KIN 268: Endocrine System

KIN 267 + KIN 268

Hormones

Hormones

substances secreted into bloodstream which stimulate a response in another cell, tissue, or organ; chemical mediators that regulate tissue/organ growth and development, metabolism and energy balance, blood, ECF, and internal envir.’s chemical composition and volume, cause contraction of cardiac and smooth muscle

Endocrine glands

secrete hormones in low amounts into interstitial fluid and capillaries; pituitary, thyroid, parathyroid, adrenal, pineal

Exocrine glands

secrete substances into ducts going to external environment (cavities, lumen, surfaces) e.g. sweat and oil glands, mucous glands (saliva), digestive glands (enzymes into GI tract)

Secreting cells

found in hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, placenta; some of these organs/tissue aren’t part of the endocrine system or classified as endocrine glands but secrete hormones

Synergistic effect

hormones work more effectively when a second is present to assist; act together so the sum of their individual effects is greater e.g. glucagon and epinephrine

Antagonistic effect

hormones oppose the action of others; one causes the opposite cellular response to the other or reduces the number of receptors e.g. insulin and glucagon

Hormone receptors

continually synthesized and broken down; down-regulated (less sensitive=less effect) in the presence of high concentrations of hormones (i.e. insulin and T2D) or up-regulated (more sensitive=more effect) in the presence of low concentrations of hormone (i.e. testosterone with age)

Local hormones

hormones that act close by; don’t enter the bloodstream or circulate

Paracrines

local hormones that act on nearby cells; Interleukin-2 is produced by T cells which helps activate other immune cells

Autocrines

local hormones that act on the same cell that secretes them; -Interleukin-2 can stimulate the T cell to produce more T cell

Water-soluble hormones

amine hormones, peptide/protein hormones; circulate freely in blood plasma; bind to cell surface receptors on plasma membrane and need help of messengers because they can’t diffuse through PM

Lipid-soluble hormones

steroid hormones, thyroid hormones, nitric oxide, eicosanoids; require transport protein (made in liver) to travel through blood; receptors inside target cell; 10% are free in the blood and penetrate plasma membrane to enter nucleus and cause reaction

Steroid hormones

lipid-soluble hormone; derived from cholesterol; each is unique due to the presence of different chemical groups attached at various sites on the four rings at the core of its structure

Nitric oxide

lipid-soluble hormone; vasodilator; hormone and neurotransmitter; secreted from endothelial cells lining blood vessels

Eicosanoid hormones

lipid-soluble local hormone; derived from a 20-carbon fatty acid –> arachidonic acid; produce prostaglandins and leukotrienes (mediate/promote inflammatory response); appear in the blood in small quantities; inactivated quickly; bind to receptors on target cells to stimulate or inhibit synthesis of second messengers

Amine hormones

water-soluble hormone; synthesized by decarboxylating and modifying amino acids; e.g. catecholamines: epinephrine, norepinephrine and dopamine (from tyrosine), histamines (from histidine) and serotonin and melatonin (from tryptophan)

Peptide and protein hormones

water-soluble hormone; amino acid polymers (small and large chain); ADH and oxytocin; hGH and insulin; TSH (glycoprotein hormones)

Secretion regulation

signals from the nervous system, chemical changes in the blood, other hormones; mostly by negative and a few positive feedback

Hypothalamus

links nervous and endocrine systems; produce 5 releasing and 2 inhibiting hormones via the hypophyseal portal system; control the release of hormones by the pituitary gland

Neurosecretory cells (hypothalamus)

secrete hypothalamic hormones in response to stimulation; axons form the hypothalamohypophyseal tract

Pituitary gland

connected to the hypothalamus by the infundibulum; composed of the anterior and posterior lobes

Anterior lobe (adenohypophysis)

75% of the pituitary’s weight; secretes 7 hormones (hGH, TSH, FSH, LH, PRL, ACTH, MSH) that circulate and target other endocrine glands (tropic hormones) from 5 types of cells (somatotrophs [-], thyrotrophs [-], gonadotrophs, lactotrophs, corticotrophs [-])

Posterior lobe (neurohypophysis)

made of hypothalamic neural axon tissue; stores/secretes 2 hormones (ADH, OT) from axon terminals made by neurosecretory cells

Releasing hormones (hypothalamus)

1. Growth hormone-releasing hormone 2. Thyrotropin releasing hormone 3. Corticotropin-releasing hormone 4. Prolactin-releasing hormone 5. Gonadotropin-releasing hormone

Inhibiting hormones (hypothalamus)

1. Growth hormone-inhibiting hormone (somatostatin) 2. Prolactin-inhibiting hormone (dopamine)

Human growth hormone

most plentiful; controlled by GHRH and GHIH; exerts its growth-promoting effect indirectly through IGFs; increases growth of bone and soft tissues (by increasing amino acid uptake and protein synthesis)

Insulin-like growth factors

small protein hormones

Antidiuretic hormone (ADH)

secretion varies with blood osmotic pressure; increase in blood volume = decrease in ADH secretion; decrease in blood volume = increase in ADH secretion

Osmoreceptors

in the hypothalamus; monitor blood osmotic pressure

Pineal gland

regulates biological clock/sleep-wake cycle; produces serotonin by day, converts it to melatonin at night (promotes sleeps); darkness stimulates serotonin secretion, lightness inhibits secretion; back roof of 3rd ventricle

