Endocrine System 1

Endocrine System 1

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Endocrine System 1...

Endocrine System 1:

The endocrine system along with the nervous system functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion and interpretation of impulses. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities. The action of the endocrine system is measured in minutes, hours, or weeks and is more generalized than the action of the nervous system. The nervous system usually acts more quickly and has short-term effects.

In general, the endocrine system and its hormones help regulate growth, the use of foods to produce energy, resistance to stress, the pH of body fluids and fluid balance, and reproduction.

In specific, what we would like to gain in knowledge from this chapter is specific functions of hormones, how they contribute to homeostasis and some of the more prevalent disorders.

COMPARISON OF ENDOCRINE AND EXOCRINE GLANDS

There are two major categories of glands in the body---endocrine and exocrine. Exocrine glands have ducts that carry their secretory product to a surface. These have a variety of functions and include the sweat, sebaceous, salivary, and mammary glands and the glands that secrete digestive enzymes.

Here is the first in a long but simple line of exercises…what is the difference between secretion and excretion? Let me know on paper the next time we meet for class.

Endocrine glands are ductless, that is they do not have ducts to take their secretions to specific sites. Instead, hormones are secreted directly into capillaries and circulate in the blood throughout the body. The secretory products of endocrine glands are called hormones.

CHARACTERISTICS AND CHEMISTRY OF HORMONES

Each hormone in the body is unique. Each one is different in it's chemical composition, structure, and action. With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.

Amines- these simple hormones are structural variation of the amino acid tyrosine. This group includes thyroxine from the thyroid gland and epinephrine and norepinephrine from the adrenal medulla.

Proteins- these hormones are chains of amino acids. Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins. Short chains of amino acids are called peptides. Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.

Steroids- cholesterol is the precursor for the steroid hormones, which include cortisol and aldosterone from the adrenal cortex, estrogen and progesterone from the ovaries, and testosterone from the testes.

So in general, it is wise to know that there are three classifications of hormones. This will serve you well on an upcoming quiz.

MECHANISM OF HORMONE ACTION

Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone.

This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. If a hormone and a receptor site do not match, then there is no reaction. All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.

Hormones bring about their characteristic effects on target cells by modifying cellular activity. Cells in a target tissue have receptor sites for specific hormones. Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.

In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell. The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm. Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP. This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity. The protein hormone, which reacts at the cell membrane, is called the first messenger. c Amp that brings about the action attributed to the hormone is called the second messenger. This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.

Now, let's put our heads together and think of last term and what chain of activities had precursors that activated a process to bring about a specific reaction. I can think of two examples how about you?????

Steroids, which are lipid or fat soluble, diffuse through the cell membrane and react with receptors inside the cell. The hormone receptor complex that is formed enters the nucleus, where it has a direct effect on specific genes within the DNA.

Protein hormones react with receptors on the surface of the cell, and the sequence of events that results in hormone action is relatively rapid. Steroid hormones typically react with receptor sites inside a cell. Because this method of action actually involves synthesis of proteins, it is relatively slow.

CONTROL OF HORMONE ACTION

Hormones are very potent substances, which means that very small amounts of a hormone may have profound effects on a metabolic process. Because of their potency, hormone secretion must be regulated within very narrow limits in order to maintain homeostasis in the body.

Hormones are secreted by endocrine glands when there is a need for them, that is, for their effects, on their target organs. The cells of the endocrine glands respond to changes in the blood. These stimuli are the information they use to increase or decrease secretion of their own hormones. When a hormone brings about its effects, that reverses the stimulus, and secretions of the hormone decreases until the stimulus reoccur. A specific example may be helpful here.

Insulin is secreted by the pancreas when the blood glucose level is high, that is hyperglycemia is the stimulus for secretion of insulin. Once circulating in the blood, insulin enables cells to remove glucose from the blood to use for energy production and enables the liver to store glucose as glycogen. As a result of these actions of insulin, blood glucose level decreases, reversing the stimulus for secretion of insulin. Insulin then decreases until the blood decreases until the blood glucose level increases again.

