Unit 8: Endocrine System

Endocrine system

second controlling system of body (fast control) that uses chemical messengers (hormones) that are released into the blood

Hormones

regulate activity of other cells and control several major processes including reproduction, growth and development, mobilization of body defenses, maintenance of much of homeostasis, and regulation of metabolism; produced by specialized cells which secrete them into extracellular fluids; blood transfers them to target sites

Endocrinology

scientific study of hormones and endocrine organs

Two classifications of hormones

Amino acid-based (proteins, peptides, amines) and lipid-based

Steroids

made from cholesterol

Prostaglandins

made from highly active lipids that act as local hormones

Hormone action

1. change plasma membrane permeability or membrane potential by opening or closing ion channels

2. activate or inactivate enzymes

3. stimulate or inhibit cell division

4. promote or inhibit secretion of a product

5. turn on or off transcription of certain genes

Direct gene activation

lipid-based and used by steroid hormones and thyroid hormone

Second-messenger system

amino acid-based and used by protein and peptide hormones; cAMP (most common secondary messenger)

Steps of direct gene activation

1. steroid hormone diffuses through plasma membrane of target cells

2. once inside cell, hormone enters nucleus

3. hormone binds to specific protein within nucleus

4. hormone receptor complex binds to specific sites on cell’s DNA

5. certain genes activated to transcribe messenger RNA

6. new proteins synthesized

Steps of second-messenger system

1. hormone (1st messenger) binds to receptor protein on plasma membrane

2. activated receptor sets off series of reactions that activates an enzyme

3. enzyme catalyzes a reaction that produces a second-messenger molecule (cAMP)

4. second messenger molecule oversees additional intracellular changes to promote specific response in target cell

Negative feedback mechanism

controls hormone levels in blood; stimulus or low hormone levels in blood trigger release of more hormone; stops once appropriate level in blood reached

Hormonal stimuli

most common category of stimuli; endocrine organs activated by other hormones; ex. hormones of hypothalamus stimulate anterior pituitary to secrete its hormones

Humeral stimuli

changing blood levels of certain ions and nutrients stimulate hormone release (various body fluids including blood and bile); ex. parathyroid hormone and calcitonin produced in response to changing levels of blood calcium levels

Neural stimuli

nerve fibers stimulate hormone release; most under control of sympathetic nervous system; ex. sympathetic stimulation of adrenal medulla to release epinephrine and norepinephrine

Major endocrine glands

hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, gonads

Mixed glands

hormone producing glands with both endocrine and exocrine functions (pancreas and gonads)

Purely endocrine glands

anterior pituitary, thyroid, adrenals, parathyroids

Ductless endocrine glands

release hormones directly into the blood or lymph

Pituitary gland

“master”, pea-sized endocrine gland that hangs by stalk from hypothalamus in brain that is protected by sella turcica of sphenoid bone; includes two functional lobes: anterior (glandular tissue) and posterior (nervous tissue); controls activities of other endocrine glands

Hypothalamus

produces releasing hormones and inhibiting hormones; released into portal circulation, which connects it to anterior pituitary; produces oxytocin and antidiuretic hormone which are carried to posterior pituitary via neurosecretory cells for storage; released in response to nerve impulses from this gland

Posterior pituitary

stores hormones made by hypothalamus

Oxytocin

only released in significant amounts during childbirth and nursing and stimulates contractions of uterus during labor, sexual relations, and breastfeeding; causes milk ejection in nursing women; only positive feedback hormone

Antidiuretic hormone

inhibits urine production (diuresis) by promoting water reabsorption by kidneys; urine volume decreases, blood volume and pressure increase; in large amounts, ADH causes constriction of arterioles leading to increased blood pressure (vasopressin); alcohol inhibits ADH secretion

Anterior pituitary hormones that affect nonendocrine targets

growth hormone (hGh) and prolactin (PRL)

Anterior pituitary tropic hormones

stimulate other endocrine glands to release hormones; follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyotropic hormone (TSH), adrenocorticotropic hormone (ACTH)

All six anterior pituitary hormones are…

proteins or peptides (amino acid-based), act through second-messenger systems, regulated by hormonal stimuli, and regulated mostly by negative feedback loops

Growth hormone (GH)

general metabolic hormone; major effects directed to growth of skeletal muscles and long bones, plays role in determining final body size, causes fats to be broken down for a source of energy

Prolactin (PRL)

stimulates and maintains milk production following childbirth; function in males unknown

Gonadtropic hormones

regulate hormonal activity of gonads

Follicle stimulating hormone (FSH)

gonadtropic hormone that stimulates follicle development in ovaries and sperm development in testes

Luteinizing hormone (LH)

gonadtropic hormone that triggers ovulation of an egg in females and testosterone production in males

Pineal gland

hangs from roof of third ventricle and secretes melatonin; believed to trigger body’s sleep/wake cycle and coordinate the hormones of fertility in humans and to inhibit the reproductive system until maturity occurs

Thyroid gland

found at base of throat, inferior to Adam’s apple and consists of 2 lobes and a connecting isthmus; follicles are hollow structures that store colloidal material (thyroid hormone derived from this colloid)

Thyroid hormone

major metabolic hormone that controls rate of oxidation of glucose to supply body heat and chemical energy (ATP); needed for tissue growth and development; composed of 2 active iodine-containing hormones (thyroxine and triiodothyronine)

