Ch. 13 Endocrine System

Hormones of the Pancreas

- Glucagon is a protein that stimulates the liver to break down glycogen into glucose (glycogenolysis) and to convert noncarbohydrates, such as amino acids, into glucose (gluconeogenesis). Glucagon also stimulates breakdown of fats into fatty acids and glycerol. In a negative feedback system, a low concentration of blood glucose stimulates release of glucagon from the alpha cells. When the blood glucose concentration returns toward normal, glucagon secretion decreases. This mechanism prevents hypoglycemia when the blood glucose concentration is relatively low, such as between meals, or when glucose is being used rapidly, such as during exercise.

Cells of the Endocrine System

- Cells of the endocrine system and the nervous system both communicate using chemical signals that bind to receptor molecules. In contrast to the nervous system, which releases neurotransmitter molecules into synapses, the endocrine system releases hormones into the bloodstream, which carries these messenger molecules everywhere. However, the endocrine system is also precise, because only target cells can respond to a particular type of hormone. A hormone's target cells have specific receptors that are not on other cells. These receptors are proteins or glycoproteins with binding sites for a specific hormone. The non-endocrine chemical messengers, paracrine and autocrine substances, also bind to specific receptors.

Diabetes Insipidus and ADH

- Diabetes insipidus, which is unrelated to diabetes mellitus (sugar diabetes), impairs ADH regulation of water balance. In one form of diabetes insipidus, the kidneys are unable to retain water because ADH V2 receptors on the kidney collecting ducts are defective. The hormone can bind, but the receptor fails to trigger cAMP formation. Tumors and injuries affecting the hypothalamus and posterior pituitary can also cause diabetes insipidus.

Diabetes Mellitus

- Diabetes mellitus is a metabolic disease that arises from a lack of insulin or an inability of cells to respond to insulin. The persistent elevated blood glucose level can damage the eyes, heart, kidneys, and peripheral nerves. For a person with diabetes, controlling blood glucose requires daily monitoring and action. According to the CDC, about 37.2 million people in the United States have diabetes, and of this number, 8.1 million do not yet know that they have the condition. Both insulin deficiency and impaired insulin response disturb carbohydrate, protein, and fat metabolism. Because insulin helps glucose cross some cell membranes, diabetes impairs movement of glucose into adipose and resting skeletal muscle cells. At the same time, the formation of glycogen, which is a long chain of glucose molecules, declines. As a result, the blood glucose concentration rises (hyperglycemia). When it reaches a certain level, the kidneys begin to excrete the excess. Glucose in the urine (glycosuria) raises the urine's osmotic pressure, and too much water is excreted. Excess water output causes dehydration and extreme thirst (polydipsia). About 90-95% of people with diabetes have type 2, in which the beta cells produce insulin but body cells lose the ability to recognize it. The condition usually has milder symptoms than type 1 diabetes but can become much more severe over time if blood glucose is not rigorously controlled. Most affected individuals are overweight when symptoms begin. Type 2 diabetes is being increasingly diagnosed in children and adolescents.

Function of Endocrine Glands

- Endocrine glands and their hormones help regulate metabolic processes. They control the rates of certain chemical reactions; aid in transporting substances through membranes; and help regulate water balance, electrolyte balance, and blood pressure. Endocrine hormones also play vital roles in reproduction, development, and growth. Small groups of specialized cells produce some hormones although these cells are primarily parts of other systems. The larger endocrine glands—the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, and pancreas—are the subject of this chapter

Cortisol

- (hydrocortisone) is a glucocorticoid, which means it affects glucose metabolism. It is produced primarily in the middle zone (zona fasciculata) of the adrenal cortex and has a molecular structure similar to aldosterone. In addition to affecting glucose, cortisol influences protein and fat metabolism. Among the more important actions of cortisol are the following: It inhibits the synthesis of protein in various tissues, increasing the blood concentration of amino acids. It stimulates liver cells to synthesize glucose from noncarbohydrates (gluconeogenesis), such as circulating amino acids and glycerol, thus increasing the blood glucose concentration. It promotes the release of fatty acids from adipose tissue, increasing the use of fatty acids and decreasing the use of glucose as energy sources, thus preserving glucose availability for the brain.

Structure of Thyroid Gland

- A capsule of connective tissue covers the thyroid gland. The gland is made up of many secretory parts called follicles. Cavities in the follicles are lined with a single layer of cuboidal epithelial cells, called follicular cells. A clear viscous substance called colloid fills the cavities. It consists primarily of a glycoprotein called thyroglobulin. The follicular cells produce and secrete hormones that are either stored in the colloid or released into nearby capillaries. Other hormone-secreting cells, called extrafollicular cells (C cells), lie outside the follicles.

Negative Feedback of Insulin

- A negative feedback system sensitive to the concentration of blood glucose regulates insulin secretion. When glucose concentration is relatively high, such as after a meal, the beta cells release insulin. By promoting formation of glycogen in the liver and entrance of glucose into adipose and muscle cells, insulin helps prevent excessive rise in blood glucose concentration (hyperglycemia). Then, when the glucose concentration falls, as it normally does between meals or during the night, insulin secretion decreases. As blood insulin levels decrease, less glucose enters fat cells and resting muscle cells, and the glucose remaining in the blood is available for cells, such as nerve cells, that do not require insulin to take up glucose.

Steroid Abuse

- Abusing steroids carries serious risks to health. Steroids hasten adulthood, stunting height and causing early hair loss. In males, excess steroid hormones lead to breast development, and in females to a deepened voice, hairiness, and a male physique. The kidneys, liver, and heart may be damaged, and atherosclerosis may develop because steroids raise LDL and lower HDL—the opposite of a healthy cholesterol profile. Steroids can also cause psychiatric symptoms, including delusions, depression, and violent behavior.

MODY

- An enzyme called glucokinase enables pancreatic cells to "sense" the blood glucose level, which is important in regulating synthesis of glucagon and insulin. In one form of a rare type of diabetes mellitus, maturity-onset diabetes of the young (MODY), a mutation in a gene encoding glucokinase impairs the ability of beta cells to assess when to produce insulin. Other mutations that cause MODY alter insulin's structure, secretion, or cell surface insulin receptors or the ability of liver cells to form glycogen in response to insulin. MODY is treated with drugs or dietary modification.