This is an example of a negative feedback mechanism, in which information, about the effects of the hormone is fed back to the gland, which then decreases its secretion of the hormone. This is why the mechanism is called “negative”; the effects of the hormone reverse the stimulus and decrease the secretion of the hormone. The secretion of many other hormones is regulated in a similar way.

Many hormones are regulated by a negative feedback mechanism; other hormones control some; and others are affected by direct nerve stimulation.

Some endocrine glands secrete hormones in response to other hormones. The hormones that cause secretion of other hormones are called tropic hormones. A hormone from gland A causes gland B to secrete its hormone. For example, thyroid-stimulating hormone (TSH) from the anterior pituitary gland (gland A) causes the thyroid gland (gland B) to secrete the hormone thyroxin.

A third method of regulating hormone secretion is by direct nervous stimulation. A nerve stimulus causes gland A to secrete its hormone. A physiological example of this mechanism is given by the adrenal medulla, which secretes epinephrine (adrenaline) in response to stimulation by the sympathetic nerves.

PITUITARY GLAND

The pituitary gland or hypophysis is a small gland about 1 centimeter in diameter of the size of a pea. Despite its size, the pituitary gland regulates many body functions. Its two major portions are the posterior pituitary gland or the neurohypophysis, which is actually an extension of the nerve tissue of the hypothalamus, and the anterior pituitary gland (adenohypophysis), which is separate glandular tissue. The gland is connected to the hypothalamus of the brain by a slender stalk called the infundibulum.

Hormones from the pituitary gland control the function of many other glands in the body, such as the ovaries, testes, thyroid gland, and the adrenal glands. The pituitary gland also secretes hormones that directly influence growth, kidney function, delivery of infants, and milk production and release by the breasts. Excessive secretion of a hormone is called hypersecretion. A deficiency of a hormone is called hyposecretion.

The two hormones of the posterior pituitary gland are actually produced in the hypothalamus and are simply stored in the posterior pituitary until needed. Their release is stimulated by nerve impulses from the hypothalamus.

Antidiuretic hormone or ADH- increases the reabsorption of water by kidney tubules, which decreases the amount of urine formed. The water is reabsorbed into the blood, so as urinary output is decreased; blood volume is increased, which helps maintain normal blood pressure. The stimulus for secretion of ADH is decreased water content of the body. If too much water is lost in sweating or diarrhea, for example, osmoreceptors in the hypothalamus detect the increased “saltiness” of body fluid. The hypothalamus then transmits impulses to the posterior pituitary to increase the secretion of ADH and decrease the loss of more water in urine. Any type of dehydration stimulates the secretion of ADH to conserve body water. Did you know that ingestion of alcoholic beverages inhibits or decreases ADH secretion and results in increased urine output? Certain drugs, called diuretics, counteract the effects of ADH and result in fluid loss. These drugs are sometimes prescribed for patients with high blood pressure of those with edema due to congestive heart failure because the drugs have the effect of removing fluid from the body.

Oxytocin- stimulates contraction of the smooth muscle in the wall of the uterus. It also stimulates the ejection of milk from the lactating breast. A commercial preparation of this hormone, called Pitocin, it is sometimes used to induce labor. One of the many reasons that healthcare providers encourage breastfeeding is due to the immediate stimulation of the postpartum uterus to contract when nursing. This helps the uterus return to its prenatal state. Pitocin may also be used to hasten the delivery of the placenta, control bleeding after delivery and to stimulate milk ejection.

