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The Endocrine System

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Major Endocrine Organs

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The Pituitary Gland ( Hypophysis ) Securely seated in the sella turcica of the sphenoid bone, the tiny pituitary gland , or hypophysis (hi- pof′i ?-sis; “to grow under”), secretes at least nine hormones. Usually said to be the size and shape of a pea, this gland is more accurately described as a pea on a stalk.

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the pituitary gland has two major lobes ; 1.neural tissue 2.glandular.

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The posterior pituitary lobe is composed largely of pituicytes ( glia -like supporting cells) and nerve fibers. It releases neurohormones (hormones secreted by neurons) received ready-made from the hypothalamus. Thus, this lobe is a hormone-storage area and not a true endocrine gland in the precise sense. The posterior lobe plus the infundibulum make up the region called the neurohypophysis ( nu″ro -hi- pof′i ?-sis), a term commonly used (incorrectly) to indicate the posterior lobe alone.

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The anterior pituitary (lobe), or adenohypophysis ( ad″e ?-no-hi- pof′i ?-sis), is composed of glandular tissue ( adeno = gland), and it manufactures and releases a number of hormones.

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Relationships of the pituitary gland and hypothalamus

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The Thyroid Gland Location and Structure The butterfly-shaped thyroid gland is located in the anterior neck, on the trachea just inferior to the larynx.Its two lateral lobes are connected by a median tissue mass called the isthmus .

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The thyroid gland is the largest pure endocrine gland in the body. Its prodigious blood supply (from the superior and inferior thyroid arteries) makes thyroid surgery a painstaking (and bloody) endeavor.

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Thyroid Hormone Often referred to as the body’s major metabolic hormone, thyroid hormone (TH) is actually two iodine-containing amine hormones, thyroxine ( thi-rok′sin ), or T 4 , and triiodothyronine ( tri″i-o″do-thi′ro-nēn ), or T 3 . T 4 is the major hormone secreted by the thyroid follicles; most T 3 is formed at the target tissues by conversion of T 4 to T 3 . Very much like one another, the hormones are constructed from two linked tyrosine amino acids. The principal difference is that T 4 has four bound iodine atoms, and T 3 has three (thus, T 4 and T 3 ).

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Synthesis of thyroid hormone. Binding of TSH to follicle cell receptors (not illustrated) stimulates synthesis of the thyroglobulin colloid and the release of thyroxine (T 4 ) and triiodothyronine (T 3 ) from the cell in the six steps illustrated here. (Only a few of the thyroglobulins in the colloid are illustrated. The rest of the colloid is represented by the unstructured yellow substance.)

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Thyroid disorders. (a) An enlarged thyroid (goiter) of a Bangladeshi boy. (b) Exophthalmos of Graves’ disease

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Calcitonin Calcitonin is a polypeptide hormone produced by the parafollicular , or C, cells of the thyroid gland. Because its most important effect is to lower blood Ca 2+ levels, calcitonin is a direct antagonist of parathyroid hormone, produced by the parathyroid glands. Calcitonin targets the skeleton, where it (1) inhibits osteoclast activity and hence bone resorption and release of Ca 2+ from the bony matrix and (2) stimulates Ca 2+ uptake and incorporation into bone matrix. Thus, calcitonin has a bone-sparing effect.

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The Parathyroid Glands The tiny, yellow-brown parathyroid glands are nearly hidden from view in the posterior aspect of the thyroid gland. There are usually four of these glands.

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Parathyroid hormone (PTH), or parathormone , the protein hormone of these glands, is the single most important hormone controlling the calcium balance of the blood. PTH release is triggered by falling blood Ca 2+ levels and inhibited by hypercalcemia . PTH increases Ca 2+ levels in blood by stimulating three target organs: the skeleton (which contains considerable amounts of calcium salts in its matrix), the kidneys, and the intestine.

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The Adrenal (Suprarenal) Glands The paired adrenal glands are pyramid-shaped organs perched atop the kidneys (ad = near; renal = kidney), where they are enclosed in a fibrous capsule and a cushion of fat. They are also often referred to as the suprarenal glands (supra = above).

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The Pancreas Located partially behind the stomach in the abdomen, the soft, triangular pancreas is a mixed gland composed of both endocrine and exocrine gland cells. Along with the thyroid and parathyroids , it develops as an outpocketing of the epithelial lining of the gastrointestinal tract. Acinar cells, forming the bulk of the gland, produce an enzyme-rich juice that is ducted into the small intestine during food digestion.

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Scattered among the acinar cells are approximately a million pancreatic islets (also called islets of Langerhans ), tiny cell clusters that produce pancreatic hormones. The islets contain two major populations of hormone-producing cells, the glucagon-synthesizing alpha (α) cells and the more numerous insulin-producing beta (β) cells.

