Endocrine System

Introduction:

The endocrine system uses hormones; the nervous system uses neurotransmitters

Overview of the Endocrine System:

The body has 4 principal mechanisms of communication between cells

• Gap junctions: pores in cell membrane allow signaling molecules, nutrients, and electrolytes to move from cell to cell

• Neurotransmitters: released from neurons to travel across synaptic cleft to second cell

• Paracrines: secreted into tissue fluids to affect nearby cells

• Hormones: chemical messengers that travel in the bloodstream to other tissues and organs

Comparison of Endocrine and Exocrine Glands

Exocrine glands

• Have ducts; carry secretion to an epithelial surface or the mucosa of the digestive tract: "external secretions"

• Extracellular effects (food digestion)

Endocrine glands

• No ducts

• Contain dense, fenestrated capillary networks which allow easy uptake of hormones into bloodstream

• "Internal secretions"

• Intracellular effects such as altering target cell metabolism

Liver cells defy rigid classification —releases hormones, releases bile into ducts, releases albumin and blood-clotting factors into blood (not hormones)

Hypothalamic Hormones 1

Eight hormones produced in hypothalamus

• Six regulate the anterior pituitary

• Two are released into capillaries in the posterior pituitary

Six releasing and inhibiting hormones stimulate or inhibit the anterior pituitary

• TRH, CRH, GRH, and GHRH are releasing hormones that promote anterior pituitary secretion of TSH, PRL, ACTH, FSH, LH, and GH

• PI inhibits secretion of prolactin, and somatostatin inhibits secretion of growth hormone and thyroid-stimulating hormone by the anterior pituitary

Posterior Pituitary Hormones 2

Two hormones are produced in hypothalamus and transported to the posterior lobe of pituitary

• Released when hypothalamic neurons are stimulated

ADH (antidiuretic hormone)

• Increases water retention, thus reducing urine volume and preventing dehydration

• Also called vasopressin because it can cause vasoconstriction

Posterior Pituitary Hormones :

Oxytocin (OT)

• Surge of hormone released during sexual arousal and orgasm

• Promotes feelings of sexual satisfaction and emotional bonding between partners

• Stimulates labor contractions during childbirth

• Stimulates flow of milk during lactation

• May promote emotional bonding between lactating mother and infant

The Thymus

Thymus plays a role in three systems: endocrine, lymphatic, immune

• lymph is fluid that leaks out of your blood

Bilobed gland in the mediastinum superior to the heart

• Goes through involution after puberty

Site of maturation of T cells important in immune defense

Secretes hormones (thymopoietin, thymosin, and thymulin) that stimulate development of other lymphatic organs and activity of T lymphocytes

The Thyroid Gland

Largest gland that is purely endocrine

• Composed of two lobes and an isthmus below the larynx

• Dark reddish brown color due to rich blood supply

Thyroid follicles: sacs that make up most of thyroid

• Contain protein-rich colloid

• Follicular cells: simple cuboidal epithelium that lines follicles

Secretes thyroxine (T, because of four iodine atoms) and triiodothyronine (T3) in response to TSH

• Increases metabolic rate, 2 consumption, heat production (calorigenic effect), appetite, growth hormone secretion, alertness, reflex speed
  Parafollicular (C or clear cells secrete calcitonin with rising blood calcium

• Stimulates osteoblast activity and bone formation in children

The Parathyroid Glands:

Usually four glands partially embedded in posterior surface of thyroid gland

• Can be found from as high as hyoid bone to as low as aortic arch

Secrete parathyroid hormone (PTH)

• Increases blood Ca?* levels

• Promotes synthesis of calcitriol

• Increases absorption of Ca2+

• Decreases urinary excretion

• Increases bone resorption

The Adrenal Medulla

Adrenal medulla: inner core, 10% to 20% of gland

Has dual nature acting as an endocrine gland and a ganglion of the sympathetic nervous system

• Innervated by sympathetic preganglionic fibers

• Consists of modified sympathetic postganglionic neurons called chromaffin cells

• When stimulated, release catecholamines (epinephrine and norepinephrine) and a trace of dopamine directly into the bloodstream

As hormones, catecholamines have multiple effects

• Increase alertness and prepare body for physical activity

• Mobilize high-energy fuels, lactate, fatty acids, and glucose

• Glycogenolysis and gluconeogenesis by liver boost glucose levels

• Epinephrine inhibits insulin secretion and so has a glucose-sparing effect

  • Muscles use fatty acids, saving glucose for brain

• Increase blood pressure, heart rate, blood flow to muscles, pulmonary airflow, and metabolic rate

• Decrease digestion and urine production

The Adrenal Cortex

Cortex surrounds medulla and secretes several corticosteroids (hormones) from three layers of glandular tissue

• Zona glomerulosa (thin, outer layer)

• Cells are arranged in rounded clusters

• Secretes mineralocorticoids-regulate the body's electrolyte balance

• Zona fasciculata (thick, middle layer)

• Cells arranged in fascicles separated by capillaries

• Secretes glucocorticoids and androgens

• Zona reticularis (narrow, inner layer)