The hormones or the anterior pituitary gland regulate many body functions. Releasing hormones from the hypothalamus regulates them in turn. These releasing hormones are secreted into capillaries in the hypothalamus and pass through the veins in the gland to another capillary network in the anterior pituitary gland. Here, the releasing hormones are absorbed and stimulate secretion of the anterior pituitary hormones. This pathway allows the releasing hormones to rapidly stimulate the anterior pituitary, without having to pass through the general circulation. Hormones of the anterior lobe or the pituitary of the adenohypophysis are:

Growth hormone- may also be called somatotropin and does indeed stimulate growth. GH stimulates the growth of bones, muscles, and other organs. This hormone drastically affects the appearance of an individual because it influences height. If there is too little growth hormone in a child, that person may become a pituitary dwarf of normal proportion but of small stature. An excess of hormone in a child results in a child results in exaggerated bone growth, and the individual becomes exceptionally tall or a giant. After ossification is complete and an increase in bone length is no longer possible, excess growth hormone causes an enlargement in the diameter of the bones. The result is a condition called acromegaly.

Thyroid stimulating hormone or TSH- may also be called thyrotropin, and its target organ is the thyroid gland. TSH stimulates the normal growth of the thyroid and the secretion of thyroxine (T4) and triiodothyronine (T3). When our metabolic rate or energy production decreases the releasing hormone for TSH is released. This hormone affects the activity of the thyroid gland.

ACTH or adrenocorticotropic hormone stimulates the secretion of cortisol and other hormones by the adrenal cortex. This hormone reacts with receptor sites in the cortex, of the adrenal gland to stimulate the secretion of the cortical hormones. Secretion of ACTH is increased by corticotropin releasing hormone (CRH) from the hypothalamus. CRH is produced in any type of physiological stress situation such as injury, disease, exercise, or hypoglycemia (being hungry is stressful).

Prolactin- or lactogenic hormone promotes glandular tissue in the breast and stimulates the production of milk. This hormone does not cause the milk to be ejected from the breast. Hyposecretion of prolactin presents no problem except in women who choose to breast feed their babies. Hypersecretion is more common and is usually the result of pituitary tumors. This causes inappropriate lactation and lack of menstruation in females. In males, it results in impotence.

Follicle stimulating hormone or FSH- is one of the gonadotropic hormones that are it has effects on the gonads: the ovaries or testes. FSH is named for one of its functions in women. Within the ovaries are ovarian follicles that contain potential egg cells. FSH stimulates the growth of ovarian follicles; that is, it initiates egg development in cycles of approximately 28 days. FSH also stimulates secretion of estrogen by follicle cells. In men, FSH initiates sperm production within the testes.

Luetinizing hormone or LH- is another gonadotropic hormone. In women, LH is responsible for ovulation, the release of a mature ovum from the ovarian follicle. LH causes ovulation and the production and secretion of the sex hormones, progesterone and estrogen, in the female. In the male, LH is sometimes called interstitial-cell-stimulating hormone because it stimulates the interstitial cells of the testes to produce and secrete the male sex hormone testosterone. Without FSH and LH, the ovaries and testes decrease in size, ova and sperm are not produced, and sex hormones are not secreted.

Melanocyte stimulating hormone or MSH- influences the melanocytes of the skin to produce melanin. Melanin reacts to sunlight and will cause the skin to darken.

THYROID GLAND

The thyroid gland is a very vascular gland that is located in the neck. It is estimated that 4-5 liters of blood pass through the gland every hour. It also consists of 2 lobes, one on each side of the trachea below the larynx. A narrow band of tissue called the isthmus connects the two lobes. The structural units of the thyroid gland are thyroid follicles. The thyroid gland secretes three hormones, thyroxine (T4), triiodothyronine (T3), and calcitonin. About 95% of the active thyroid hormone is thyroxine. Both of these hormones require iodine for their synthesis. The thyroid hormone secretion is regulated by a negative feedback system that involves the amount of circulating hormones. If there is an iodine deficiency, the thyroid cannot make sufficient hormone. This stimulates the anterior pituitary to secrete TSH, which causes the thyroid gland to increase in size in a useless attempt to produce more hormones. But then again, it cannot produce more hormones because it does not have the necessary raw material or iodine to do this. A simple goiter is an enlarged thyroid gland resulting from a deficiency of iodine in the diet.

