Chapter 10 The Endocrine System

Chapter 10
The Endocrine System

MECHANISMS OF HORMONE ACTION

˜Endocrine glands secrete chemicals (hormones) into the blood (Figure 10-1)

˜Hormones perform general functions of communication and control but a slower,
longer-lasting type of control than that provided
by nerve impulses

˜Cells acted on by hormones are called target cells; organs containing target cells are target organs

 

REGULATION OF HORMONE SECRETION

 

˜Hormone secretion is controlled by homeostatic feedback

˜Negative feedback—mechanisms that reverse the direction of a change in a physiological system (Figure 10-4)

˜Positive feedback—(uncommon) mechanisms that amplify physiological changes



 

 

PROSTAGLANDINS

 

˜Prostaglandins (PGs) are powerful substances found in a wide variety of body tissues

˜PGs are often produced in a tissue and diffuse only a short distance to act on cells in that tissue

˜Several classes of PGs include prostaglandin A (PGA), prostaglandin E (PGE), and prostaglandin F (PGF)

˜PGs influence many body functions, including respiration, blood pressure, gastrointestinal secretions, and reproduction



 

PITUITARY GLAND
(Figure 10-5)

˜Anterior pituitary gland (adenohypophysis)

ØNames of major hormones

•Thyroid-stimulating hormone (TSH)

•Adrenocorticotropic hormone (ACTH)

•Follicle-stimulating hormone (FSH)

•Luteinizing hormone (LH)

•Growth hormone (GH)

•Prolactin (PRL) (lactogenic hormone)

 

˜Anterior pituitary gland (cont.)

ØFunctions of major hormones

•TSH—stimulates growth of the thyroid gland; also stimulates it to secrete thyroid hormone

•ACTH—stimulates growth of the adrenal cortex and stimulates it to secrete glucocorticoids (mainly cortisol)

•FSH—initiates growth of ovarian follicles each month in the ovary and stimulates one or more follicles to develop to the stage of maturity and ovulation; FSH also stimulates estrogen secretion by developing follicles; stimulates sperm production in the male

•LH—acts with FSH to stimulate estrogen secretion and follicle growth to maturity; causes ovulation; causes luteinization of the ruptured follicle and stimulates progesterone secretion by corpus luteum; causes interstitial cells in the testes to secrete testosterone in the male

•GH—stimulates growth by accelerating protein anabolism; also accelerates fat catabolism and slows glucose catabolism; by slowing glucose catabolism, tends to increase blood glucose to higher than normal level (hyperglycemia)

•PRL, or lactogenic hormone—stimulates breast development during pregnancy and secretion of milk after the delivery of the baby

˜Posterior pituitary gland (neurohypophysis)

ØNames of hormones

•Antidiuretic hormone (ADH)

•Oxytocin

ØFunctions of hormones

•ADH—accelerates water reabsorption from urine in the kidney tubules into the blood, thereby decreasing urine secretion

•Oxytocin (OT)—stimulates the pregnant uterus to contract; may initiate labor; causes glandular cells of the breast to release milk into ducts; enhances social bonding

HYPOTHALAMUS

˜Actual production of ADH and oxytocin occurs in the hypothalamus

˜After production in the hypothalamus, hormones pass along axons into the pituitary gland

˜The secretion and release of posterior pituitary hormones is controlled by nervous stimulation

˜The hypothalamus controls many body functions related to homeostasis (temperature, appetite, and thirst)

 

 

THYROID GLAND
(Figure 10-6)

˜Names of hormones

ØThyroid hormones—thyroxine (T4) and triiodothyronine (T3)

ØCalcitonin (CT)

˜Functions of hormones

ØThyroid hormones—accelerate catabolism (increase the body’s metabolic rate)

ØCT—decreases the blood calcium concentration by inhibiting breakdown of bone, which would release calcium into the blood

    

PARATHYROID GLANDS
(Figure 10-8)

˜Name of hormone—parathyroid hormone (PTH)

˜Function of hormone—increases blood calcium concentration by increasing the breakdown of bone with the release of calcium into the blood

ADRENAL GLANDS
(Figure 10-9)

