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Anatomy & Physiology- CH 15- The Endocrine System

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Endocrine System
Works together with nervous system to maintain homeostasis. Primary function is to produce hormones. Hormones act on specific target cells to produce specific effects. All cells and tissues of the body are targets of hormones.
Hormones
Chemical messengers produced by endocrine glands and secreted directly into blood vessels. Produce effects when they find their specific receptors in or on cells, lock and key model (or softball and a glove). A cell that has receptors for a particular hormone is referred to as a target of that hormone.
Control of Hormone Secretion: Negative Feedback Systems
When the level of a specific hormone drops below needed levels, the appropriate endocrine gland is stimulated to produce more of that hormone. Once the proper hormone level is present in the bloodstream, stimulation of that endocrine gland is reduced and production of that hormone is reduced.
Control of Hormone Secretion: Direct stimulation from the nervous system
Secretion of some hormones is stimulated by sympathetic nerve impulses when an animal feels threatened. "Fight or Flight" response. Adrenaline is released from the adrenal gland during a stressful event and this will cause increased heart rate.
Major Endocrine Glands
Anterior Pituitary. Posterior Pituitary. Thyroid. Parathyroid. Adrenal Cortex. Adrenal Medulla. Pancreas (Islets). Testis. Ovary.
Hypothalamus
Located on the ventral aspect of the brain. Help control appetite, body temperature, and sleep-wake cycles. Controls activities of the pituitary gland, attached to the hypothalamus by a stalk(stalk contains nerve fibers). Portal system of blood vessels links hypothalamus with anterior portion of pituitary gland. Neurons in the hypothalamus secrete releasing and inhibiting hormones into these blood vessels which cause the anterior pituitary to release or inhibit release of other hormones. Neurons also secrete antidiuretic hormone (ADH) and oxytocin. Transported to posterior pituitary for storage. Released into the bloodstream by nerve impulses from the hypothalamus.
Pituitary Gland
Hypophysis, "master gland". Two separate glands with different structures, functions, and embryological origins.
Pituitary Gland: Anterior Pituitary
Adenohypophysis; rostral portion. Stimulated by hypothalamus and direct feedback from target organs and tissues to produce its hormones.
Pituitary Gland: Posterior Pituitary
Neurohypophysis; caudal portion. Stores and releases hormones produced in hypothalamus. Doesn't produce any hormones.
Anterior Pituitary: Hormones
1. Growth Hormone. 2. Prolactin. 3. Thyroid-Stimulating Hormone. 4. Adrenocorticotropic Hormone. 5. Follicle-Stimulating Hormone. 6. Luteinizing Hormone. 7. Melanocyte-Stimulating Hormone.
Growth Hormone (GH)
Also known as somatotropin and somatotropic hormone. Promotes body growth in young animals, especially bone and muscle. Helps regulate the metabolism of proteins, carbohydrates, and lipids in all the body cells. Promotes anabolism/synthesis of proteins. Promotes release of lipids from adipose tissue and catabolism/breakdown of lipids inside cells. Decreases the use of glucose by cells (thus promoting hyperglycemia)/
Prolactin
Helps trigger and maintain lactation (secretion of milk by the mammary glands). Production and release continues as long as the teat or nipple continues to by stimulated by nursing or milking. When stimulation stops, milk production stops and the mammary gland shrinks back to its nonlactating size. No effect in the male.
Thyroid- Stimulating Hormone (TSH)
Also known as Thyrotrophic Hormone. Stimulates growth and development of thyroid gland and causes it to produce its hormones. Secretion is regulated by feedback from the thyroid gland (its target organ) via direct effects on anterior pituitary gland and changes in TSH releasing factor (TRH) produced by the hypothalamus. TRH from the hypothalamus acts on the anterior pituitary. Pituitary releases TSH which acts on the thyroid gland. Thyroid gland releases thyroid hormone into the blood. When the level of thyroid hormone gets high enough or too high the pituitary decreases TSH release. Less TSH causes the thyroid to decrease release of thyroid hormone into the blood stream.
Adrenocorticotropic Hormone (ACTH)
Stimulates growth and development of the adrenal cortex and release of some of its hormones. Regulated by feedback from the hormones of the adrenal cortex. During times of stress, ACTH can also be released quickly as a result of stimulation of the hypothalamus by other parts of the brain. Hypothalamus sends a burst ACTH-releasing factor to the anterior pituitary through the portal system causing ACTH to be released quickly
Follicle-Stimulating Hormone (FSH)
In females stimulates growth and development of ovarian follicles. Also stimulates cells lining the follicles to produce and secrete estrogens used for breeding and pregnancy. In the male stimulates spermatogenesis.