Thyroxine controls the rate of metabolism, heat production, and oxidation of most cells. To summarize, the functions of thyroxine are as follows:

controlling the rate of metabolism; how cells utilize oxygen to produce heat and energy

is a diuretic and promotes water loss through urine

promotes protein synthesis or building and thus helps with tissue growth

stimulates the breakdown of liver glycogen

stimulates the cellular breakdown of glucose

Hyposecretion of thryoxine in a newborn has devastating effects on the growth of the child. Without thyroxine, physical growth is diminished, as is mental development. This condition is called cretinism, characterized by severe physical and mental retardation. If the thyroxine deficiency is detected shortly after birth, the child may be treated with thyroid hormones to promote normal development.

Hyposecretion of thyroxine in an adult is called myxedema. Without thyroxine, the metabolic rate (energy production) decreases, resulting in lethargy, muscular weakness, and slow heart rate, a feeling of cold, weight gain, and characteristic puffiness of the face. The administration of thyroid hormones will return the metabolic rate to normal.

Grave's disease is a hypersecretion of thyroxine that is believed to be an immune disorder. The symptoms are those that would be expected when the metabolic rate is abnormally elevated: weight loss accompanied by an increase in appetite, increased sweating, fast heart rate, feeling of warmth, and fatigue. A goiter may also be present along with exopthalmos or bulging of the eye. Treatment is aimed at decreasing the secretion of thyroxine by the thyroid, and medications or radioactive iodine may be used to accomplish this.

Calcitonin- controls the calcium concentration in the body by maintaining a proper calcium level in the bloodstream. Calcium is an essential body mineral. Approximately 99% of the calcium in the body is stored in the bones. The rest is located in the blood and tissue fluids. Calcium is necessary of blood clotting, holding cells together, and neuromuscular functions. Calcitonin decreases the reabsorption of calcium and phosphate from the bones to the blood thereby lowering blood levels of these minerals. This function of calcitonin helps maintain normal blood levels of these minerals while helping to maintain a strong bone matrix. It is believed that calcitonin exerts its most important influence during the childhood years when the bones are growing at significant rates.

Hypercalcemia is when the blood levels of calcium are high. This triggers the stimulation of calcitonin and means that no more calcium will be removed from the bones until there is a real need for more calcium in the blood.

PARATHYROID GLANDS

Parathyroid glands are usually four in number and are very tiny about the size of grains of rice. They are attached to the posterior surface of the thyroid gland and secrete the hormone, parathormone or PTH.

PTH or parathormone- is the most important regulator of blood calcium levels. It is secreted in response to low blood calcium levels and it' affect is to increase those levels. Parathormone increases the stimulation of the number and the size of specialized bone cells called osteoclasts. Bone calcium is bonded to phosphorus in a compound that is called calcium phosphate. So when calcium is released into the bloodstream, phosphorus is released along with it.

PTH and calcitonin have opposite or antagonistic effects to one another.

Hypoparathyroidism or insufficient secretion of PTH leads to increased nerve excitability. The low blood calcium levels trigger spontaneous and continuous nerve impulses, which trigger muscle contraction.

If you have had the muscular system your mind should be racing right now!!! What exactly does the role of calcium have to do with muscle contraction or if you haven't had this system yet…you know that for years you have seen and heard athletes talking about milk and calcium and how important this is.

Well think of this…. We know that we need calcium to help provide us with a strong foundation for our bone growth. We also need to have calcium to help with muscle contraction. Muscle fibers are made from thin and thick fibers in order for them to intertwine and have smooth and fluid movement when contracting. Calcium is one of the chemicals that are involved in changing the way that the fibers intertwine and then ultimately allow for the muscle fibers to shorten and contract.

Hyperparathyroidism or and excessive secretion of PTH leads to an increase in the osteoclast activity that removes calcium from the bones and excretes it into the bloodstream. This increase in calcium in the blood may lead to kidney stones or a buildup of mineral deposits that can be found in abnormal places. It may also lead to a bone disease called osteitis fibrosa cystica where the bone mass decreases, decalcification occurs, cyst like cavities appear in the bone and spontaneous fractures result.