 

˜Adrenal cortex

ØNames of hormones (corticoids)

•Glucocorticoids (GCs)—chiefly cortisol (hydrocortisone)

•Mineralocorticoids (MCs)—chiefly aldosterone

•Sex hormones—small amounts of male hormones (androgens) secreted by adrenal cortex of both sexes

ØThree cell layers (zones)

•Outer layer, secretes mineralocorticoids

•Middle layer, secretes glucocorticoids

•Inner layer, secretes sex hormones

ØMineralocorticoids—increase blood sodium and decrease body potassium concentrations by accelerating kidney tubule reabsorption
of sodium and excretion of potassium

 

˜Adrenal cortex (cont.)

ØFunctions of glucocorticoids

•Help maintain normal blood glucose concentration by increasing gluconeogenesis—the formation of “new” glucose from amino acids produced by the breakdown of proteins, mainly those in muscle tissue cells; also the conversion to glucose of fatty acids produced by the breakdown of fats stored in adipose tissue cells

•Play an essential part in maintaining normal blood pressure—make
it possible for epinephrine and norepinephrine to maintain a normal degree of vasoconstriction, a condition necessary for maintaining normal blood pressure

•Act with epinephrine and norepinephrine to produce an antiinflammatory effect, to bring about normal recovery from inflammations of various kinds

•Produce anti-immunity, antiallergy effect; bring about a decrease in the number of lymphocytes and plasma cells and therefore a decrease
in the amount of antibodies formed

•Secretion of glucocorticoid quickly increases when the body is thrown into a condition of stress; high blood concentration of glucocorticoids,
in turn, brings about many other stress responses (Figure 10-10)

 

˜Adrenal medulla

ØNames of hormones—epinephrine (Epi),
or adrenaline, and norepinephrine (NR)

ØFunctions of hormones—help the body resist stress by intensifying and prolonging the effects of sympathetic stimulation; increased epinephrine secretion is the first endocrine response to stress

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PANCREATIC ISLETS

(Figure 10-11)

 

 

˜Names of hormones

ØGlucagon—secreted by alpha cells

ØInsulin—secreted by beta cells

˜Functions of hormones

ØGlucagon increases the blood glucose level by accelerating liver glycogenolysis (conversion
of glycogen to glucose)

ØInsulin decreases the blood glucose by accelerating the movement of glucose out
of the blood into cells, which increases glucose metabolism by cells

 

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FEMALE SEX GLANDS

 

˜The ovaries contain two structures that secrete hormones—the ovarian follicles and the corpus luteum; see Chapter 20

˜Effects of estrogen (feminizing hormone)

ØDevelopment and maturation of breasts
and external genitals

ØDevelopment of adult female body contours

ØInitiation of menstrual cycle

 

 

MALE SEX GLANDS

 

˜The interstitial cells of testes secrete the male hormone testosterone; see Chapter 20

˜Effects of testosterone (masculinizing hormone)

ØMaturation of external genitals

ØBeard growth

ØVoice changes at puberty

ØDevelopment of musculature and body contours typical of the male



THYMUS

˜Name of hormone—thymosin

˜Function of hormone—plays an important role in the development and function of the body’s immune system

PLACENTA

˜Name of hormones—chorionic gonadotropins, estrogens, and progesterone

˜Functions of hormones—maintain the corpus luteum during pregnancy

PINEAL GLAND

˜A small gland near the roof of the third ventricle of the brain

ØGlandular tissue predominates in children and young adults

ØBecomes fibrous and calcified with age

˜Called third eye because its influence on secretory activity is related to the amount of light entering the eyes

˜Secretes melatonin, which:

ØInhibits ovarian activity

ØRegulates the body’s internal clock

 

 

OTHER ENDOCRINE STRUCTURES

˜Many organs (e.g., the stomach, intestines, and kidney) produce endocrine hormones

ØStomach lining produces ghrelin, which affects appetite and metabolism

˜The atrial wall of the heart secretes atrial natriuretic hormone (ANH), which stimulates sodium loss from the kidneys

˜Fat-storing cells secrete leptin, which controls how full or hungry one feels

 

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