Thyrotropic hormone (TH) or thyroid-stimulating hormone (TSH)

anterior pituitary hormone that influences growth and activity of thyroid gland

Adrenocorticotropic hormone (ACTH)

anterior pituitary hormone that regulates endocrine activity of adrenal cortex

Thyroxine

(T4) secreted by thyroid follicles

Triiodothyronine

(T3) formed from conversion of T4 at target tissues

Calcitonin

decreases blood calcium levels by causing calcium deposition on blood; released in response to rising blood calcium levels; antagonistic to parathyroid hormone; produced by parafollicular cells found between the follicles

Parathyroid glands

tiny masses on posterior of thyroid gland that secretes parathyroid hormone and hypercalcemic hormone

Parathyroid hormone (PTH)

most important regulator of calcium ion homeostasis of blood

Hypercalcemic hormone

causes increased blood calcium levels, stimulates osteoclasts to remove calcium from bone, and stimulates kidneys and intestine to absorb more calcium

Thymus

located in upper thorax, posterior to sternum; largest in infants and children and decreases in size throughout adulthood; produces thymosin

Thymosin

produced by thymus; matures some types of white blood cells and is important in developing immune system

Adrenal glands

sit on top of kidneys and made up of two regions: the cortex and medulla

Adrenal cortex

outer glandular region that has three layers that produce corticosteroids; mineralocorticoids, glucocorticoids, and sex hormones

Mineralocorticoids

(mainly aldosterone) produced in outer adrenal cortex, regulates mineral (salt) content in blood, particularly sodium and potassium ions; regulate water and electrolyte balance, and target organ is the kidney

Glucocorticoids

(including cortisone and cortisol) produced by middle layer of adrenal cortex; promote normal cell metabolism, help resist long-term stressors by increasing blood glucose levels (hyperglycemic hormones), anti-inflammatory properties (decrease edema and pain), released in response to increased blood levels of ACTH

Sex hormones

produced in inner layer of adrenal cortex; small amounts made throughout life, most hormones produced are androgens, but some estrogens formed

Adrenal medulla

produces two similar hormones (catecholamines): epinephrine (adrenaline) ad norepinephrine (noradrenaline) which prepare body to deal with short-term stress (fight-or-flight) by increasing heart rate, blood pressure, blood glucose levels, and dilating small passageways of lungs; these events result in more oxygen and glucose in blood and faster circulation to organs

Pancreas

located in abdomen, close to stomach; mixed gland with both endocrine and exocrine functions

Pancreatic islets (islets of Langerhans)

produce hormones such as insulin and glucagon

Insulin

produced by beta cells and released when blood glucose levels are high and increases rate of glucose uptake and metabolism by body cells; effects are hypoglycemic

Glucagon

produced by alpha cells and released when blood glucose levels are low, stimulates liver to release glucose to blood to increase blood glucose levels; effects are hypoglycemic

Gonads

produce sex cells (exocrine function) and sex hormones

Ovaries

female gonads located in pelvic cavity that produce ova or eggs and have two groups of steroid hormones (estrogens and progesterone)

Testes

male gonads suspended outside the pelvic cavity that produce sperm and androgens such as testosterone

Estrogens

stimulate the development of secondary female characteristics and mature the female reproductive organs; with progesterone, promotes breast development and regulate menstrual cycle

Progesterone

acts with estrogen to bring about the menstrual cycle, helps in the implantation of an embryo in uterus, and helps prepare breasts for lactation

Testosterone

responsible for adult male secondary sex characteristics, promotes growth and maturation of male reproductive system, and required for sperm cell production

Other hormone producing tissues and organs

nonendocrine in function but also secrete hormones; stomach, small intestine, kidneys, heart

Placenta

produces hormones that maintain pregnancy, some hormones play part in delivery of baby, produces human chorionic gonadotropin (hCG) in addition to estrogen, progesterone, and other hormones; human placental lactogen (hPL) prepares breasts for lactation, and relaxin relaxes pelvic ligaments and pubic symphysis for childbirth

Prostaglandins

located in plasma membrane of almost all body cells; increased blood pressure, cause constriction of respiratory passageways, stimulate muscles of uterus, promoting menstrual pain and labor, enhance blood clotting, promote inflammation and pain

Gastrin

located in stomach and stimulates glands to release HCl

Intestinal gastrin

target organ is stomach and stimulates gastric glands and motility

Secretin

stimulates release of bicarbonate-rich juice in pancreas, increases release of bile in liver, and reduces secretions and motility in stomach

Cholecystokinin

stimulates release of enzyme rich juice in pancreas, stimulates expulsion of stored bile in gallbladder, and causes sphincter to relax, allowing pancreatic juice and bile to enter duodenum

Erythropoietin

stimulates production of red blood cells in bone marrow

Active vitamin D3

stimulates active transport of dietary calcium ions across intestinal cell membranes

Atrial natriuretic peptide (ANP)

inhibits sodium ion reabsorption and renin release in kidney and inhibits secretion of aldosterone which decreases blood volume and pressure

Leptin

suppresses appetite and increases energy expenditure in brain

Resistin

antagonizes insulin’s action on liver cells in fat, muscle, and liver

Developmental aspects of endocrine system

in absence of disease, efficiency of endocrine system remains high until old age, decreasing function of female ovaries at menopause leads to symptoms such as osteoporosis, increased chance of heart disease, and mood changes, efficiency of all endocrine glands gradually decrease with aging leading to increase incidence of diabetes mellitus, immune system depression, lower metabolic rate, and cancer rates in some areas