THYMUS GLAND

The thymus gland is both an endocrine gland and a lymphatic organ. Fairly large during childhood, it begins to atrophy and disappear in puberty and is essentially non-functional into early adulthood. Recent research has discovered that this gland secretes a large number of hormones. These hormones help to stimulate the lymphoid cells that are responsible for the production of antibodies against certain diseases.

The hormone thymosin enhances the development of T cell lymphocytes, which help to protect the body from foreign invaders. If an infant is born without a thymus gland, the immune system does not properly develop and the body is highly susceptible to infections.

Did you ever wonder why with advancing age we are constantly on the look out for tumors or irregularities within our bodies? Well, we know that the thymus produces special cells that fight off foreign invaders. The majority of our cells are produced while the thymus is functioning. These special cells have a life span of about 60 years within our bodies. So as our age advances are lymphocytes decrease naturally and now we have a non-functioning thymus so therefore we cannot replace the loss.

ADRENAL OR SUPRARENAL GLANDS

The adrenal or suprarenal glands are paired with one gland located near the upper portion of each kidney. The glands are embedded in the fat that surrounds the kidneys. Each gland is divided into an outer region or cortex, and an inner region, the adrenal medulla. The cortex and the medulla of the adrenal gland develop from different embryonic tissues and secrete different hormones.

The hypothalamus of the brain influences both of the adrenal gland but by different mechanisms. The medulla receives direct stimulation from nerve impulses that originate in the hypothalamus and then travel through the brain stem, spinal cord and sympathetic nerves.

Adrenal medullas secrete epinephrine and norepinephrine, which collectively are called catecholamines. Both epinephrine (adrenalin) and norepinephrine (noradrenalin) are both secreted in stress situations and help prepare the body for “fight or flight”.

Norepinephrine- is secreted in small amounts, and its most significant function is to cause vasoconstriction in the skin, viscera, and skeletal muscles, which raises blood pressure.

Epinephrine- is secreted in larger amounts, increases heart rate and force of contraction and stimulates vasoconstriction in skin and viscera and vasodilation in skeletal muscles. It also dilates the bronchioles, decreases peristalsis, stimulates the liver to change glycogen to glucose, increases the use of fats for energy, and increases the rate of cell respiration. Epinephrine is actually more effective than sympathetic stimulation, because the hormone increases energy production and cardiac output to a greater extent.

The adrenal cortex secretes three types of steroid hormones:

mineralocorticoids

glucocorticoids

sex hormones

Mineralocorticoid- Aldosterone is the most abundant of the mineralocorticoids. The target organs of aldosterone are the kidneys, but there are important secondary effects as well. While Aldosterone primarily affects the kidneys, it also acts on the intestines, salivary glands, and sweat glands. In general, its effect is to conserve sodium ions and water in the body and to eliminate potassium ions. Aldosterone increases the reabsorption of sodium and the excretion of potassium by the kidney tubules. Sodium ions are returned to the blood, and potassium ions are excreted in urine. The principal mineralocorticoid is aldosterone, which acts to conserve sodium ions and water in the body.

A tumor of the adrenal cortex may lead to hypersecretion of mineralocorticoid. If this results in excessive potassium depletion, neurons and muscle fibers become less responsive to stimuli. Symptoms of this condition include paralysis, muscle weakness, and cramps.

Glucocorticoid is secreted by them middle region of the adrenal cortex. The principal glucocorticoid is cortisol, which increases blood glucose levels. Cortisol increases the use of fats and excess amino acids for energy, and decreases the use of glucose. Cortisol is secreted in any type of physiological stress situation: disease, physical injury, hemorrhage, fear or anger, exercise, and hunger. Cortisol also has an anti-inflammatory effect. During inflammation, histamine from damaged tissue makes capillaries more permeable, and the lysosomes of damaged cells release their enzymes, which help breakdown-damaged tissue but may also cause destruction of nearby healthy tissue.