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These cells act as tiny fuel sensors, secreting glucagon and insulin appropriately during the fasting and fed states. Thus insulin and glucagon are intimately but independently involved in the regulation of blood glucose levels. Their effects are antagonistic: Insulin is a hypoglycemic hormone , whereas glucagon is a hyperglycemic hormone .

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Regulation of blood glucose levels by insulin and glucagon.

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Glucagon Glucagon ( gloo′kah-gon ), a 29-amino-acid polypeptide, is an extremely potent hyperglycemic agent. One molecule of this hormone can cause the release of 100 million molecules of glucose into the blood! The major target of glucagon is the liver, where it promotes the following actions :

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1. Breakdown of glycogen to glucose ( glycogenolysis ) 2. Synthesis of glucose from lactic acid and from noncarbohydrate molecules ( gluconeogenesis ) 3. Release of glucose to the blood by liver cells, which causes blood glucose levels to rise

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Insulin Insulin is a small (51-amino-acid) protein consisting of two amino acid chains linked by disulfide (–S–S–) bonds. It is synthesized as part of a larger polypeptide chain called proinsulin . The middle portion of this chain is then excised by enzymes, releasing functional insulin. This “clipping” process occurs in the secretory vesicles just before insulin is released from the beta cell.

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Symptomatic results of insulin deficit (diabetes mellitus).

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Type 1 diabetics totally lack insulin activity, and their disease is difficult to control. Typically they develop long-term vascular and neural problems. The lipidemia and high blood cholesterol levels typical of the disease can lead to severe vascular complications including atherosclerosis, strokes, heart attacks, renal shutdown, gangrene, and blindness. Consequences of neuropathies include loss of sensation, impaired bladder function, and impotence. Female type 1 diabetics also tend to have lumpy breasts and to undergo premature menopause, which increases their risk for cardiac problems. Some diabetics can delay renal damage by taking drugs to manage high blood pressure.

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type 1 diabetes mellitus , formerly called insulin-dependent diabetes mellitus (IDDM). Symptoms appear suddenly, usually before age 15. However, a long asymptomatic period during which the beta cells are systemically destroyed by an autoimmune response precedes these symptoms.

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Over 90% of known DM cases are type 2 diabetes mellitus , formerly called non-insulin-dependent diabetes mellitus (NIDDM) , which grows increasingly common with age. About 12 million people in the U.S. have been diagnosed with type 2 diabetes, and roughly half as many are believed to be undiagnosed victims. Although ketoacidosis is not a major problem for type 2 diabetics, they are at risk for the same complications as type 1 diabetics—heart disease, amputations, kidney failure, and blindness.

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The Gonads The male and female gonads produce steroid sex hormones , identical to those produced by adrenal cortical cells.

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The paired ovaries are small, oval organs located in the female’s abdominopelvic cavity. Besides producing ova, or eggs , the ovaries produce several hormones, most importantly estrogens and progesterone.

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The male testes , located in an extra-abdominal skin pouch called the scrotum, produce sperm and male sex hormones, primarily testosterone ( tes-tos′té-rōn ).

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The Pineal Gland The tiny, pine cone–shaped pineal gland hangs from the roof of the third ventricle in the diencephalon. Its secretory cells, called pinealocytes , are arranged in compact cords and clusters.

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Its only major secretory product is melatonin ( mel″ah-to′nin ), a powerful antioxidant and amine hormone derived from serotonin. Melatonin concentrations in the blood rise and fall in a diurnal (daily) cycle. Peak levels occur during the night and make us drowsy; lowest levels occur around noon.

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The Thymus Located deep to the sternum in the thorax is the lobulated thymus gland . Large and conspicuous in infants and children, the thymus diminishes in size throughout adulthood. By old age, it is composed largely of adipose and fibrous connective tissues.

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Hormones Hormones are chemical substances , secreted by cells into the extracellular fluids, that regulate the metabolic function of other cells in the body. Although a large variety of hormones are produced, nearly all of them can be classified chemically as either amino acid based or steroids.

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Mechanisms of Hormone Action A given hormone influences the activity of only certain tissue cells, referred to as its target cells. Hormones bring about their characteristic effects on target cells by altering cell activity; that is, they increase or decrease the rates of normal cellular processes. The precise response depends on the target cell type.

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For example, when the hormone epinephrine binds to certain smooth muscle cells in blood vessel walls, it stimulates them to contract . Epinephrine binding to cells other than muscle cells may have a different effect, but it does not cause those cells to contract.

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Interaction of Hormones at Target Cells 1.Permissiveness 2.synergism 3.antagonism

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Permissiveness is the situation when one hormone cannot exert its full effects without another hormone being present.