Adrenal Sex Hormones

- Cells in the inner zone (zona reticularis) of the adrenal cortex mostly produce sex hormones. These hormones are male (adrenal androgens), but some of them are converted into female hormones (estrogens) by the skin, liver, and adipose tissues. The amounts of these adrenal hormones are very small compared to the supply of sex hormones from the gonads, but they may contribute to early development of the reproductive organs. Adrenal androgens may also play a role in the female sex drive.

Stress and Stressor

- Factors that change the body's internal or external environment are potentially life threatening. Sensory receptors detecting such changes trigger impulses that reach the hypothalamus, initiating physiological responses that resist a loss of homeostasis. These responses include increased activity in the sympathetic division of the autonomic nervous system and increased secretion of adrenal hormones. A factor capable of stimulating such a response is called a stressor, and the condition it produces in the body is called stress.

Aldosterone Affects

- Cells in the outer zone (zona glomerulosa) of the adrenal cortex synthesize aldosterone. This hormone is called a mineralocorticoid because it helps regulate the concentration of mineral electrolytes, such as sodium and potassium ions. Specifically, aldosterone causes the kidney to conserve sodium ions and to excrete potassium ions. The cells that secrete aldosterone respond directly to changes in the composition of blood plasma. However, whereas an increase in plasma potassium strongly stimulates these cells, a decrease in plasma sodium only slightly stimulates them. Control of aldosterone secretion is indirectly linked to plasma sodium level by the renin-angiotensin system.

Booze and Urination

- Frequent and copious urination often follows drinking alcoholic beverages, because ethyl alcohol inhibits ADH secretion. A person must replace the lost body fluid to maintain normal water balance. Drinking too much beer can actually lead to dehydration because the body loses more water than it takes in.

Hyperparathyroidism

Symptoms/Mechanism: Fatigue, muscular weakness, painful joints, altered mental functions, depression, weight loss, bone weakening. Increased PTH secretion overstimulates osteoclasts. Cause: Tumnor Treatment: Remove tumor, correct bone deformities

Anterior Lobe of Pituitary Gland

The anterior lobe secretes a number of hormones, including growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). The cells that make up the posterior lobe (pituicytes) do not synthesize hormones. The anterior pituitary consists of glandular cells rather than neurons, but it, too, is under the brain's control. Axon terminals of another population of hypothalamic neurons secrete hormones called releasing hormones (or in some cases release-inhibiting hormones). These releasing hormones are carried in the blood via a capillary bed associated with the hypothalamus. The vessels merge to form the hypophyseal portal veins that pass downward along the pituitary stalk and give rise to a second capillary bed in the anterior lobe. In this way, substances released into the blood from the hypothalamus are carried directly to target cells in the anterior lobe. The hypothalamus, therefore, functions as an endocrine gland, yet it also controls other endocrine glands. This dual activity is also true of the anterior pituitary.

Hormones of the Adrenal Medulla

The chromaffin cells of the adrenal medulla produce, store, and secrete two closely related hormones, epinephrine, also called adrenalin, and norepinephrine, also called noradrenalin. Both of these substances are a type of amine hormone called a catecholamine, and they have similar molecular structures and physiological functions. Epinephrine is synthesized from norepinephrine.

hyper

above: hyperthyroidism—condition resulting from an above-normal secretion of thyroid hormone.

Parathyroid Glands

are on the posterior surface of the thyroid gland. Most individuals have four of them—a superior and an inferior gland associated with each of the thyroid's bilateral lobes. The parathyroid glands secrete a hormone that regulates the concentrations of calcium and phosphate ions in the blood. Structure: Each parathyroid gland is a small, yellowish brown structure covered by a thin capsule of connective tissue. The body of the gland consists of many tightly packed secretory cells closely associated with capillary networks Secretions: The parathyroid glands secrete a protein, parathyroid hormone (PTH), or parathormone. This hormone increases the blood calcium ion concentration and decreases the blood phosphate ion concentration through actions in the bones, kidneys, and intestines.

cort

bark, rind: adrenal cortex—outer portion of an adrenal gland.

hypo

below: hypothyroidism—condition resulting from a below-normal secretion of thyroid hormone.

para

beside: parathyroid glands—set of glands on the posterior surface of the thyroid gland.

toc

birth: oxytocin—hormone that stimulates the uterine muscles to contract during childbirth.

lact

milk: prolactin—hormone that promotes milk production.

diure

to pass urine: diuretic—substance that promotes urine production.

crin

to secrete: endocrine—internal secretion.

Non-Steroid Hormone Action Steps

1. Non-Steroid Hormone reaches its target cell 2. Non-Steroid Hormones bind to receptor molecule on the cell membrane 3. Hormone binding activates molecules of adenylate cyclase 4. Adenylate Cyclase catalyzes conversion of ATP into cyclic adenosine monophosphate (cAMP) 5. cAMP activates existing proteins causing a series of reactions leading to cellular changes associated with the hormone's action.

Endocrine System Control

1. The hypothalamus, which constantly receives information about the internal environment, controls the anterior pituitary gland's release of hormones. Many anterior pituitary hormones affect the activity of other glands. Hormones that act on other glands are called tropic hormones 2. The nervous system directly stimulates some glands. The adrenal medulla, for example, secretes its hormones (epinephrine and norepinephrine) in response to impulses from preganglionic sympathetic neurons. The secretory cells replace the postganglionic sympathetic neurons, which would normally secrete norepinephrine alone as a neurotransmitter 3. Another group of glands responds directly to changes in the composition of the internal environment. For example, when the blood glucose level rises, the pancreas secretes insulin, and when the blood glucose level falls, it secretes glucagon --> In each of these cases, as hormone levels rise in the blood and the hormone exerts its effects, negative feedback inhibits the system and hormone secretion decreases. Then, as hormone levels in the blood decrease and the hormone's effects wane, inhibition of the system ceases, and secretion of that hormone increases again. As a result of negative feedback, hormone levels in the bloodstream remain relatively stable, fluctuating slightly around an average value

Renin

- Groups of specialized kidney cells (juxtaglomerular cells) are able to respond to changes in blood pressure and the plasma sodium ion concentration. If the level of either of these factors decreases, these kidney cells release an enzyme called renin. Renin reacts with a blood protein called angiotensinogen to catalyze the partial breakdown of angiotensinogen into a peptide called angiotensin I. Another enzyme (angiotensin-converting enzyme, or ACE), found primarily in lung blood vessels, catalyzes a reaction that converts angiotensin I into another form, angiotensin II, which is carried in the bloodstream. When angiotensin II reaches the adrenal cortex, it stimulates the release of aldosterone. ACTH is necessary for the adrenal gland to respond to this and other stimuli.

