Biology 212 Anatomy & Physiology Endocrine System

Endocrine System Isolated organs & tissues (“endocrine glands”) which secrete chemical messages (“hormones”) into the blood which cause changes in cells and tissues (“targets”) elsewhere in the body. Endocrine glands are ductless: they have a rich network of capillaries, and hormones are carried to target tissues & organs by the blood.

Hormones: Three major chemical groups: 1) Peptides – Chains / rings of amino acids

Hormones: Three major chemical groups: 2) Amines – Small molecules containing one or more amine groups

Hormones: Three major chemical groups: 3) Steroids – Formed from the lipid cholesterol by adding specific sidechains

Endocrine System Target cells & tissues respond to hormones relatively slowly (seconds, minutes, days) compared to much more rapid (milliseconds) responses to nervous stimulation

Hormones can produce a variety of changes in their target cells: - Stimulate or inhibit cell differentiation - Stimulate or inhibit cell division - Activate or deactivate enzymes - Stimulate or inhibit protein synthesis - Stimulate or inhibit secretion - Change permeability of plasma membrane

A specific hormone can only affect cells which have specific receptors for it on their plasma membranes.

Any single hormone can have only one effect on any specific target cells, but it may affect different types of target cells in different ways. For example, epinephrine can: Stimulate contraction of smooth muscle cells in arterioles of intestines Inhibit contraction of smooth muscle cells in arterioles of leg muscles Stimulate cardiac muscle cells to contract more quickly Stimulate sweat glands to secrete sweat Stimulate liver cells to release glucose into the blood Stimulate fat cells to break down and release fats into the blood

- Changing the amount of hormone circulating in the blood The effect of a hormone on its target cells can be changed (increased or decreased) by: - Changing the amount of hormone circulating in the blood - Changing the number of receptors which the target cells have for the hormone. - Changing the specificity and affinity which these receptors have for the hormone Upregulation Downregulation

Pituitary Gland also called Hypophysis Located in sella turcica of sphenoid bone Two developmentally and functionally distinct parts, both under control of hypothalamus of brain

Anterior pituitary or adenohypophysis Secretes 6 peptide hormones Secretory cells regulated (stimulated or inhibited) by hormones produced in the hypothalamus and carried to it by hypothalamic – hypophyseal portal veins

1) Growth Hormone (GH): Stimulates growth of many organs 2) Thyroid Stimulating Hormone (TSH): Stimulates thyroid gland to secrete its hormones 3) Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to secrete its hormones 4) Prolactin (PRL): Stimulates milk production by breast 5) Follicle Stimulating Hormone (FSH): Stimulates development or eggs / sperm 6) Luteinizing Hormone (LH): Stimulates testes to produce testosterone Stimulates follicle of ovary to change to corpus luteum

Posterior pituitary or neurohypophysis Secretes 2 peptide hormones Secretory cells are really the axons whose cell bodies are located in the hypothalamus and whose axons pass through the hypothalamic-hypophyseal tract

1) Oxytocin (OT): Stimulates contraction of smooth muscle in breast (lactation) and uterus (labor) 2) Antidiuretic Hormone (ADH): Increases absorption of water in the kidney to concentrate urine and decrease its volume

Thyroid Gland: Located in neck, anterior to trachea.

Thyroid Gland: Located in neck, anterior to trachea. Secretes 2 similar amine hormones Triiodothyronine Tetraiodothyronine and 1 peptide hormone Calcitonin

Thyroid Gland: Located in neck, anterior to trachea. Secretes 2 similar amine hormones Triiodothyronine Tetraiodothyronine and 1 peptide hormone Calcitonin Secretory cells for T3 and T4 arranged in hollow spheres called follicles; Stimulated by thyroid-stimulating hormone from adenohypophysis. Secretory cells for calcitonin located between follicles; Regulated by concentration of calcium in blood

Triiodothyronine (T3) & tetraiodothyronine (T4) increase metabolic rates of cells throughout the body by stimulating energy production by mitochondria. Calcitonin (CT) stimulates osteoblasts to store additional calcium in the extracellular matrix of bone, thus lowering calcium levels in the blood

Parathyroid Glands: Four small glands on posterior surface of thyroid Secretes one peptide: Parathyroid Hormone (PTH) PTH increases concentration of calcium in blood: - Stimulates osteocytes & osteoclasts to reabsorb calcium from bone - Inhibits kidneys from losing calcium through urine

