhood before adult body size has been reached.

Actions of the thyroid gland

Thyroxine, the major hormone secreted by the thyroid follicles, is often considered to be the body's major metabolic hormone. When the body's metabolic rate decreases, the anterior pituitary secretes thyroid-stimulating hormone, which triggers the thyroid to secrete thyroxine. Thyroxine then stimulates energy production in cells in the body, increasing the rate at which they consume oxygen and utilize carbohydrates, fats, and proteins. When cells increase their energy production, they generate more heat as a result. This is important when the body is trying to adapt to cold temperatures. In children, thyroxine is essential to the normal development of the muscular, nervous, and skeletal systems. In adults, it is important for continued tissue growth and development. Iodine is an important component of thyroxine. Without the proper amount of iodine in an individual's diet, thyroxine would not be produced, and physical and mental growth and abilities would then diminish.

It has been recorded since the time of the ancient Greeks that the varying seasons have had an effect on people's moods and behavior. Generally, the short, dark days of late autumn and winter dampen many people's spirits, and the longer and lighter days of spring and summer have the opposite effect.

While members of the medical community have noted this annual winter depression, they did not fully explore its reasons until the early 1980s. Since then, medical researchers have concluded that the lack of sunlight from November through March is indeed responsible for what is commonly referred to as the "winter blues."

Many people feel mildly "depressed" during the winter, but some suffer from severe symptoms. These include daytime drowsiness, fatigue and low energy level, diminished concentration, irritability, carbohydrate craving and increased appetite, weight gain, and social withdrawal. This mood disorder that affects people only during the autumn and winter seasons is called seasonal affective disorder or SAD.

SAD is a very real problem that affects approximately 10 million people each year in the United States (women suffering from SAD outnumber men four to one). Researchers believe that people with SAD have lost the natural rhythm that signals the body to fall asleep and to awake at the proper times. Melatonin, secreted by the pineal gland when light is low, helps bring the body to rest. Daylight signals the gland to stop producing the hormone to allow the body to come awake.

Researchers do not know why some people are affected more than others. They have discovered, however, that an effective treatment for SAD sufferers is light, particularly morning light. When exposed to a light box that emits bright, artificial sunlight (called phototherapy or light therapy) for thirty minutes a day, almost 80 percent of SAD patients showed marked improvement in their moods.

Calcitonin, the second important hormone secreted by the thyroid, helps maintain normal levels of calcium in the blood. It is secreted directly into the bloodstream when the thyroid detects high levels of calcium in the blood. Calcitonin travels to the bones, stimulating the bone-building cells to absorb calcium from the blood. It also targets the kidneys, stimulating them to absorb and excrete the excess calcium. When blood calcium levels return to normal, the thyroid stops secreting calcitonin.

Actions of the parathyroid glands

Like the thyroid's calcitonin, the parathyroid's parathyroid hormone (PTH) also regulates the levels of calcium in the blood. However, its stimulus and effect are just the opposite. Thus, calcitonin and PTH are antagonistic: they work against each other to maintain the normal levels of calcium in the bloodstream.

The parathyroids secrete PTH when blood calcium levels are low. Like calcitonin, PTH targets the bones and the kidneys. In the bones, PTH stimulates the bone-dissolving cells to break down bone, thus releasing calcium (a component of bone) into the bloodstream. In the kidneys, PTH decreases the amount of calcium that is excreted in the urine. Both of these actions raise the levels of calcium in the blood. When those levels have returned to normal, the parathyroids stop secreting PTH.

Actions of the thymus

The thymus and its collective hormones, thymosins, play an important role in helping the body develop immunity (the ability to resist disease). In a fetus and infant, immature or not fully developed lymphocytes (type of white blood cell) are produced in the bone marrow, the spongylike material that fills the cavities inside most bones. A certain group of these lymphocytes then travel to the thymus. There, thymosins changed them into T lymphocytes or T cells (the letter T refers to the thymus). While maturing, dividing, and multiplying in the thymus, T cells are "programmed" to recognize the difference between cells that belong to the body and those that are foreign or abnormal. Once they are fully mature, T cells leave the thymus and enter the bloodstream. They circulate to the spleen, lymph nodes, and other lymphatic tissue where they await the call to defend the body.