Growth Hormone (GH)

- Growth hormone (GH) Action: Stimulates increase in size and rate of division of body cells; enhances movement of amino acids through membranes; promotes growth of long bones Source of Control: Secretion inhibited by somatostatin (SS) and stimulated by growth hormone-releasing hormone (GHRH) from the hypothalamus

Growth Hormone

- Growth hormone secretion varies during the day, peaking during sleep. Two biochemicals from the hypothalamus control its secretion. They are released alternately, exerting opposite effects. Growth hormone-releasing hormone (GHRH) stimulates secretion of GH, and somatostatin (SS), also called GH-inhibiting hormone (GHIH), inhibits secretion. Nutritional state can affect control of GH. More GH is released in response to an abnormally low blood glucose concentration. Conversely, when the blood glucose concentration increases, growth hormone secretion decreases. Increased levels of some amino acids stimulate growth hormone secretion. Growth hormone can stimulate elongation of bone tissue directly, but its effect on cartilage requires a mediator protein, insulin-like growth factor-1 (IGF-1). Growth hormone releases IGF-1 from the liver and other tissues

Hormones

- Hormones are organic compounds. They are of two major types: steroids, or steroidlike substances; and nonsteroids, which include amines, peptides, proteins, and glycoproteins. Hormones can stimulate changes in target cells even in extremely low concentrations.

Prostaglandins

- Prostaglandins are paracrine substances, acting locally. They are present in small amounts, but are potent. They are not stored in cells; instead they are synthesized just before they are released. They are rapidly inactivated. Prostaglandins produce a variety of effects. Some prostaglandins relax smooth muscle in the airways of the lungs and in the blood vessels, dilating these passageways. Yet other prostaglandins can contract smooth muscle in the walls of the uterus, causing menstrual cramps and labor contractions. They stimulate secretion of hormones from the adrenal cortex and inhibit secretion of hydrochloric acid from the wall of the stomach. Prostaglandins also influence movements of sodium ions and water in the kidneys, help regulate blood pressure, and have powerful effects on both male and female reproductive physiology. When tissues are injured, prostaglandins promote inflammation. Drugs such as aspirin and certain steroids that relieve the joint pain of rheumatoid arthritis inhibit production of prostaglandins in the synovial fluid of affected joints. Daily low doses of aspirin may reduce the risk of heart attack by altering prostaglandin activity.

Exocrine Glands

- Secretions from exocrine glands enter tubes or ducts that lead to body surfaces. - Two examples of exocrine secretions are stomach acid reaching the lumen of the digestive tract and sweat released at the skin's surface

Somatostatin

- Somatostatin (similar to the hypothalamic hormone), which the delta cells release, may help regulate glucose metabolism by inhibiting secretion of glucagon and insulin, but its inhibitory effect in the pancreas is not strong.

Steroid and Thyroid Hormones

- Steroid hormones are poorly soluble in water. They are carried in the bloodstream bound to plasma proteins in a way that they are released in sufficient quantity to affect their target cells. Unlike amine, peptide, and protein hormones, steroid hormones are soluble in the lipids that make up the bulk of cell membranes. For this reason, these hormones can diffuse into cells relatively easily and are able to enter any cell in the body, although only target cells will respond. Once inside a target cell, steroid and thyroid hormones combine (usually in the nucleus) with specific protein receptors. The resulting hormone-receptor complex binds to particular DNA sequences, either activating or repressing specific genes. Activated genes are transcribed into messenger RNA (mRNA) molecules. The mRNAs enter the cytoplasm, where they direct the synthesis of specific proteins, which may be enzymes, transport proteins, or even hormone receptors. The activities of these hormones produce the cellular changes associated with the particular hormone

Types of Stress

- Stressors may be physical or psychological, or a combination. Physical stress threatens tissues. Extreme heat or cold, decreased oxygen concentration, infections, injuries, prolonged heavy exercise, and loud sounds inflict physical stress. Unpleasant or painful sensations often accompany physical stress. Psychological stress results from thoughts about real or imagined dangers, personal losses, unpleasant social interactions (or lack of social interactions), or any threatening factors. Feelings of anger, fear, grief, anxiety, depression, and guilt cause psychological stress. Psychological stress may also stem from pleasant stimuli, such as friendly social contact, feelings of joy or happiness, or sexual arousal. The factors that produce psychological stress vary greatly from person to person. A situation that is stressful to one person may not affect another, and what is stressful at one time may not be at another time

Anterior Pituitary Hormones

- The anterior lobe of the pituitary gland is enclosed in a capsule of connective tissue and largely consists of epithelial tissue organized in blocks around many thin-walled blood vessels. The epithelial tissue has five types of secretory cells. They are somatotropes that secrete GH, mammatropes that secrete PRL, thyrotropes that secrete TSH, corticotropes that secrete ACTH, and gonadotropes that secrete FSH and LH In males, LH (luteinizing hormone) is also referred to as ICSH (interstitial cell-stimulating hormone) because it affects the interstitial cells of the testes

Portal System

- The arrangement of two capillaries in series is unusual and is called a portal system. It exists in three places in the body: the hepatic portal vein connects intestinal capillaries to special liver capillaries called sinusoids, the efferent arteriole of kidney nephrons connects two sets of capillaries, and the hypophyseal portal vein gives rise to capillaries in the anterior lobe of the pituitary gland.

Hormones of the Adrenal Cortex

- The cells of the adrenal cortex produce more than thirty different steroids, including several hormones (corticosteroids). Unlike the adrenal medullary hormones, without which a person can survive, some of those released by the cortex are vital. In the absence of these adrenal cortical secretions, without extensive electrolyte therapy a person usually dies within a week. The most important adrenal cortical hormones are aldosterone, cortisol, and certain sex hormones.

Cellular Response to hormone w/ second messenger

- The cellular response to a hormone operating through a second messenger is greatly amplified. This is possible because many second messenger molecules can be activated in response to just a few hormone-receptor complexes, and the enzymes that are activated as a result can repeatedly catalyze reactions. Because existing proteins are activated, the response is fast. Cells are highly sensitive to changes in the concentrations of nonsteroid hormones because of such rapid amplification. Cellular response to a steroid hormone (and thyroid hormones) is directly proportional to the number of hormone-receptor complexes that form. Some amplification occurs because more than one mRNA may be transcribed from an activated gene and each mRNA may be translated into multiple copies of a protein. The response is much slower than the response to hormones acting through second messengers because protein synthesis takes some time, although the response lasts longer.