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For example , the development of the reproductive system is largely regulated by reproductive system hormones, as we might expect. However, thyroid hormone is necessary (has a permissive effect) for normal timely development of reproductive structures; without thyroid hormone, reproductive system development is delayed.

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Synergism of hormones occurs in situations where more than one hormone produces the same effects at the target cell and their combined effects are amplified.

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For example, both glucagon (produced by the pancreas) and epinephrine cause the liver to release glucose to the blood; when they act together , the amount of glucose released is about 150% of what is released when each hormone acts alone.

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When one hormone opposes the action of another hormone, the interaction is called antagonism .

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For example, insulin , which lowers blood glucose levels, is antagonized by the action of glucagon , which acts to raise blood glucose levels. Antagonists may compete for the same receptors, act through different metabolic pathways, or even, as noted in the progesterone-estrogen interaction at the uterus, cause down-regulation of the receptors for the antagonistic hormone.

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Control of Hormone Release The synthesis and release of most hormones are regulated by some type of negative feedback system . In such a system, hormone secretion is triggered by some internal or external stimulus. As hormone levels rise, they cause target organ effects and inhibit further hormone release. As a result, blood levels of many hormones vary only within a narrow range.

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Endocrine Gland Stimuli three major types of stimuli:1. humoral 2. Neural 3. hormonal.

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Humoral Stimuli Some endocrine glands secrete their hormones in direct response to changing blood levels of certain critical ions and nutrients.

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hormonal stimuli , which are also bloodborne chemicals . The term humoral harks back to the ancient use of the term humor to refer to various body fluids (blood, bile, and others).

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Humoral stimulus: Low blood calcium levels trigger parathyroid hormone release from the parathyroid glands, which causes blood Ca 2+ levels to rise by stimulating release of Ca 2+ from bone.

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Neural Stimuli in a few cases, nerve fibers stimulate hormone release. The classic example of neural stimuli is sympathetic nervous system stimulation of the adrenal medulla to release catecholamines ( norepinephrine and epinephrine) during periods of stress.

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Neural stimulus: The stimulation of adrenal medullary cells by sympathetic nervous system (SNS) fibers triggers the release of catecholamines to the blood.

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Hormonal stimulus: Hormones released by the hypothalamus stimulate the anterior pituitary to release hormones that stimulate other endocrine organs to secrete hormones.

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Hormonal Stimuli Finally, many endocrine glands release their hormones in response to hormones produced by other endocrine organs, and the stimuli in these cases are called hormonal stimuli.

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SYSTEM CONNECTIONS: Homeostatic Interrelationships Between the Endocrine System and Other Body Systems

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Integumentary System Androgens cause activation of sebaceous glands; estrogen increases skin hydration. Skin produces cholecalciferol ( provitamin D).

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Skeletal System PTH and calcitonin regulate calcium blood levels; growth hormone, T 3 , T 4 , and sex hormones are necessary for normal skeletal development. The skeleton provides some protection to endocrine organs, especially to those in the brain, chest, and pelvis.

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Muscular System Growth hormone is essential for normal muscular development; other hormones ( thyroxine and catecholamines ) influence muscle metabolism. Muscular system mechanically protects some endocrine glands; muscular activity elicits catecholamine release.

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Nervous System Many hormones (growth hormone, thyroxine , sex hormones) influence normal maturation and function of the nervous system. Hypothalamus controls anterior pituitary function and produces two hormones.

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Cardiovascular System Several hormones influence blood volume, blood pressure, and heart contractility; erythropoietin stimulates red blood cell production. Blood is the main transport medium of hormones; heart produces atrial natriuretic peptide.

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Lymphatic System/Immunity Lymphocytes “programmed” by thymic hormones seed the lymph nodes; glucocorticoids depress the immune response and inflammation. Chemical messengers of the immune system stimulate the release of cortisol and ACTH; lymph provides a route for transport of hormones.

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Respiratory System Epinephrine influences ventilation (dilates bronchioles). Respiratory system provides oxygen; disposes of carbon dioxide; converting enzyme in lungs converts angiotensin I to angiotensin II.

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Digestive System GI hormones and paracrines influence GI function; activated vitamin D necessary for absorption of calcium from diet; catecholamines influence digestive motility and secretory activity. Digestive system provides nutrients to endocrine organs.

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Urinary System Aldosterone and ADH influence renal function; erythropoietin released by kidneys influences red blood cell formation. Kidneys activate vitamin D (considered a hormone).

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Reproductive System Hypothalamic, anterior pituitary, and gonadal hormones direct reproductive system development and function; oxytocin and prolactin involved in birth and breast-feeding. Gonadal hormones feed back to influence endocrine system function.