Actions of the pancreas

Although the islets of Langerhans make up but a small part of the pancreas, they work tirelessly, acting like an organ within an organ. The main hormones they secrete—glucagon and insulin—are vital to the normal functioning of the body. They regulate blood glucose (sugar) levels in the same way that parathyroid hormone and calcitonin regulate blood calcium levels.

GLUCAGON. Glucagon is secreted by the islets of Langerhans in response to low blood glucose levels. To raise those levels (and the body's energy), glucagon then travels to the liver. The liver performs a multitude of functions in the body. One of those is to store excess glucose that is not immediately required by the body's cells for energy. In order to store that glucose, the liver converts it to glycogen (a starch form of the sugar glucose made up of thousands of glucose units). Glucagon stimulates the liver to change glycogen back into glucose and secrete it into the bloodstream for use by the cells for energy production. When glucose levels rise to normal, the islets of Langerhans stop secreting glucagon.

INSULIN. Insulin has the opposite effect: it lowers blood glucose levels that are too high. When those high levels are detected, the islets of Langerhans secrete insulin, which then travels to almost all cells in the body. Insulin stimulates the cells to take in more glucose and use it to produce energy. Insulin also stimulates the liver to take in more glucose and store it as glycogen for later use by the body. After they break down glucose, the cells use the energy created to build proteins and enhance their energy reserves. Insulin is the only hormone that decreases blood glucose levels and is absolutely necessary in order for the cells to utilize glucose. Without it, the cells cannot take in glucose. After glucose levels return to normal, the islets of Langerhans stop secreting insulin.

Actions of the adrenal glands

The small adrenal glands, capping the kidneys, control numerous activities in the body. The hormones they secrete aid in cell metabolism, adjust the water balance, and increase cardiovascular and respiratory activity.

CORTISOL. In times of physical stress (injury, exercise, anger, fear), the hypothalamus secretes a releasing hormone that causes the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH, in turn, targets the adrenal cortex, stimulating it to secrete cortisol. Like insulin, cortisol stimulates most body cells to increase their energy production. Unlike insulin, cortisol causes the cells to increase energy output by using fats and amino acids (proteins) instead of glucose. In stressful situations, this is extremely important because glucose is conserved for use by the brain (glucose is the sole source of energy for neurons or cells in nervous tissue).

Cortisol also has an anti-inflammatory effect, suppressing the activities of white blood cells and other components in the body's defense line. Inflammation is an important first step in tissue repair, but if left unchecked, will lead to excessive tissue destruction. Cortisol limits the inflammation process to what is necessary for immediate tissue repair by blocking the effects of histamine (a chemical released by damaged cells that brings more blood flow to the area).

ALDOSTERONE. Aldosterone, another steroid hormone secreted by the adrenal cortex, targets the kidney cells that regulate the formation of urine. A decrease in blood pressure or volume, a decrease in the sodium (salt) level in blood, and an increase in the potassium level in blood all stimulate the secretion of aldosterone. Once released, aldosterone spurs the kidney cells to reabsorb sodium from the urine and to excrete potassium instead. Sodium is then returned to the bloodstream. When sodium is reabsorbed into the blood, water in the body follows it, thus increasing blood volume and pressure. Aldosterone also reduces the amount of sodium and water lost through the sweat and salivary glands. When normal blood, sodium, and potassium levels are all reached, the adrenal cortex stops releasing aldosterone.