The Digestive Glands

- The digestive glands that secrete hormones are generally associated with the linings of the stomach and small intestine. The small intestine alone produces dozens of hormones, many of which have not been well studied.

Endocrine System

- The endocrine system is so named because the cells, tissues, and organs that compose it, collectively called endocrine glands, secrete substances into the internal environment. The secreted substances, called hormones, diffuse from the interstitial fluid into the bloodstream and eventually act on cells, called target cells, some distance away. Hormones can effect changes in target cells even in extremely low concentrations.

PTH Function

- The extracellular matrix of bone tissue contains a considerable amount of calcium phosphate and calcium carbonate. PTH stimulates bone resorption by osteoclasts and inhibits the activity of osteoblasts - As bone resorption increases, calcium and phosphate ions are released into the blood. At the same time, PTH causes the kidneys to conserve blood calcium ions and to excrete more phosphate ions in the urine. This combined action restores the blood calcium ion concentration without increasing the blood phosphate ion concentration. - PTH also indirectly stimulates absorption of calcium ions from food in the intestine by influencing metabolism of vitamin D. Vitamin D (cholecalciferol) synthesis begins when intestinal enzymes convert dietary cholesterol into the inactive form, provitamin D (dehydrocholesterol). This provitamin is largely stored in the skin, and exposure to the ultraviolet wavelengths of sunlight changes it to vitamin D. Some vitamin D also comes from foods. - The liver changes vitamin D to hydroxycholecalciferol, which is carried in the bloodstream or stored in tissues. When PTH is present, hydroxycholecalciferol can be changed in the kidneys into the active form of vitamin D (dihydroxycholecalciferol), which stimulates absorption of calcium ions from the intestine

Insulin

- The hormone insulin is also a protein, and its main effect is exactly opposite that of glucagon. Insulin stimulates the liver to form glycogen from glucose and inhibits conversion of noncarbohydrates into glucose. Insulin also has the special effect of promoting the facilitated diffusion of glucose through the membranes of cells bearing insulin receptors. These cells include those of adipose tissue, liver, and resting skeletal muscle (glucose uptake by active muscle is not dependent on insulin). Insulin action decreases the concentration of blood glucose toward normal, promotes transport of glucose and amino acids into cells, and increases glycogen and protein synthesis. It also stimulates adipose cells to synthesize and store fat.

Responses to Stress

- The hypothalamus controls the response to stress, termed the general adaptation syndrome (GAS). This response initially proceeds through two stages: the short-term "alarm" stage and the long-term "resistance" stage. The response to stress works to maintain homeostasis.

Calcium Ion Homeostasis

- The opposite effects of calcitonin and PTH maintain calcium ion homeostasis. This is important in a number of physiological processes. For example, if the blood calcium ion concentration drops below the normal range (hypocalcemia), the nervous system becomes abnormally excitable, and impulses may be triggered spontaneously. As a result, muscles, including the respiratory muscles, may undergo tetanic contractions, and the person may suffocate. In contrast, an abnormally high concentration of blood calcium ions (hypercalcemia) depresses the nervous system. Consequently, muscle contractions are weak and reflexes are sluggish.

Negative Feedback

Generally, hormone secretion is controlled in three ways, all of which employ negative feedback In each case, an endocrine gland or the system controlling it senses the concentration of the hormone the gland secretes, a process the hormone controls, or an action the hormone has on the internal environment

Glucagon, Insulin and Somatostatin Action

Glucagon Action: Stimulates the liver to break down glycogen and convert noncarbohydrates into glucose; stimulates breakdown of fats Source of Control: Blood glucose concentration Insulin Action: Promotes formation of glycogen from glucose, inhibits conversion of noncarbohydrates into glucose, and enhances movement of glucose through adipose and muscle cell membranes, decreasing blood glucose concentration; promotes transport of amino acids into cells; enhances synthesis of proteins and fats Source of Control: Blood glucose concentration Somatostatin Action: Helps regulate carbohydrates Source of Control: Not determined

Hormones that require cAMP

Hormones whose actions require cyclic AMP include releasing hormones from the hypothalamus; thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) from the anterior pituitary gland; antidiuretic hormone (ADH) from the posterior pituitary gland; parathyroid hormone (PTH) from the parathyroid glands; norepinephrine and epinephrine from the adrenal glands; calcitonin from the thyroid gland; and glucagon from the pancreas. Certain nonsteroid hormones use second messengers other than cAMP. For example, a second messenger called diacylglycerol (DAG), like cAMP, activates a protein kinase, leading to a cellular response.

Growth Hormone - Dwarfism

Insufficient secretion of human growth hormone (HGH) during childhood produces hypopituitary dwarfism. Body proportions and mental development are normal, but because secretion of other anterior pituitary hormones is also below normal, additional hormone deficiency symptoms may appear. For example, a child with growth hormone deficiency might not develop adult sexual features without hormone therapy.

Pineal Gland

Other organs that produce hormones are part of the endocrine system, too. They include the pineal gland; the thymus; reproductive organs; and certain cells of the digestive tract, the heart, and the kidneys. The pineal gland is a small, oval structure deep between the cerebral hemispheres, where it is attached to the upper portion of the thalamus near the roof of the third ventricle. It largely consists of specialized pineal cells and supportive neuroglia. The pineal gland secretes a hormone, melatonin, that is synthesized from serotonin. Varying patterns of light and dark outside the body control the gland's activities. In the presence of light, more action potentials from the retina travel to the hypothalamus, which then suppresses melatonin secretion from the pineal gland. In the absence of light, impulses from the retina decrease and secretion of melatonin increases.

The Pituitary Gland

(hypophysis), at the base of the brain, is about one centimeter in diameter. It is attached to the hypothalamus by the pituitary stalk, or infundibulum, and lies in the sella turcica of the sphenoid bone The pituitary gland consists of two distinct portions: an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis).

ACE Inhibitors

- ACE inhibitors are a class of drugs used to treat some forms of high blood pressure (hypertension). They work by binding to the active site on angiotensin-converting enzyme, blocking formation of angiotensin II and preventing inactivation of bradykinin, a vasodilator. Both effects dilate blood vessels, lowering blood pressure toward normal.