Luteinizing Hormone (LH)
Completes process of follicle development in ovary. Increasing amounts of estrogens from the follicle feed back to the anterior pituitary and cause reduced production of FSH and increased production of LH. LH levels reach peak when follicle is fully mature, usually causes ovulation (rupture of the follicle to release the oocyte). After ovulation LH stimulates cells in the empty follicle to multiply and develop into the Corpus Luteum (CL). Corpus Luteum produces progestin hormones (like progesterone) necessary for maintenance for pregnancy. In the male LH stimulates interstitial cells in the testes to develop and produce testosterone.
Melanocyte-Stimulating Hormone (MSH)
Associated with control of color changes in the pigment cells (melanocytes) of reptiles, fish, and amphibians. Animals that can change color/color patterns rapidly. Administration of artificially large amounts of MSH to higher mammals can cause darkening of the skin from melanocyte stimulation.
The Posterior Pituitary
Stores two hormones in the Hypothalamus. Antidiuretic Hormone (ADH) and Oxytocin. Transported from hypothalamus to posterior pituitary and stored in nerve endings. Periodically released into the bloodstream in response to nerve impulses from hypothalamus.
Antidiuretic Hormone (ADH)
Helps prevents diuresis (decreases water loss and subsequently causes concentrated urination). Receptors in hypothalamus detect changes in osmotic pressure of blood that result from dehydration/hemoconcentration. Generates nerve impulses-causes release of ADH. ADH travels to the kidney-causes kidney to reabsorb more water from the urine and return it to the blood stream (dilutes out the blood). ADH deficiency causes diabetes insipidus.
Oxytocin: Effects on the uterus
causes contraction of the myometrium at the time of breeding and at the time of breeding and at parturition. Induces uterine contractions that aid transport of spermatozoa to the oviducts. Stimulates uterine contractions that aid in the delivery of the fetus and the placenta.
Oxytocin: Effect on Active Mammary Glands
Causes movement of milk down to the lower parts of the glands(milk letdown). Stimulation of the teat or nipple by nursing or milking causes oxytocin to be released into the bloodstream. Causes contraction of the myoepithelial cells around the mammary gland alveoli and small ducts. Squeezes milk into the lower parts of the gland
The Thyroid Gland
Consists of two lobes on either side of larynx. Produces thyroid hormone and calcitonin. Thyroid hormone is produced in thousands of thyroid follicles. Simple cuboidal glandular cells surrounding globule of thyroid hormone precursor.
Thyroid Hormones
T3 (triiodothyronine) and T4 (tetraiodothyronine or thyroxine). Produced when TSH from anterior pituitary reaches the thyroid gland. T4 produced in greater abundance then T3 but is mostly converted to T3 before producing effects on target cells.
Effects of Thyroid Hormones
Influences development and maturation of CNS; growth and development of muscles and bones. Caloigenic Effect- regulates the metabolic rate of all body's cells (rate at which the body burns nutrients to produce energy). Allows animal to generate heat and maintain constant internal body temperature. Production increases with exposure to cold and then increases metabolic rate to generate more heat. Affects metabolism of proteins, carbohydrates, and lipids much like GH does. Promotes anabolism of protein, catabolism of lipids and hyperglycemia.
Calcitonin
Produced by C Cells located between thyroid follicles. Maintain blood calcium levels. Prevents hypercalcemia by encouraging excess calcium to be deposited in the bones, bones are the bodies "calcium bank". Calcium is involved in muscle contraction, blood clotting, milk secretion, and formation and maintenance of the skeleton.
Parathyroid Hormone (PTH)
Also called parathormone. Produced by the parathyroid glands, small pale nodules in on or near the thyroid glands. Helps maintain blood calcium levels, oppsite effect of calcitonin. Prevents hypocalcemia, causes kidneys to retain calcium and the intestine to absorb calcium from food; withdraws calcium from bones.
The Adrenal Glands
Located near the cranial ends of the kidneys. Consists of two glands: Adrenal Cortex and Adrenal Medulla.
Adrenal Cortex
ACTH stimulates the cortex to produce numerous steroid hormones classified into three main groups: 1. Glucocorticoids 2. Mineralocorticoids 3. Sex Hormones.
Adrenal Cortex: Glucocorticoids
Cortisone, cortisol and corticosterone. Hyperglycemic effect. Stimulates glcuconegenesis in the liver (conversion of protein and fat breakdown products into glucose). Also help maintain blood pressure and help the body resist the effects of stress.
Adrenal Cortex: Mineralocorticoids
(e.g., Aldosterone). Regulate the levels of important electrolytes in the body. Salt is reabsorbed and potassium and hydrogen are secreted.