Normal cortisol secretion seems to limit the inflammation process to what is useful for tissue repair, and to prevent excessive tissue destruction. Too much cortisol, however, decreases the immune response, leaving the body susceptible to infection and significantly slowing the healing of damaged tissue.

Cortisol also helps to counteract the inflammatory response. For this reason, it is used clinically to reduce the inflammation in certain allergic reactions, bursitis, arthritis, infections, and some types of cancer.

Persons with inflamed joints often receive injections of pharmaceutical glucocorticoid, cortisone, to relieve the pain and inflammation. Over the counter creams and ointments containing hydrocortisone are available to relieve the itching and inflammation of skin rashes.

Gonadocorticoids or sex hormones are the third group of steroids secreted by the adrenal cortex.

Male hormones, androgens, and female hormones, estrogens, are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and the ovaries.

In women, the masculinization effect of androgen secretion may become evident after menopause, when estrogen levels from the ovaries decrease. Tumors that result in hypersecretion of gonadocorticoids may have dramatic effects in prepubertal boys and girls. There is a rapid onset of puberty and sex drive in males. Females develop the masculine distribution of body hair, including a beard, and the clitoris enlarges to become more like a penis.

Hyposecretion of the adrenal cortex leads to a condition known as Addison's disease. The lack of mineralocorticoids causes low blood sodium, high blood potassium, and dehydration and the lack of glucocorticoid causes low blood glucose levels. Aldosterone deficiency leads to retention of potassium and excretion of sodium and water in urine. The result is severe dehydration, low blood volume and low blood pressure. Without treatment circulatory shock and death will follow. Treatment involves administration of hydrocortisone; in high doses this will also compensate for the aldosterone deficiency. In addition, there is increased pigmentation in the skin. If untreated, hyposecretion of the adrenal cortex leads to death in a few days.

Hypersecretion of the adrenal cortex causes Cushing's syndrome. Excessive cortisol promotes fat deposition in the trunk of the body, while the extremities remain thin. The skin becomes thin and fragile and healing after an injury is slow. The bones also become fragile and osteoporosis is accelerated. This is characterized by elevated blood glucose levels, retention of sodium ions and water with subsequent puffiness or edema, loss of potassium ions, and in females, masculinization. . Treatment is aimed at removal of the cause of the hypersecretion, whether it is a pituitary or adrenal tumor.

GONADS (TESTES AND OVARIES)

The gonads, the primary reproductive organs, are the testes in the male and the ovaries in the female. These organs are responsible for the sperm and the ova but they also secrete hormones and are considered to be endocrine glands.

Androgens- male sex hormones. The principal androgen is testosterone, which is secreted by the testes. Production of testosterone begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty. This steroid hormone is responsible for:

the growth and development of the male reproductive structures

increased skeletal and muscular growth

enlargement of the larynx accompanied by voice changes

growth and distribution of body hair

increased male sex drive

closure of the epiphyseal discs in the long bones of the body

Testosterone secretion is regulated by a negative feedback system that involves releasing hormones from the hypothalamus and gonadotropins from the anterior pituitary.

The testes produce androgens, primarily testosterone, which are responsible for the development and maintenance of male secondary sex characteristics.

OVARIES

The ovaries are located in the pelvic cavity, one on each side of the uterus. Two groups of sex hormones are produced in the ovaries.

Estrogen - is secreted by the follicle cells of the ovary: secretion is stimulated by FSH from the anterior pituitary gland. Estrogen promotes the maturation of the ovum in the ovarian follicle and stimulates the growth of blood vessels in the endometrium of the uterus in preparation for possible fertilized eggs.

Estrogen promotes:

the development of breast tissue

distribution of fat evidenced in the hips, legs, and breasts

maturation of the reproductive organs such as the uterus and the vagina (menstruation)

the closure of the epiphyseal discs in long bones

Estrogen is also believed to lower blood cholesterol levels and triglycerides. Before the age of menopause this is beneficial in that it decreases the risk of atherosclerosis and coronary artery disease.