Graves Disease

- Autoantibodies (against self) bind TSH receptors on thyroid cell membranes, mimicking action of TSH, overstimulating gland (hyperthyroidism); exopthalmia (protrusion of the eyes) and goiter

GnRH

- Gonadotropin secretion is under the control of gonadotropin-releasing hormone (GnRH). Gonadotropins are absent in the body fluids of infants and children. Secretion of GnRH, and the resulting increase in blood levels of gonadotropins, mark the the onset of puberty.

Erythropoietin

- Increasing the number of red blood cells can increase oxygen delivery to muscles and thereby enhance endurance. Athletes introduced "blood doping" in 1972. The athletes would have blood removed a month or more prior to performance, then reinfuse the blood shortly before a competition, boosting the number of red blood cells. Easier than blood doping is to take erythropoietin (EPO), a hormone secreted from the kidneys and the liver that signals the bone marrow to produce more red blood cells. EPO is used to treat certain forms of anemia. Using it to improve athletic performance is ill advised. In 1987, EPO abuse led to heart attacks and death in twenty-six cyclists. Some runners and swimmers also abuse EPO.

The Reproductive Organs

- The reproductive organs that secrete important hormones include the testes, which produce testosterone; the ovaries, which produce estrogens and progesterone; and the placenta, which produces estrogens, progesterone, and a gonadotropin.

Non-Steroid Hormones Function

A nonsteroid hormone, such as an amine, peptide, or protein, combines with specific receptor molecules on the target cell membrane. Each receptor molecule is a protein that has a binding site and an activity site. The hormone combines with the binding site, which causes the receptor's activity site to interact with other membrane proteins. The hormone that triggers this first step in what becomes a cascade of biochemical activity is considered a first messenger. The biochemicals in the cell that induce the changes leading to the hormone effect are called second messengers. The entire process of chemical communication, from outside cells to inside, is called signal transduction. Many hormones use cyclic adenosine monophosphate (cyclic AMP, or cAMP) as a second messenger. Cyclic AMP activates another set of enzymes called protein kinases that transfer phosphate groups from ATP molecules to protein substrate molecules, a process called phosphorylation. Phosphorylation activates some proteins, and renders others inactive.

Half-Life

A value called the half-life is used to indicate the rate of removal of a substance, such as a neurotransmitter or a hormone. Half-life is the time it takes for half of the hormone molecules to be removed from the plasma. For example, a hormone with a half-life of ten minutes would start out at 100% of its blood concentration, and if secretion were to stop, it would drop to 50% in ten minutes, 25% in another ten minutes, 12.5% in another ten minutes, and so on.

Cellular Response to second messenger activation

Cellular responses to second messenger activation include altering membrane permeabilities, activating enzymes, promoting synthesis of certain proteins, stimulating or inhibiting specific metabolic pathways, promoting cellular movements, and initiating secretion of hormones and other substances. A specific example is the action of epinephrine to raise blood sugar during periods of physical stress. Epinephrine acts through the second messenger cAMP to increase the activity of the enzyme that breaks down liver glycogen, increasing the number of glucose molecules that can diffuse out of liver cells and enter the bloodstream. Another enzyme, phosphodiesterase, quickly and continuously inactivates cAMP, so its action is short-lived. For this reason, a continuing response in a target cell requires a continuing signal from hormone molecules binding receptors in the target cell membrane.

Simple Goiter

Deficiency of thyroid hormones due to iodine deficiency; because no thyroid hormones inhibit pituitary release of TSH, thyroid is overstimulated and enlarges but functions below normal (hypothyroidism)

Aldosterone

Ex of steroid hormone: which is secreted from the adrenal glands and stimulates the kidneys to retain sodium. In response to aldosterone, cells that form tubules in the kidney begin to synthesize more Na+/K+ pumps, which are the proteins that actively transport sodium ions out of the cell, in this case returning sodium to the bloodstream.

Intermediate Lobe (Fetus)

In the fetus, a narrow region develops between the anterior and posterior lobes of the pituitary gland. Called the intermediate lobe (pars intermedia), it produces melanocyte-stimulating hormone (MSH), which regulates the synthesis of melanin—the pigment in skin and in parts of the eyes and brain. In most adults this intermediate lobe is no longer a distinct structure, but its secretory cells persist in the two remaining lobes.

Thymus

The thymus lies in the mediastinum posterior to the sternum and between the lungs. It is large in young children but shrinks with age. The thymus secretes a group of hormones, called thymosins, that affect production and differentiation of certain white blood cells (T lymphocytes). The thymus plays an important role in immunity

horm

impetus, impulse: hormone—substance that a cell secretes that affects another cell.

tropic

influencing: adrenocorticotropic hormone—a hormone secreted by the anterior pituitary gland that stimulates the adrenal cortex.

endo

inside: endocrine gland—gland that internally secretes into a body fluid.

med

middle: adrenal medulla—inner portion of an adrenal gland.

exo

outside: exocrine gland—gland that secretes to the outside through a duct.

vas

vessel: vasopressin—hormone that helps maintain blood pressure by constricting blood vessels.

Non-Steroid Hormones

- Hormones called amines, including norepinephrine and epinephrine, are derived from the amino acid tyrosine. These hormones are synthesized in the adrenal medulla (the inner portion of the adrenal gland) Protein hormones, like all proteins, are composed of long chains of amino acids that are linked and folded into specific molecular structures. They include the hormone secreted by the parathyroid gland and some of those secreted by the anterior pituitary gland. Certain other hormones secreted from the anterior pituitary gland are glycoproteins, which consist of carbohydrates joined to proteins. The peptide hormones are short chains of amino acids. This group includes hormones associated with the posterior pituitary gland and some produced in the hypothalamus. Hypothalamus: Corticotropin-releasing hormone -CRH Amines: Amino acids, Ex: Norepinephrine, epinephrine Peptides: Amino acids, Ex: ADH, OT, TRH, SS, GnRH Another group of compounds, called prostaglandins, are paracrine substances. They regulate neighboring cells. Prostaglandins are lipids (20-carbon fatty acids that include 5-carbon rings) and are synthesized from a type of fatty acid (arachidonic acid) in cell membranes. Prostaglandins are produced in a wide variety of cells, including those of the liver, kidneys, heart, lungs, thymus, pancreas, brain, and reproductive organs.

