The Endocrine System Mosby items and derived items

Learning Objectives Describe the negative feedback system that controls production of hormones. List the major endocrine glands and the hormones they produce. Describe the structure and functions of the pituitary gland. Describe the effect(s) of growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, melanocytestimulating hormone, antidiuretic hormone, and oxytocin. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Learning Objectives Describe the structure of the thyroid gland. Describe the effects of thyroid hormone, calcitonin, and parathormone. Differentiate between T 3 and T 4. List the three categories of hormones produced by the adrenal cortex. List the hormones produced by the pancreatic islet cells and describe the effect(s) of each. Describe the effect(s) of androgens, estrogens, and progestins. List the hormones produced by the kidneys, stomach, small intestine, placenta, thymus, and pineal body. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Endocrine System Works together with nervous system to maintain homeostasis Primary function: produces hormones Hormones act on specific target cells to produce specific effects. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 A cell that has receptors for a particular hormone is referred to as a target of that hormone. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 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. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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. Ø “Fight or flight" response Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Major Endocrine Glands Anterior pituitary Posterior pituitary Thyroid Parathyroid Adrenal cortex Adrenal medulla Pancreas (islets) Testis Ovary Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Hypothalamus Part of diencephalon section of the brain Controls activities of the pituitary gland Portal system of blood vessels links hypothalamus with anterior portion of pituitary gland Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Hypothalamus Modified neurons in the hypothalamus secrete releasing and inhibiting factors into portal blood vessels Ø Specific for a particular anterior pituitary hormone Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Hypothalamus Modified neurons in hypothalamus also secrete antidiuretic hormone and oxytocin Transported to posterior pituitary for storage Released into the bloodstream by nerve impulses from the hypothalamus Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Pituitary Gland (Hypophysis) Two separate glands with different structures, functions, and embryological origins Anterior pituitary - adenohypophysis; rostral portion Ø Stimulated by hypothalamus and direct feedback from target organs and tissues to produce its hormones Posterior pituitary - neurohypophysis; caudal portion Ø Stores and releases hormones produced in hypothalamus Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

The Anterior Pituitary Known 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 Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Growth Hormone (GH) Also known as somatotropin and somatotropic hormone Promotes body growth in young animals Helps regulate the metabolism of proteins, carbohydrates, and lipids in all the body's cells Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Prolactin Helps trigger and maintain lactation Ø Secretion of milk by the mammary glands Prolactin production and release continues as long as the teat or nipple continues to be stimulated by nursing or milking. When prolactin stimulation stops, milk production stops, and the mammary gland shrinks back to its nonlactating size. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Thyroid-Stimulating Hormone (TSH) Also known as thyrotrophic hormone Stimulates growth and development of thyroid gland causes it to produce its hormones Secretion is regulated by feedback from the thyroid gland Ø Has direct effects on anterior pituitary gland through changes in TSH-releasing factor produced by the hypothalamus Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 ACTH can also be released quickly as a result of stimulation of the hypothalamus by other parts of the brain. Ø Hypothalamus sends a burst of ACTH-releasing factor to the anterior pituitary through the portal system causing ACTH to be released quickly Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Follicle-Stimulating Hormone (FSH) In female, stimulates growth and development of ovarian follicles oogenesis Ø Also stimulates cells lining the follicles to produce and secrete estrogens In the male, stimulates spermatogenesis Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Luteinizing Hormone (LH) Completes process of follicle development in ovary Increasing amounts of estrogens feedback to the anterior pituitary and cause reduced production of FSH and increased production of LH LH levels reach a peak when follicle is fully mature Ø Usually causes ovulation Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Luteinizing Hormone (LH) After ovulation, LH stimulates cells in the empty follicle to multiply and develop into the corpus luteum. Ø Corpus luteum produces progestin hormones necessary for the maintenance of pregnancy In the male, LH stimulates interstitial cells to develop and produce testosterone. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Melanocyte-Stimulating Hormone (MSH) Associated with control of color changes in the pigment cells (melanocytes) of reptiles, fish, and amphibians Administration of artificially large amounts of MSH to higher mammals can cause darkening of the skin from melanocyte stimulation Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

The Posterior Pituitary Stores two hormones produced in the hypothalamus Antidiuretic hormone and oxytocin Transported from hypothalmus to posterior pituitary and stored in nerve endings Periodically released into the bloodstream in response to nerve impulses from hypothalmus Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Antidiuretic Hormone (ADH) Helps prevent diuresis Receptors in hypothalamus detect changes in osmotic pressure of blood that result from dehydration/hemoconcentration Generates nerve impulses - causes release of ADH Ø ADH travels to the kidney - causes kidneys to reabsorb more water from the urine and return it to the bloodstream ADH deficiency causes diabetes insipidus. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Oxytocin Effects on uterus - causes contraction of the myometrium at the time of breeding and at parturition Induces uterine contractions that aid transport of spermatozoa to the oviducts Ø Stimulates uterine contractions that aid in the delivery of the fetus and the placenta Ø Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Oxytocin Effect on active mammary glands - causes movement of milk down to the lower parts of the gland (milk letdown) Stimulation of the teat or nipple by nursing or milking causes oxytocin to be released into the bloodstream. Ø Causes contraction of the myoepithelial cells around the mammary gland alveoli and small ducts Ø Forces milk into the lower parts of the gland Ø Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