• Cells in branching network

• Secretes glucocorticoids and sex steroids

Mineralocorticoids: from zona glomerulosa

• Steroid hormones that regulate electrolyte balance

• Aldosterone stimulates Na* retention and K* excretion

• Water is retained with sodium by osmosis, so blood volume and blood pressure are maintained

• Part of the renin-angiotensin-aldosterone (RAA) system

Glucocorticoids

• Secreted by zona fasciculata and zona reticulata in response to ACTH

• Regulate metabolism of glucose and other fuels

• Cortisol and corticosterone stimulate fat and protein catabolism, gluconeogenesis (glucose from amino acids and fatty acids), and release of fatty acids and glucose into blood

• Help body adapt to stress and repair tissues

• Anti-inflammatory effect becomes immune suppression with long-term use

Sex steroids

• secreted by zona fasciculata and zona reticularis

• androgens: set libido through life; large role in prenatal male development

The Pancreatic Islets:

Glucagon: secreted by alpha (a) cells or glucagon cells

• Released between meals when blood glucose concentration is falling

• In liver, stimulates gluconeogenesis, glycogenolysis, and the release of glucose into the circulation raising blood glucose leve

• In adipose tissue, stimulates fat catabolism and release of free fatty acids

• Glucagon also released in response to rising amino acid levels in blood, promotes amino acid absorption, and provides cells with raw material for gluconeogenesis

Insulin secreted by beta (B) cells or insulin cells

• Secreted during and after meal when glucose and amino acid blood levels are rising

• Stimulates cells to absorb these nutrients and store or metabolize them, lowering blood glucose levels

• Promotes synthesis of glycogen, fat, and protein

• Suppresses use of already-stored fuels

• Brain, liver, kidneys, and RBCs absorb glucose without insulin, but other tissues require insulin

• Insufficiency or inaction is cause of diabetes mellius

• Also produce amylin to reduce spikes in blood sugar

Somatostatin secreted by delta (o) cells or somatostatin cells

• Partially suppresses secretion of glucagon and insulin

• Inhibits nutrient digestion and absorption which prolongs absorption of nutrients

Pancreas also has PP cells that secrete pancreatic polypeptide

(PP) that inhibits vagal stimulation of pancreas after a meal Hyperglycemic hormones raise blood glucose concentration (includes hormones from other glands)

• Glucagon, growth hormone, epinephrine, norepinephrine, cortisol, and corticosterone

Hypoglycemic hormones lower blood glucose

Endocrine Functions of Other Tissues and Organs:

Skin

• Keratinocytes convert a cholesterol-like steroid into cholecalciferol using UV from sun

Liver: involved in the production of at least five hormones

• Converts cholecalciferol into calcidiol

• Secretes angiotensinogen (a prohormone)

• Precursor of angiotensin Il (a regulator of blood pressure)

• Secretes 15% of erythropoietin (stimulates bone marrow)

• Source of IGF-I that controls action of growth hormone

• Hepcidin: promotes intestinal absorption of iron

Kidneys: play role in production of three hormones

• Convert calcidiol to calcitriol, the active form of vitamin D

• Increases Ca?* absorption by intestine and inhibits loss in the urine

• Secrete renin that converts angiotensinogen to angiotensin I

• Angiotensin Il created by converting enzyme in lungs

• Constricts blood vessels and raises blood pressure

• Produces 85% of erythropoietin

• Stimulates bone marrow to produce RBCs

Heart

• Atrial muscle secretes two natriuretic peptides in response to an increase in blood pressure

• These decrease blood volume and blood pressure by increasing Na* and H20 output by kidneys and oppose action of angiotensin Il

• Lowers blood pressure

Stomach and small intestine secrete at least 10 enteric hormones secreted by enteroendocrine cells

• Coordinate digestive motility and glandular secretion

• Cholecystokinin, gastrin, ghrelin, and peptide YY (PYY)

Adipose tissue secretes at least hormones including leptin

• Slows appetite

Osseous tissue: osteocalcin secreted by osteoblasts

• Increases number of pancreatic beta cells, pancreatic output of insulin, and insulin sensitivity of body tissues

• Inhibits weight gain and onset of type 2 diabetes mellitus

Placenta

• Secretes estrogen, progesterone, and others

• Regulate pregnancy, stimulate development of fetus and mammary glands

• Saturated when all receptor molecules are occupied by hormone molecules

Hormone Receptors and Mode of Action

Hormones stimulate only those cells that have receptors for them

Receptors are protein or glycoprotein molecules

• On plasma membrane, in the cytoplasm, or in the nucleus

Receptors act like switches turning on metabolic pathways when hormone binds to them

Usually each target cell has a few thousand receptors for a given hormone

Receptor-hormone interactions exhibit specificity and saturation

• Specific receptor for each hormone

• Saturated when all receptor molecules are occupied by hormone molecules

Peptides and catecholamines

• Cannot penetrate target cell

• Bind to surface receptors and activate intracellular processes through second messengers

Steroids and thyroid hormone

• Penetrate plasma membrane and bind to internal receptors (usually in nucleus)

• Influence expression of genes of target cell

• Take several hours to days to show effect due to lag for protein synthesis