Actions of Hormones

- Hormones exert their effects by altering metabolic processes. A hormone might change the activity of an enzyme necessary for synthesizing a particular substance or alter the rate at which particular chemicals are transported through cell membranes. A hormone delivers its message to a cell by uniting with the binding site of its receptor. The more receptors the hormone binds on its target cells, the greater the response. The number of receptors on target cells may change. Upregulation is an increase in the number of receptors on a target cell, which often occurs as a response to a prolonged decrease in the level of a hormone. Downregulation is the opposite, a decrease in the number of receptors in response to a prolonged increase in hormone levels. Therefore, the number of receptors changes in ways that maintain an appropriate response to hormone level.

Posterior Lobe of Pituitary Gland

- However, specialized neurons, whose axon endings enter the posterior lobe of the pituitary, secrete into the bloodstream two important hormones: antidiuretic hormone (ADH) and oxytocin (OT). The cell bodies of these neurons are in the hypothalamus. - the posterior pituitary is actually part of the nervous system. The axons of certain neurons whose cell bodies are in the hypothalamus extend down into the posterior pituitary gland. Impulses on these axons trigger the release of chemicals from their axon terminals, which then enter the bloodstream as posterior pituitary hormones. Note that although these chemicals from the hypothalamus are released by neurons, they are not considered neurotransmitters. Because they enter the bloodstream, they are considered hormones.

Hypoglycemia

- Hypoglycemia, or a low blood glucose level due to excess insulin in the bloodstream, causes episodes of shakiness, weakness, and anxiety. Hypoglycemia is most often seen when a person with diabetes mellitus injects too much insulin, but it can also reflect a tumor of the insulin-producing cells of the pancreas, or it may occur transiently following strenuous exercise. Following a diet of frequent, small meals low in carbohydrates and high in protein can often control symptoms by preventing surges of insulin that lower the blood glucose level.

Melatonin

- Melatonin secretion is part of the regulation of circadian rhythms, which are patterns of repeated activity associated with cycles of night and day, such as sleep/wake rhythms. Melatonin binds to two types of receptors on brain neurons, one that is abundant and one that is scarce. The major receptors are on cells of the suprachiasmatic nucleus, a region of the hypothalamus that regulates the circadian clock. Binding to the second, less abundant type of receptor induces sleepiness.

Other Organs that Produce Hormones

- Other organs that produce hormones include the heart, which secretes two natriuretic peptides, the kidneys, which secrete the active form of vitamin D, and the liver, which, along with the kidneys, secretes erythropoietin (EPO), a hormone that stimulates red blood cell production

Growth Hormone - Gigantism

- Oversecretion of growth hormone in childhood may result in gigantism, in which height may eventually exceed 8 feet. Gigantism is usually caused by a tumor of the pituitary gland, which secretes excess pituitary hormones, including HGH. As a result, a person with gigantism may have other metabolic disturbances. An inherited form of gigantism is seen in families in Ireland. Growth hormone oversecretion in an adult after the epiphyses of the long bones have ossified causes a condition called acromegaly. The person does not grow taller, but soft tissues continue to enlarge and bones thicken, producing a large tongue, nose, hands, and feet, and a protruding jaw. The heart and thyroid enlarge. Early symptoms include headache, joint pain, fatigue, and depression. A pituitary tumor or abuse of growth hormone (as a drug) can cause acromegaly.

Oxytocin and ADH

- Oxytocin has an antidiuretic action, but less so than ADH. In addition, oxytocin can contract smooth muscle in the uterine wall, playing a role in the later stages of childbirth. The uterus becomes more sensitive to oxytocin's effects during pregnancy. Stretching of uterine and vaginal tissues late in pregnancy, as the fetus grows, initiates sensory impulses to the hypothalamus, which then signals the posterior pituitary to release oxytocin, which, in turn, stimulates the uterine contractions of labor. Oxytocin may be given intravenously to stimulate uterine contractions, inducing labor, if the uterus is not sufficiently contracting to expel a fully developed fetus. Oxytocin may also be administered to the mother following childbirth to ensure that the uterine smooth muscle contracts enough to squeeze broken blood vessels closed, minimizing bleeding.

Cushing's Syndrome

- is hypersecretion of cortisol from any cause, such as an adrenal tumor or oversecretion of ACTH by the anterior pituitary. The condition may also result from taking corticosteroid drugs for many years, such as to treat asthma or rheumatoid arthritis. Tissue protein level plummets, due to muscle wasting and loss of bone tissue. Blood glucose level remains elevated, and excess sodium is retained. As a result, tissue fluid increases, blood pressure rises, and the skin appears puffy. The skin may appear thin due to inhibition of collagen synthesis by the excess cortisol. Adipose tissue deposited in the face and back produce a characteristic "moon face" and "buffalo hump." Increase in adrenal sex hormone secretion may masculinize a female, causing growth of facial hair and a deepening voice. Other symptoms include extreme fatigue, sleep disturbances, skin rashes, headache, and leg muscle cramps.

Posterior Pituitary Hormones

- The posterior lobe of the pituitary largely consists of neuroglia (pituicytes) and the axons of hypothalamic neurons. This is different from the anterior lobe, which is primarily glandular epithelium. The neuroglia support the axons. The secretions of these neurons function not as neurotransmitters but as hormones, because they enter the bloodstream The hormones associated with the posterior pituitary are antidiuretic hormone (also known as vasopressin) and oxytocin. These hormones are transported down axons through the pituitary stalk to the posterior pituitary and are stored in vesicles (secretory granules) near the ends of the axons. The hormones are released into the blood in response to action potentials conducted on the axons of the neurosecretory cells. Therefore, posterior pituitary hormones are synthesized in the hypothalamus, and their secretion is controlled by the hypothalamus, but they are named for where they enter the bloodstream. Antidiuretic hormone and oxytocin are short polypeptides with similar sequences. A diuretic is a chemical that increases urine production. An antidiuretic, then, is a chemical that decreases urine production. ADH produces its antidiuretic effect by reducing the volume of water that the kidneys excrete. In this way, ADH plays an important role in regulating the concentration of body fluids

Calcitonin

- The thyroid gland produces calcitonin, which is not referred to as "thyroid hormone" because it is synthesized by the C cells, distinct from the gland's follicles. Calcitonin plays a role in the control of blood calcium and phosphate ion concentrations. It helps lower concentrations of calcium and phosphate ions by decreasing the rate at which they leave the bones and enter extracellular fluids by inhibiting the bone-destroying activity of osteoclasts. At the same time, calcitonin increases the rate at which calcium and phosphate ions are deposited in bone matrix by stimulating activity of osteoblasts. Calcitonin also increases the excretion of calcium ions and phosphate ions by the kidneys. A high blood calcium ion concentration stimulates calcitonin secretion. This may occur following absorption of calcium ions from a recent meal. Certain hormones also prompt calcitonin secretion, such as gastrin, released from active digestive organs. Calcitonin helps prevent prolonged elevation of the blood calcium ion concentration after eating.