Progesterone- causes the uterine lining to thicken in preparation for pregnancy. This thickening of the endometrium allows for further growth of blood vessels, which will eventually become a placenta. Progesterone also promotes the storage of glycogen. The secretory cells of the mammary glands also develop under the influence of progesterone.

Both estrogen and progesterone are secreted by the placenta during pregnancy.

INHIBIN IS HORMONE THAT IS SECRETED BY BOTH THE OVARIES AND THE TESTES. THE FUNCTION OF INHIBIN IS TO DECREASE THE SECRETION OF FSH. THE INTERACTION OF INHIBIN, TESTOSTERONE, AND OTHER HORMONES MAINTAIN SPERMATOGENESIS OR THE MANUFACTURING OF SPERM.

PANCREAS

The pancreas is located in the LUQ of the abdomen lying transversely on the posterior abdominal wall. The pancreas is a digestive organ as well as an endocrine gland. The endocrine portion of the pancreas consists of the pancreatic islets of Langerhans, which secrete alpha cells, which secrete glucagon. The beta cells produce insulin.

Glucagon- stimulates the liver to change glycogen to glucose and to increase the use of fats and excess amino acids for energy production. Hypoglycemia or a low blood glucose level stimulates the secretion of glucagon. Such a state may occur between meals or during physiological stress situations such as exercise. The principal action of glucagon, from the alpha cells, is to raise blood glucose levels.

Insulin- beta cells in the pancreatic islets secrete the hormone insulin in response to a high concentration of glucose in the blood. The action of insulin is the opposite or antagonistic to glucagon. It promotes the permeability of cell membranes to glucose. Once inside the cells, glucose is used in cell respiration to produce energy. Insulin also enables cells to take in fatty acids and amino acids to use in the building of lipids and proteins. Insulin is a vital hormone; we cannot survive for very long without it. With respect to blood glucose levels, insulin decreases its level by promoting the use of glucose for energy production.

A deficiency of insulin or in its functioning is called diabetes mellitus. There are two types of diabetes mellitus. Type 1 is called insulin dependent diabetes (IDDM), and its onset is usually in childhood or juvenile onset. Type 2 is called non-insulin dependent diabetes (NIDDM), and its onset is usually later in life or mature onset.

Type 1 diabetes is characterized by destruction of the beta cells of the islets of Langerhans and a complete lack of insulin. Destruction of the beta cells is believed to be and autoimmune response, perhaps triggered by a virus, onset of diabetes is usually abrupt. This form of diabetes occurs most often in children, and there may be a genetic predisposition in juvenile diabetics than in other children. Insulin by injection is essential to control this form of diabetes. Recently, however, an immunosuppressant medication has been found to slow or stop the progression of type 1 diabetes if given as the disease is developing. By blocking the autoimmune response, this drug seems to permit the survival of some of the beta cells, which will continue to produce insulin.

In type 2 diabetes, insulin is produced but cannot exert its effects because of a deficiency of insulin receptors on cell membranes. Onset of type 2 diabetes is usually gradual, and risk factors include of family history of diabetes and being overweight. Control may not require insulin, but rather medications that enable insulin to react with the remaining cell membrane receptors. Some of the newer medications seem to stimulate the same reaction that insulin does. Without insulin, blood glucose levels remain high, and glucose is lost in the urine. Since more water is lost as well, symptoms include greater urinary output (polyuria), and thirst (polydipsia.)

