Anatomy & Physiology Chapter 16

Endocrine System

Endocrine vs. Nervous System

See Table 16-1

Classification of hormones by chemical structure

Steroid hormones Derived from

cholesterol Lipid-soluble Pass through plasma

membranes easily

Nonsteroid hormones Synthesized primarily from amino acids Protein hormones: long chains of amino acids ex:

insulin, PTH Glycoproteins: protein hormones with a

carbohydrate group attached ex: FSH, LH Peptide hormones: short chain of amino acids ex:

oxytocin, ADH Amino acid derivatives: derived from a single

amino acid Amine hormones: derived from tyrosine ex:epinephrine Iodine added to tyrosine ex: thyroid hormones

General Principles of Hormone Action Bind to a specific receptor on cell by “lock-

and-key” mechanism Some hormones are attached to plasma

proteins in bloodstream Since blood carries hormones most

everywhere lots more produced than makes it to its target

Mechanism of steroid hormone action (Mobile-receptor hypothesis)

Attach to soluble plasma proteins in blood

Receptors usually found within the cell

Regulate cells by regulating production of certain critical proteins

Amount of steroid hormone present determines magnitude of target cell’s response

Response to steroid hormones often slow

Mechanisms of Nonsteroid Hormones Second messenger mechanism: also called

fixed-membrane-receptor hypothesis

Nuclear receptor mechanism

Second messenger mechanism Binds to receptors on

target cell’s plasma membrane

Then second messenger within cell triggers appropriate cellular changes

Most use cAMP as second messenger

Operates more quickly than steroid mechanism

Nuclear receptor mechanism Small iodinated amino acids (T3 & T4) Enter target cell & bind to receptors

associated with DNA which triggers transcription of mRNA

Regulation of hormone secretion Usually part of a negative feedback loop

and is called endocrine reflexes Endocrine cells often sensitive to changes

produced by its target cells May be regulated by hormone produced

by another gland, esp pituitary gland May be influenced by nervous system

input

Prostaglandins Lipid molecules Tissue hormones: secretion produced in

tissues & diffuse short distance only to other cells in same tissue

Tissues known to secrete prostaglandins: kidneys, lungs, iris, brain, thymus

Vocabulary Words Hormone Synergism Permissiveness Antagonism Tropic hormones Sex hormones Anabolic hormones

Pituitary Gland

Pituitary gland (Hypophysis) Lies within sella

turcica of skull Connected to the

hypothalamus by a stalk, infundibulum

Consists of 2 different glands Adenohypohysis or

anterior pituitary Neurohypophysis or

posterior pituitary

Anterior Pituitary Two parts

pars anterior: major part pars intermedia

5 types of secretions Somatrophs-secrete GH Corticotrophs-secrete ACTH Thyrotrophs: secrete TSH Lactotrophs: secrete prolactin Gondatrophs: secrete LH and FSH

Growth Hormone (GH) Promotes protein anabolism thus

promotes growth of bones, muscles Promotes lipid mobilization & catabolism Indirectly inhibits glucose metabolism Indirectly increases blood glucose levels

Prolactin (PRL) During pregnancy promotes breast

development After birth stimulates mammary gland to

begin milk secretion

Tropic hormones Have stimulating effect on other endocrine

glands

Thyroid stimulating hormone: TSH Adrenocorticotropic hormone: ACTH Follicle stimulating hormone: FSH Luteinizing hormone: LH

TSH Maintains growth & development of

thyroid gland & causes it to secrete its hormones

ACTH Promotes & maintains normal growth of

cortex of adrenal gland & stimulates it to secrete some of its hormones

Gonadotropins Stimulate growth & development of

gonads FSH: stimulates follicles (with ovum) to

maturity, also stimulates follicle to secrete estrogen; in male: stimulates development of seminiferous tubules

LH: stimulates formation of corpus luteum; in males: stimulates interstitial cells in testes to develop & secrete testosterone

Control of secretion of anterior pituitary

Hypophyseal portal system: complex of small blood vessels between hypothalamus & anterior pituitary

Hypothalamus secretes releasing hormones through this system

During times of stress cerebral cortex can send impulses to hypothalamus to secrete releasing hormones thus mind-body link