Pancreas

acini cells are exocrine, islet cells are endocrine: 1) alpha cells increase blood glucose (secrete glucagon); 2) beta cells decrease blood glucose (secrete insulin) 3) delta (D) cells secrete somatostatin 4) F cells secrete pancreatic polypeptides; scattered islets of Langerhans contain secretin cells

Somatostatin

acts as a paracrine and inhibits insulin and glucagon release; also inhibits secretion of hGH

Pancreatic polypeptide

inhibits somatostatin secretion, gall bladder contraction and release of digestive enzymes

Parathyroid glands

parathyroid hormone increases blood Ca2+; decreases urinary excretion; promotes synthesis of calcitriol (active vit D) which increases absorption of Ca²⁺ from foods and bone

Chief/principal cells

parathyroid gland cell; produce PTH/parathormone

Oxiphil cells

parathyroid gland cell; function not known in normal parathyroid glands but secrete excess PTH in cases of parathyroid cancer

Thyroid glands

calcitonin decreases blood Ca++; TSH stimulates thyroid hormones (T4 and T3); increase BMR; help maintain temp; stimulate protein synthesis; increase use of glucose and fatty acids for ATP production; upregulate β receptors that attach to epinephrine and norepinephrine; accelerate growth with GH and insulin

Adrenal glands

suprarenal cortex (80-90%) secretes steroid hormones: weak sex-specific androgens, glucocorticoids (cortisol), mineralocorticoids (aldosterone); suprarenal medulla stimulated by sympathetic preganglionic neurons of ANS secretes 3 catecholamines (epinephrine, norepinephrine, dopamine); covered by capsule; vascularized

Zona glomerulosa

secretes mineralocorticoids used to regulate mineral homeostasis (aldosterone)

Zona fasciculata

secretes glucocorticoids that affect glucose homeostasis (primarily cortisol [hydrocortisone]- the most produced, cortisone and corticosterone)

Zona reticularis

secretes weak androgens (hormones with masculinizing effects)

Renin-angiotensin-aldosterone (RAA) pathway

controls secretion of aldosterone; stimulated by dehydration, Na+ deficiency or hemorrhage (all decrease blood volume) → BP decreases stimulating secretion of renin (from juxtaglomerular cells) → renin converts angiotensin I to angiotensin II → adrenal cortex secretes aldosterone

Chromaffin cells

adrenal medulla; 80% secrete epinephrine and 20% norepinephrine (still controlled by hypothalamus); fight-or-flight response; increase heart rate, cardiac output, BP, blood flow to liver, respiration, dilation of airways, blood glucose, breakdown of glycogen, fat to fatty acids, proteins (nutrient delivery to muscles)

Gonads

ovaries and testes; produce oocytes and sperm (gametes); in conjunction with anterior pituitary secretion of FSH and LH (stimulated by GnRH); ovaries produce estrogens (estradiol and estrone), progesterone, relaxin, and inhibin; testes produce testosterone

Thymus

secretes thymic factor, thymic humoral factor, thymopoietin, and thymosin which regulate development, maturation, and later activation of T-lymphocytes (WBC that destroys microbes and foreign substances)

Growth factors

hormones that stimulate cell growth and division (from G0 to G1); newly discovered; involved in tissue development, growth, and repair

Eustress

helpful, everyday stress that prepares us to meet challenges

Distress

any type of harmful stress that may be damaging; body works to maintain homeostasis to counteract

Two-stage stress response

controlled by hypothalamus; 1. fight-or-flight response stimulates resources to prepare for immediate activity 2. resistance reaction lasts longer and can result in exhaustion if it lasts too long

Stress Response 1: Stressor

something threatening or exhilarating happens

Stress Response 2: Alarm

body initially responds to stressor by lowering resistance

Stress Response 3: resistance

stressor continues and the body mobilizes to withstand the stress and return to homeostasis

Stress Response 4a: Exhaustion

ongoing, extreme stressor eventually deplete the body’s resources so we function less than normal before finally returning to normal

Stress Response 4b: Illness/Death

body’s resources are not replenished and/or additional stressors occur so the the body suffers breakdowns

Pituitary gigantism/acromegaly

pituitary gland disorder caused by excess secretion of GH during adulthood

Dwarfism

pituitary gland disorder caused by low GH production

Goiter

pituitary gland disorder caused by reduced production of thyroid hormone (TSH); causes enlarged thyroid gland

Diabetes insipidus

pituitary gland disorder caused by defects in ADH which leads to excess urine production and dehydration

Graves disease

pituitary gland disorder that develops due to excess thyroid hormone

Cushing’s syndrome

hypersecretion of cortisol from adrenal cortex; characterized by loss of muscle mass, body weight, immune system gets tired

Addison’s disease

hyposecretion of glucocorticoids and aldosterone from adrenal gland; characterized by weakness, weak immune system, low blood volume and pressure

Diabetes mellitus

pancreatic islet disorders; most common endocrine disorder, caused by the inability to produce insulin; type I: autoimmune disease where beta cells are destroyed; type II (insulin resistance): more common, linked to lifestyle (obesity, inactivity), tissue don’t respond to normal-high blood sugar