Adrenal Glands: Located in abdomen Superior to kidneys Each adrenal actually two functionally distinct glands, one (adrenal cortex) wrapped around the other (adrenal medulla)

Adrenal Medulla: Actually a ganglion of the sympathetic division of the autonomic nervous system Secretory cells stimulated by preganglionic sympathetic neurons and Secrete two related amine hormones called catecholamines

Since epinephrine and norepinephrine are the neurotransmitters normally secreted by postganglionic sympathetic neurons, they produce all of the effects of sympathetic nerve stimulation on various organs

Adrenal Cortex: Secretes more than twenty different steroid hormones, collectively called the corticosteroids Secretory cells arranged in three concentric layers, each of which secretes a specific group of corticosteroids. From superficial to deep: - Zona glomerulosa - Zona fasciculata - Zona reticularis

Cells in zona glomerulosa secrete mineralocorticoids which regulate concentrations of various minerals (ions, electrolytes) in the blood by controlling their retention and removal by the kidneys. Most common mineralocorticoid = Aldosterone

Cells in zona fasciculata secrete glucocorticoids which Regulate metabolism in many cells by: a) stimulating breakdown of fat b) stimulating breakdown of proteins c) increasing concentrations of fatty acids, amino acids, and glucose in the blood Stimulate repair of damaged tissue Most common glucocorticoids = cortisol & corticosterone

Cells in zona reticularis secrete gonadocorticoids, or sex hormones, which supplement those secreted by testes and ovaries. Mostly androgens (male hormones) Some estrogens and progesterone (female hormones) Most common = dehydroepiandrosterone & testosterone

Pancreas: Located in abdomen Posterior & inferior to stomach Body of pancreas anterior to aorta & inferior vena cava Tail of pancreas extends left

Pancreas: Primarily an exocrine gland 99% of cells secretes digestive enzymes into small intestine but 1% = groups of endocrine cells called pancreatic islets or islets of Langerhans Secrete two peptide hormones Insulin Glucagon which are antagonistic to each other in regulating blood glucose

Pancreatic Islets: Alpha cells produce glucagon Stimulated by low blood sugar Causes increased blood sugar by glycogen breakdown gluconeogenesis Beta cells produce insulin Stimulated by high blood sugar Causes decreased blood sugar by glycogen synthesis

Diabetes Mellitus Two forms: INSULIN DEPENDENT (Type I; Juvenile-onset) - Beta cells destroyed by body’s immune system, not producing enough insulin NON-INSULIN DEPENDENT (Type II; Adult-onset) - Beta cells still producing insulin, but: Insulin is defective & can’t act on target cells; or - Target cells are defective & can’t recognize insulin Hyperglycemia: Beta cells not producing enough insulin Alpha cells producing too much glucagon Hypoglycemia: Beta cells producing too much insulin Alpha cells not producing enough glucagon

Gonads: Produce most of the steroid sex hormones

Gonads: Produce most of the steroid sex hormones Testes: Interstitial (Leydig) cells produce androgens mostly testosterone Stimulated by luteinizing hormone from adenohypophysis Causes development of male sexual characteristics & their maintenance

Gonads: Produce most of the steroid sex hormones Ovaries: As oocyte develops, it is surrounded by follicular cells which produce estrogens, primarily estradiol Stimulated by Follicle Stimulating Hormone from adenohypophysis. Causes development of female sexual characteristics Prepares uterus for pregnancy Prepares breast for lactation

Gonads: Produce most of the steroid sex hormones Ovaries: After ovulation, follicle changes to corpus luteum, secretes progesterone. Stimulated by luteinizing hormone from adenohypophysis Maintains thick lining with uterus for pregnancy

Many other organs also contain cells & tissues which secrete hormones. Examples: Kidney: Renin (blood pressure) Erythropoietin (erythrocyte development) Calciferol (blood calcium) Heart: Atrial natriuretic hormone (blood sodium) Liver: Angiotensin (blood pressure) Thymus: Thymopoietin (lymphocyte development) Stomach: Cholecystokinin (gall bladder contraction) & Secretin (pancreas secretion) Intestine: Gastrin (stomach function)