Posterior pituitary Storage & release site

for: ADH (antidiuretic

hormone) Oxytocin

Hormones are not made within the pituitary but within the supraoptic or paraventricular nuclei of hypothalamus

Release of these hormones thus controlled by nervous stimulation

Antidiuretic hormone Prevents formation of a large volume of

urine Release triggered by osmoreceptors near

supraoptic nucleus

Oxytocin Stimulates contraction of uterine muslces Causes ejection of milk from breasts Regulated by a positive feedback

mechanism

Pineal gland Located on dorsal surface of brain’s

diencephalon Member of both nervous system &

endocrine system Produces melatonin, functions to support

biological clock

Thyroid gland In neck, on anterior &

lateral surface of trachea just below the larynx

Composed of structural units called follicles

Hormones: Thyroid hormones Calcitonin

Thyroid hormones T3: principal thyroid hormone T4: most abundant, precursor of T3 Thyroid gland stores thyroid hormone as

thyroglobulins Regulate metabolic rate of all cells

Calcitonin Produced by parafollicular cells of thyroid Influences processing of calcium by bone

cells by decreasing blood calcium levels Antagonist to parathyroid hormone

Parathyroid gland 4-5 of them

embedded on posterior surface of thryoid’s lateral lobes

Hormone: PTH (parathryroid hormone)

PTH Antagonist to calcitonin Acts on bones & kidneys to increase blood

calcium Bones: calcium & phosphate released from

bones Kidneys: calcium reabsorbed, phosphate

secreted, activates Vitamin D in kidney which permits Ca to be absorbed in intestine

Adrenal glands On top of kidneys Adrenal cortex: outer

part Zona glomerulosa:

mineralocorticoids Zona fasiculata:

glucocorticoids Zona reticularis:

gonadocorticoids Adrenal medulla: inner

portion

Mineralocorticoids Aldosterone: important

mineralocorticoid Primary function is

maintenance of sodium homeostasis by increasing sodium reabsorption in kidneys

Increase water retention & promotes loss of potassium

Secretion controlled by renin-angiotensin & blood potassium concentration

Glucocorticoids Cortisol most significant Affect every cell in body Protein mobilizing, lipid catabolism as

energy source, secretion increase in times of stress, essential for maintaining normal blood pressure, decrease in number of WBCs

Gonadocorticoids Secretes small amounts of male hormones Released from zona fasiculata & zona

glomerulosa Influences appearance of pubic & axillary

hair

Adrenal medulla Composed of neurosecretory tissue Produce epinephrine & norepinephrine Prolong & enhance effects of sympathetic

stimulation (fight or flight response)

Pancreas Endocrine portion: pancreatic islets (or

islets of Langerhans) Alpha cells: secrete glucagon Beta cells: secrete insulin Delta cells: secrete somatostatin Pancreatic polypeptide cells: secrete

pancreatic polypeptide

Glucagon Increase blood glucose levels by

stimulating conversion of glycogen to glucose in liver cells

Also stimulates gluconeogenesis in liver Hyperglycemic effect

Insulin Promotes movement of glucose, amino

acids, fatty acids into tissue cells Lowers blood glucose

Somatostatin Regulates other endocrine cells of

pancreatic islets by inhibiting secretion

Gonads Primary sex organs

Males: testes Females: ovaries

Testes Interstitial cells within testes produce

testosterone Responsible for growth & maintenance of

male sexual characteristics & for sperm production

Ovaries Estrogens: secreted by follicles, promote

development & maintenance of female sexual characteristics, breast development, menstrual cycle

Progesterone: secreted by corpus luteum, maintains lining of uterus necessary for pregnancy

Placenta Produces human chorionic gonadotropin

(hCG), serves as a signal to maintain uterine lining for pregnancy

Temporary endocrine gland

Thymus Atrophies at puberty Produces thymosin & thymopoietin Stimulate production of T cells

Gastric & Intestinal Mucosa Secretin: reduce acid secretion, triggers

pancreas to produce bicarbonate CCK: trigger pancreas to release digestive

enzymes, gall bladder contraction Ghrelin: stimulates hypothalamus to boost

appetite

Heart Atrial natriuretic hormone (ANH):

promotes loss of sodium in urine thus it opposes increase in blood volume or blood pressure

Antagonistic to ADH & aldosterone