The Thyroid Gland Consists of two lobes on either side of larynx Ø Lobes may be connected by isthmus Thyroid hormone is produced in follicles Ø Simple cuboidal glandular cells surrounding globule of thyroid hormone precursor Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Thyroid Hormones T 3 (triiodothyronine) and T 4 (tetraiodothyronine, or thyroxine) Produced when TSH from anterior pituitary reaches the thyroid gland T 4 produced in greater abundance than T 3 but is mostly converted to T 3 before producing effects on target cells Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Effects of Thyroid Hormones Influences development and maturation of CNS; growth and development of muscles and bones Calorigenic effect - regulates the metabolic rate of all the body's cells Allows animal to generate heat and maintain constant internal body temperature Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Effects of Thyroid Hormones 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 Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Calcitonin Produced by C cells located between thyroid follicles Helps maintain blood calcium levels Prevents hypercalcemia by encouraging excess calcium to be deposited in the bones Calcium is involved in muscle contraction, blood clotting, milk secretion, and formation and maintenance of the skeleton Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Parathyroid Hormone (PTH) Also called parathormone Produced by the parathyroid glands Ø Small, pale nodules in, or near the thyroid glands Helps maintain blood calcium levels Prevents hypocalcemia Ø Causes kidneys to retain calcium and intestine to absorb calcium from food; withdraws calcium from bones Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

The Adrenal Glands Located near the cranial ends of the kidneys Consist of two glands: 1. Adrenal cortex 2. Adrenal medulla Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Adrenal Cortex Produces numerous steroid hormones classified into three main groups: Glucocorticoids 2. Mineralocorticoids 3. Sex hormones 1. Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Adrenal Cortex 1. Glucocorticoids: cortisone, cortisol, and corticosterone Ø Ø 2. Hyperglycemic effect Also help maintain blood pressure and help the body resist the effects of stress Mineralocorticoids (e. g. , aldosterone) Regulate the levels of important electrolytes (mineral salts) in the body Sex hormones - androgens and estrogens Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Adrenal Medulla Produces epinephrine and norepinephrine Secretion is controlled by the sympathetic portion of the autonomic nervous system Ø “Fight or flight" response Increases heart rate and output, increases blood pressure, dilates air passageways in the lungs, and decreases GI function Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Pancreas Located near the duodenum Has both exocrine and endocrine functions Endocrine component - small percentage of the total volume of the organ Pancreatic islets (islets of Langerhans): groups of cells scattered throughout the pancreas Alpha cells - produce glucagon Ø Beta cells - produce insulin Ø Delta cells - produce somatostatin Ø Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Pancreatic Hormones Insulin: lowers blood glucose levels by causing glucose, amino acids, and fatty acids to be absorbed into body cells Glucagon: lowers blood glucose levels by stimulating liver cells to convert glycogen to glucose and by stimulating gluconeogenesis Somatostatin: inhibits secretion of insulin, glucagon, and GH; diminishes GI activity Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Testes Interstitial cells: clumps of endocrine cells Produce androgens when stimulated by LH Ø Interstitial cell stimulating hormone (ICSH) in the male Testosterone: primary androgen Provides for development of male secondary sex characteristics and accessory sex glands Ø Activates spermatogenesis Ø Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Ovaries Produce ova and hormones in cycles Controlled by follicle stimulating hormone (FSH) and luteinizing hormone (LH) Hormone groups produced in the ovaries: 1. Estrogens 2. Progestins Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Estrogens FSH stimulates ovarian follicles to develop Cells of follicles produce and release estrogens Ø Amount of estrogen produced increases as follicle grows Ø Increasing estrogen levels accelerate physical and behavioral changes Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Estrogens Feedback to anterior pituitary Ø Cause anterior pituitary gland to reduce the production of FSH and increase the production of LH When follicle is fully mature, LH level peaks Ø In most animal species, ovulation occurs when LH level peaks Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Progestins Group of hormones produced by corpus luteum High LH level stimulates cells of empty follicle to multiply and develop into corpus luteum Progesterone - principal progestin Ø Helps prepare uterus to receive the fertilized ovum; needed to maintain pregnancy Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Progestins In pregnant female, hormone signal is sent from uterus, and corpus luteum is maintained If no pregnancy occurs, lack of hormone signal causes corpus luteum to shrink and disappear Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Kidneys Produce erythropoietin - stimulates red bone marrow to increase production of red blood cells Production of erythropoietin stimulated by hypoxia As red blood cell production increases, more oxygen feeds back to kidneys and slows the production of erythropoietin Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Small Intestine Secretin and cholecystokinin produced by cells in lining of small intestine Secretion occurs in response to presence of chyme in duodenum Secretin stimulates pancreas to secrete fluid to neutralize acidic chyme after it passes out of the stomach Cholecystokinin stimulates pancreas to release digestive enzymes into the duodenum Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 Ø Chorionic gonadotropin (some species) Ø Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

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 Function involves hormones or hormonelike chemical substances (e. g. , thymosin and thymopoietin) Seems to cause certain cells to be transformed into T-lymphocytes Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Pineal Body Located at caudal end of the cleft that separates the two cerebral hemispheres Influences cyclic activities in the body Melatonin - hormonelike substance called that seems to affect moods and wakesleep cycles Ø May also play a role in the timing of seasonal estrous cycles in some species Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.

Prostaglandins Hormonelike substances derived from unsaturated fatty acids Produced in a variety of body tissues (skin, intestine, brain, kidney, lungs, reproductive organs, and eyes) Nine main groups - designated A through I Ø Subscript numbers and Greek letters designate subgroups Influence blood pressure, blood clotting, inflammation, GI, respiratory, reproductive, and kidney function Mosby items and derived items © 2008 by Mosby, Inc. , an affiliate of Elsevier Inc.