Hormones of the Thyroid Gland

- The thyroid gland produces three important hormones. The follicular cells synthesize two of these, which have marked effects on the metabolic rates of body cells. The extrafollicular cells produce the third type of hormone, which influences blood concentrations of calcium and phosphate ions. The two thyroid hormones that affect cellular metabolic rates are thyroxine and triiodothyronine, collectively referred to as thyroid hormone. Thyroxine, or tetraiodothyronine, is also called T4 because it includes four atoms of iodine. Triiodothyronine is also called T3 because it includes three atoms of iodine Thyroid hormone helps regulate the metabolism of carbohydrates, lipids, and proteins. It increases the rate at which cells release energy from carbohydrates, enhances the rate of protein synthesis, and stimulates breakdown and mobilization of lipids. Thyroid hormone is the major factor determining how many calories the body must consume at rest to maintain life, which is measured as the basal metabolic rate (BMR). It is essential for normal growth and development and for maturation of the nervous system. TSH from the anterior pituitary gland controls the level of thyroid hormone. Thyroxine (T4) accounts for at least 95% of circulating thyroid hormone, but once in the blood, most of the T3 and T4 combine with blood proteins (alpha globulins). It is the small fraction of hormone molecules that are not protein-bound (so called "free" hormone) that act on target cells. Thus T3, which has a 50-fold higher free concentration in the plasma, is physiologically more important. Additionally, T3 is nearly five times more potent than T4, and about a third of T4 is converted to T3 in peripheral tissues.

Thyroid Disorders

- Thyroid disorders may produce underactivity (hypothyroidism) or overactivity (hyperthyroidism) of the glandular cells. One form of hypothyroidism, infantile hypothyroidism, results from insufficient thyroid hormone during infancy and early childhood. In the absence of treatment, symptoms may include stunted growth, abnormal bone formation, slowed mental development, low body temperature, and sluggishness. Hypothyroidism is also common among older adults, producing fatigue and weight gain. Hyperthyroidism produces an elevated metabolic rate, restlessness, and overeating. In Graves' disease, the most common cause of hyperthyroidism, the eyes may protrude (exophthalmia) because of swelling in the tissues behind them Depending on the specific disease, both hypothyroidism and hyperthyroidism may result in an enlarged thyroid gland. This typically is visible as a bulge in the neck called a goiter. The presence of a goiter does not by itself indicate the nature of the thyroid disorder.

Thyroid - Stimulating Hormone (TSH)

- Thyroid-stimulating hormone, also called thyrotropin, is a glycoprotein. It controls secretion of certain hormones from the thyroid gland. TSH can also stimulate growth of the gland, and abnormally high TSH levels may lead to an enlarged thyroid gland, or goiter. The hypothalamus stimulates TSH secretion by secreting thyrotropin-releasing hormone (TRH). Circulating thyroid hormones help regulate TSH secretion by inhibiting release of TRH and TSH. Therefore, as the blood concentration of thyroid hormones increases, secretion of TRH and TSH declines

Type 1 Diabetes

- Type 1 diabetes mellitus usually appears before age twenty. It is an autoimmune disease: the immune system destroys the beta cells of the pancreas. People with type 1 diabetes must carefully monitor their blood glucose levels. They do this in two ways. Every three months, a laboratory test checks the levels of hemoglobin molecules in the blood that have glucose bound to them. This measurement is called "A1C" and should be between 6% and 7%. A1C represents the blood glucose level over the preceding three months, which is the life span of the red blood cells that transport hemoglobin.

Steroids

- lipids that include complex rings of carbon and hydrogen atoms All steroid hormones are derived from cholesterol. They include sex hormones such as testosterone and the estrogens, and secretions of the adrenal cortex (the outer portion of the adrenal gland), including aldosterone and cortisol. Vitamin D is a modified steroid and when converted to the active form in the kidneys and liver becomes a hormone

Life-Span Changes

- With age, the glands of the endocrine system generally decrease in size and increase in the proportion of each gland that is fibrous in nature. At the cellular level, lipofuscin pigment accumulates as glands age. Hormone levels change with advancing years. Treatments for endocrine disorders associated with aging supplement deficient hormones, remove part of an overactive gland, or use drugs to block the action of an overabundant hormone. Aging affects different hormones in characteristic ways. For growth hormone, the surge in secretion that typically occurs at night lessens with age. Lower levels of GH are associated with declining strength in the skeleton and muscles with advancing age. Levels of antidiuretic hormone increase with age due to slowed elimination by the liver and kidneys, rather than increased synthesis, stimulating the kidneys to reabsorb more water. The thyroid gland shrinks with age, as individual follicles shrink and more abundant fibrous connective tissue separates them. Thyroid nodules, which may be benign or cancerous, become more common with age, and are often first detected upon autopsy. Although blood levels of T3 and T4 may diminish with age, in general, the thyroid gland's control over the metabolism of various cell types is maintained throughout life. Calcitonin levels decline with age, which raises the risk of osteoporosis. Parathyroid function differs between the sexes with age. Secretion peaks in males at about age fifty, whereas in women, the level of parathyroid hormone decreases until about age forty, after which it rises and contributes to osteoporosis risk. Fat accumulates between the cells of the parathyroid glands. The most obvious changes in the aging endocrine system involve blood glucose regulation. The pancreas may be able to maintain secretion of insulin and glucagon, but lifestyle changes, such as increase in fat intake and less exercise, may increase the blood insulin level. The development of insulin resistance—the decreased ability of muscle, liver, and fat cells to take in glucose even in the presence of insulin—reflects impaired ability of these target cells to respond to the hormone, rather than compromised pancreatic function. Blood glucose buildup may signal the pancreas to secrete more insulin, setting the stage for type 2 diabetes mellitus. The thymus begins to noticeably shrink before age twenty, with accompanying declining levels of thymosins. By age sixty, thymosin secretion is nil. The result is a slowing of the maturation of B and T cells, which increases susceptibility to infections as a person ages.