Adrenal Cortex: Sex Hormones
Androgens and estrogens.
Adrenal Medulla
Produces epinephrine and norepinephrine. Secretion is controlled by the sympathetic portion of the autonomic nervous system. "Fight or Flight" response-preparation of intense physical activity. Increases heart rate and cardiac output, increase blood pressure, dilates air passageways in the lungs, and decreases GI function.
Pancreas
Located near the duodenum. Has both endocrine functions. Pancreatic islets (islets of Langerhans): groups of scattered throughout the pancreas. Alpha Cells- produce glucagon. Beta Cells- Produce insulin. Delta Cells- produce somatostatin.
Pancreatic Hormones: Insulin
Lowers blood glucose levels by causing glucose, amino acids, and fatty acids to be absorbed into body cells.
Pancreatic Hormones: Glucagon
Raises blood glucose levels by stimulating liver cells to convert glycogen to glucose and by stimulating gluconeogenesis.
Pancreatic Hormones: Somatostatin
Inhibits secretion of insulin, glucagon, and GH; diminishes GI activity.
Testes
Paired, housed in the scrotum. Made up of coiled seminiferous tubules where spermatozoa are produced. Interstitial cells: Clumps of endocrine cells scattered between the tubules. Produce androgens when stimulated by LH.
Testes: Testosterone
Primary androgen. Provides for development of male secondary sex characteristics and accessory sex glands. Activates spermatogenesis. Anabolic-stimulates build up of protein in muscle and bone.
Ovaries
Paired. Produce ova and hormones in cycles. Controlled by follicle stimulating hormone (FSH) and luteinizing hormone (LH). Hormone groups produced in the ovaries- Estrogens and Progestin's.
Estrogens
FSH stimulates ovarian follicles to develop, follicles are fluid-filled structures in which oocytes develop, cells of follicles produce and release estrogens, amount of estrogen produced increases as follicle grows. Increasing estrogen levels accelerate physical and behavioral changes. Feedback to anterior pituitary increasing estrogen levels cause anterior pituitary gland to reduce the production of FSH and increase the production of LH. When follicle is fully mature LH levels peaks, in most animal species ovulation occurs when LH level peaks, Follicle ruptures releasing the egg. High LH level stimulates cells of empty follicle to multiply and develop into corpus luteum.
Progestins
Group of hormones produced by corpus luteum. Progesterone- principal progestin. Helps prepare uterus to receive the fertilized ovum; needed to maintain pregnancy once the ovum implants. In pregnant female hormone signal is sent from uterus, and corpus luteum in maintained. If no pregnancy occurs lack of hormone signal causes corpus luteum to shrink and disappear.
Kidneys
Produce erythropoietin- stimulates red bone marrow to increase production of red blood cells.
Stomach
Gastrin: produced by cells in the wall of the stomach. Secretion stimulated by presence of food in the stomach. Stimulates gastric glands to secrete hydrochloric acid and digestive enzymes. Encourages muscular contractions of the stomach wall
Small Intestine
Secretin and Cholecystokinin produced by cells in lining of small intestine. Secretion occurs in response to presence of chime in duodenum. Secretin stimulates pancreas to secrete bicarbonate to neutralize acidic chime after it passes out of the stomach. Cholecystokinin stimulates pancreas to release digestive enzymes into the duodenum. Both enzymes inhibit gastric gland secretion , gastric motility, stimulate gallbladder to contract releasing bile salts.
Placenta
Surrounds a developing fetus during pregnancy. Acts as an interface with the maternal circulation. Produces hormones to help support and maintain pregnancy , estrogen and progesterone, Chorionic gonadotropin (some species) - Pregnancy tests
Thymus
Extends cranially from the level of the heart up into neck region along both sides of the trachea. Large in young animals atrophies later in life, kick starts the immune system early in life. Function involves hormones or hormone- like chemical substances (e.g.., thymosin and thymopoietin.) Seems to cause certain to be transformed into T-lymphocytes (T-cells).
Pineal Body
Part of the brain located at caudal end of the cleft that separates the two cranial hemispheres. Influences cyclic activities in the body/biological clock.
Pineal Body: Melatonin
Hormone like substance that seems to affect mood and wake-sleep cycles. May also play a role in timing of seasonal estrous cycles in some species.
Prostaglandins (PG)
Hormone like substances derived from unsaturated fatty acids. Produced in variety of body tissues like skin, intestine, brain, kidney, lungs, reproductive organs, and eyes. Nine main groups-designed A though I. Influence blood pressure, blood clotting, inflammation, GI, respiratory , reproductive and kidney function. PGE-plays a role in inflammation. PGF-2a- destroys the CL (luteolysis)