EPINEPHRINE AND NOREPINEPHRINE. When an individual is (or feels) threatened physically or emotionally, the hypothalamus readies the body to "fight" or "take flight" by sending impulses to the adrenal medulla. In response, the medulla secretes norepinephrine (in small amounts) and epinephrine (in larger amounts). Norepinephrine causes blood vessels in the skin and skeletal muscles to constrict, raising blood pressure. Epinephrine causes an increase in heart rate and contraction, stimulates the liver to change glycogen to glucose for use as energy by the cells, and stimulates fatty tissue to break down and release stored fats for use as energy by the cells as well. The actions of both hormones bring about increased levels of oxygen and glucose in the blood and a faster circulation of blood to the body organs, especially the brain, muscles, and heart. Reflexes and body movements quicken and the body is better able to handle a short-term emergency situation.

German-born American biologist Berta Scharrer (1906–1995) and her biologist husband Ernst Scharrer pioneered the field of neuroendocrinology, the study of the interaction between the nervous system and the endocrine glands and their secretions. Fighting against accepted scientific beliefs about cells—as well as against prejudice toward women in the sciences—Scharrer established the concept of neurosecretion, or the releasing of substances such as hormones by nerve cells.

Prior to the discoveries of Scharrer and her husband, scientists believed that neurons or nerve cells could not have a dual function. They either secreted hormones, in which case they were endocrine cells belonging to the endocrine system, or they conducted electrical impulses, making them nerve cells belonging to the nervous system.

In the 1930s, after having come to America, Scharrer and her husband set out to prove their theories with no real professional standing and therefore lacking a budget for lab animals. Scharrer reportedly collected cockroaches in the basement of the lab and used them for experiments. Soon she began experimenting on South American cockroaches she had discovered scurrying around in the bottom of a cage of lab monkeys that had arrived from South America. Scharrer found that they made better research subjects because they were slower than the American cockroach. From that point forward, she used the South American cockroaches, which traveled with her wherever she and her husband moved.

By 1950, Scharrer's research and theories on neurosecretion had become accepted as fact by the scientific community. For her pioneering scientific work, Scharrer received many honors. Included among these was the naming of a cockroach species, scharrerae , in her honor.

Actions of the ovaries

The ovaries do not begin to function until puberty, usually between the ages of eleven and fourteen in girls. At this time, the anterior pituitary gland secretes follicle-stimulating hormone, which causes follicles or tiny saclike structures to grow and mature in an ovary. Ova or eggs within these specialized structures also begin to mature. While an egg is developing in an ovarian follicle, the follicle cells surrounding the egg secrete estrogens. Increased levels of estrogens then signal the anterior pituitary gland to secrete luteinizing hormone, which causes the ovary to release a single mature egg—a process called ovulation.

After ovulation has occurred, a structure in the ovary secretes progesterone, which prevents another egg from beginning to develop and causes the lining of the uterus to grow thicker with blood vessels (estrogens also help in this latter action). The mature egg then travels through a fallopian tube to the uterus. If the egg has not fertilized by male sperm, it breaks down. About ten days later, the lining of the uterus begins to break apart and is shed outside the body during the monthly process called menstruation.

If the egg has been fertilized, it attaches to the wall of the uterus and pregnancy occurs. High levels of estrogens and progesterone are then produced to prevent another egg from maturing. In addition, progesterone prevents the muscles of the uterus from contracting so that the developing embryo will not be disturbed. Estrogens and progesterone both prepare the mammary glands to produce milk.

At puberty, the estrogens released by the follicle cells also bring about the female secondary sex characteristics. The breasts enlarge and their duct system to carry milk develops, the uterus enlarges, fat is deposited in the hips and thighs, and hair develops under the arms and in the genital area.

Actions of the testes

Puberty in boys usually occurs between the ages of twelve and sixteen. At this time, the anterior pituitary gland releases luteinizing hormone, which stimulates the testes to produce testosterone. This hormone produces many growth changes in an adolescent boy: growth of all the reproductive organs, growth of facial and body hair, growth of the larynx (resulting in a deeper voice), and growth of the skeletal muscles. Follicle-stimulating hormone, also secreted from the anterior pituitary, initiates the production of sperm in the testes. Testosterone then helps the sperm mature. This process, begun at puberty, continues throughout life.



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