Adrenocorticotropic (ACTH)

- also called "corticotropin," is a peptide that controls the manufacture and secretion of certain hormones from the outer layer (cortex) of the adrenal gland. The secretion of ACTH is stimulated by corticotropin-releasing hormone (CRH), which the hypothalamus releases in response to decreased concentrations of adrenal cortical hormones. Stress can increase secretion of ACTH by stimulating release of CRH.

Adrenal Glands

- are closely associated with the kidneys. A gland sits atop each kidney like a cap and is embedded in the mass of adipose tissue that encloses the kidney. Adrenal hormones play roles in maintaining the blood sodium level and responding to stress. They also include certain sex hormones. Structure: The adrenal glands are shaped like pyramids. Each adrenal gland is vascular and consists of two parts. The central portion is the adrenal medulla, and the outer part is the adrenal cortex. These regions are not sharply divided, but they are distinct in that they secrete different hormones. The Adrenal Cortex makes up the bulk of the adrenal gland. It is composed of closely packed masses of epithelial layers that form outer, middle, and inner zones—the zona glomerulosa, the zona fasciculata, and the zona reticularis, respectively

Follicle Stimulating Hormone and Luteinizing Hormone

- are glycoproteins and are called gonadotropins, which means they act on the gonads or reproductive organs. FSH controls growth and development of follicles that house developing oocytes in the ovaries. It also stimulates the follicular cells to secrete a group of female sex hormones, collectively called estrogen (or estrogens). In males, FSH stimulates the production of sperm cells in the testes. LH promotes secretion of sex hormones in both males and females and in females is essential for release of mature oocytes from the ovaries.

Catacholamine Synthesis

- begins with the amino acid tyrosine. In the first step of the pathway, an enzyme (tyrosine hydroxylase) in the secretory cells catalyzes a reaction that converts tyrosine into a substance called dopa. A second enzyme (dopa decarboxylase) catalyzes a reaction that modifies dopa into dopamine, and a third enzyme (dopamine betahydroxylase) catalyzes a reaction that alters dopamine to form norepinephrine. Still another enzyme (phenylethanolamine N-methyltransferase) then catalyzes conversion of norepinephrine to epinephrine. About 15% of the norepinephrine is stored unchanged. The hormones occupy tiny vesicles (chromaffin granules), much like neurotransmitters are stored in vesicles in neurons.

Adrenal Medulla

- consists of irregularly shaped cells grouped around blood vessels. These cells are intimately connected with the sympathetic division of the autonomic nervous system. The adrenal medullary cells (chromaffin cells) are modified sympathetic postganglionic neurons, and preganglionic sympathetic nerve fibers control their secretions

Pancreas

- consists of two major types of secretory tissues. This organization reflects the dual function of the pancreas as an exocrine gland that secretes digestive juice through a duct, and an endocrine gland that releases hormones that control the blood glucose level. Structure: The pancreas is an elongated, somewhat flattened organ located posterior to the stomach and partly between the parietal peritoneum and the posterior abdominal wall (retroperitoneal). A duct that attaches the pancreas to the first section of the small intestine (duodenum) transports its digestive juice into the intestine. The endocrine portion of the pancreas consists of cells grouped around blood vessels. These groups, called pancreatic islets (islets of Langerhans), include three distinct types of hormone-secreting cells—alpha cells, which secrete glucagon; beta cells, which secrete insulin; and delta cells, which secrete somatostatin

"Local Hormones"

- function similarly as messenger molecules and are sometimes termed "local hormones." These include paracrine secretions, which enter the interstitial fluid but affect only nearby cells, and autocrine secretions, which affect only the cell secreting the substance.

Prolactin

- is a protein, and as its name suggests, it promotes milk production. No normal physiological role for this hormone in human males has been firmly established. Abnormally elevated levels of the hormone can disrupt sexual function in both sexes.

Thyroid Gland

- is a vascular structure that consists of two large lateral lobes connected by a broad isthmus (is′mus). The thyroid lies just inferior to the larynx (voicebox) on either side and anterior to the trachea (windpipe). The gland is specialized to remove iodide from the blood.

Addison's Disease

- the adrenal cortex does not secrete hormones sufficiently. This may be due to immune system attack (autoimmunity) or an infection such as tuberculosis. Signs and symptoms include decreased blood sodium, increased blood potassium, low blood glucose level (hypoglycemia), dehydration, low blood pressure, frequent infections, fatigue, nausea and vomiting, loss of appetite, and increased skin pigmentation. Some sufferers experience salt cravings—one woman reported eating many bowls of salty chicken noodle soup, with pickles and briny pickle juice added! Without treatment, death comes within days from severe disturbances in electrolyte balance.

ADH

A sufficient concentration of ADH contracts certain smooth muscle, including the smooth muscle in the walls of blood vessels. As a result, vascular resistance and blood pressure may increase. (This is why ADH is also called vasopressin.) Although ADH is seldom at high enough levels to cause high blood pressure, its secretion increases following severe blood loss. In this situation, ADH's vasoconstrictor effect may help to minimize the drop in blood pressure that results from profuse bleeding and help return blood pressure toward normal. The binding of ADH to V1 receptors increases the concentration of the second messenger inositol triphosphate, which increases the intracellular calcium ion concentration in the smooth muscle of blood vessel walls, leading to vasoconstriction. The second receptor, V2, is on cells that make up the kidneys' microscopic structures called collecting ducts. ADH binding there activates the cAMP second messenger system, which ultimately causes the collecting ducts to reabsorb water that would otherwise be excreted in the urine. The hypothalamus regulates ADH secretion. Certain neurons in this part of the brain, called osmoreceptors, sense changes in the concentration of body fluids. For example, if a person is dehydrating due to a lack of water intake, the solutes in blood become more concentrated. The osmoreceptors, sensing the resulting increase in osmotic pressure, signal the posterior pituitary to release ADH, which acts on target cells in the kidneys, causing the kidneys to retain water. On the other hand, if one drinks water in excess of one's needs, body fluids become more dilute, which inhibits the release of ADH. In response, the kidneys excrete a more dilute urine until the concentration of body fluids returns to normal.

Aldosterone Action

Action: Helps regulate the concentration of extracellular electrolytes by conserving sodium ions and excreting potassium ions Factors Regulating Secretion: Plasma potassium and sodium ion concentrations and renin-angiotensin system

Thyroxin (T)

Action: Increases rate of energy release from carbohydrates; increases rate of protein synthesis; accelerates growth; necessary for normal nervous system maturation Source of Control: TSH from the anterior pituitary gland