The long-term affects of hyperglycemia produce distinctive vascular changes. The capillary walls thicken, and an exchange of gases and nutrients diminishes. The most damaging effects are seen in the skin, the retina, and the kidneys. Poorly controlled diabetes may lead to dry gangrene, blindness, and severe kidney damage. In NIDDM, atherosclerosis is quite common. It is now possible for diabetics to prevent much of this tissue damage by precise monitoring of blood glucose levels and more frequent administration of insulin. The new insulin pumps are able to more closely mimic the natural secretion of insulin. A very serious potential problem for the insulin dependent diabetic is ketoacidosis. When glucose cannot be used for energy, the body turns to fat and proteins, which are converted by the liver to ketones. Ketones are organic acids that can be used by the cells. Cells cannot utilize them rapidly, so they accumulate in the blood. Since ketones are acids, they lower the pH of blood as they accumulate. The kidneys excrete excess ketones, but in doing so excrete more water as well, which leaks to dehydration and worsens the acidosis. Without administration of insulin to permit the use of glucose for energy, and IV fluids to restore blood volume to normal, ketoacidosis will progress to coma and death.

PINEAL GLAND

The pineal gland (can be pronounced as PIE-nee-al or PIN-e-al) is a small cone shaped that extends posteriorly from the third ventricle of the brain. Melatonin is a hormone produced by pineal gland. The secretion of melatonin is greatest during darkness and decreases when light enters the eye and the retina signals the hypothalamus. A recent discovery is that the retina also produces melatonin, which seems to indicate that the eyes and pineal gland work with the biological clock of the hypothalamus. In other mammals, melatonin helps regulate seasonal reproductive cycles. For people, melatonin definitely stimulates the onset of sleep and increases its duration.

Melatonin production appears to be related to the amount of light that enters through the eye. People who work at night and sleep during the day have a reversed cycle of melatonin production. The high levels occur during the day while they are asleep and the low levels are at night when they are working and light is entering the eye.

PROSTAGLANDINS

Prostaglandins are potent chemical regulators that are produced in minute amounts and are found widely distributed in cells throughout the body. Prostaglandins are made by virtually all cells from the phospholipids of their cell hormones. They differ from other hormones in that they do not circulate in the blood to target organs, but rather exert their effects locally, where they are produced.

They are similar to hormones but different enough that they are not classified as hormones. Whereas hormones are produced by specialized cells grouped together in structures called endocrine glands, prostaglandins are produced by cells that are widely distributed throughout the body. In contrast to a hormone, which is transported in the blood and may have an effect far distant from its point of origin, a prostaglandin has a localized effect on or near the cell in which it is made. For this reason it is sometimes called a local hormone. In addition to being localized, the effect is immediate and short term. These compounds cannot be stored in the body but must be synthesized on demand and are readily inactivated.

Non steroidal anti-inflammatory drugs, such as aspirin, ibuprofen, and acetaminophen, inhibit or block the synthesis of prostaglandins. Because prostaglandins promonte the inflammatory response, inhibiting their production will reduce the inflammation of rheumatoid arthritis, bursitis, tennis elbow, and a wide variety of other inflammatory disorders.

In the stomach, prostaglandins inhibit hydrochoric acid secretion. Drugs that inhibit prostaglandin syntheses may make an individual susceptible to peptic ulcers by increasing acid secretion.

OTHER HORMONES

The gastric mucosa of the stomach lining produces a hormone called gastrin in response to food in the stomach.

The mucosa of the small intestine secrets the hormone secretin and cholecystokinin. Secretin stimulates the pancreas to produce a bicarbonate-rich substance that neutralizes the stomach acid. Cholecystokinin stimulates the contraction of the gall bladder, which releases bile. It also stimulates the pancreas to secrete digestive enzymes.

Surprisingly, the heart acts as an endocrine organ in addition to its major role of pumping blood. Special cells in the wall of the upper chambers of the heart called atrialnatriuretic hormone. The primary effect of this hormone is the loss of sodium and water in the urine. The result of this action is a decrease in blood volume and blood pressure.

The placenta develops in the pregnant female as a source of nourishment and gas exchange for the developing fetus. It also serves as a temporary endocrine gland. One of hormones it secretes is human chorionic gonadotropin (HCG), which signals the mother's ovaries to maintain the uterine lining so that it does not degenerate and slough off in menstruation. HCG reaches high levels during pregnancy, then decreases. The placenta also produces estrogen and progesterone during pregnancy.