Pancreas—extends from curve of duodenum
to the spleen. Islets of Langerhans contain
alpha cells and beta cells (see Figs. 10–1 and
10–8 and Table 10–5)
- Glucagon—secreted by alpha cells. Stimulates
liver to change glycogen to glucose; increases use
of fats and amino acids for energy. Result: raises
blood glucose level. Stimulus: hypoglycemia. - Insulin—secreted by beta cells. Increases use of
glucose by cells to produce energy; stimulates
liver and muscles to change glucose to glycogen;
increases cellular intake of fatty acids and amino
acids to use for synthesis of lipids and proteins.
Result: lowers blood glucose level. Stimulus: hy-
perglycemia. - Somatostatin—inhibits secretion of insulin and
glucagon.
Adrenal Glands—one on top of each kidney;
each has an inner adrenal medulla and an
outer adrenal cortex (see Fig. 10–1)
- Adrenal Medulla—produces catecholamines in
stress situations (Table 10–6 and Fig. 10–9).
- Norepinephrine—stimulates vasoconstriction
and raises blood pressure. - Epinephrine—increases heart rate and force,
causes vasoconstriction in skin and viscera and
vasodilation in skeletal muscles; dilates bronchi-
oles; slows peristalsis; causes liver to change
glycogen to glucose; increases use of fats for
energy; increases rate of cell respiration. Stimu-
lus: sympathetic impulses from the hypothala-
mus.
- Adrenal Cortex—produces mineralocorticoids,
glucocorticoids, and very small amounts of sex hor-
mones (function not known with certainty) (Table
10–7).
- Aldosterone—(Fig. 10–10) increases reabsorp-
tion of sodium and excretion of potassium by
the kidneys. Results: hydrogen ions are excreted
in exchange for sodium; chloride and bicarbon-
ate ions and water follow sodium back to the
blood; maintains normal blood pH, blood vol-
ume, and blood pressure. Stimulus: decreased
blood sodium or elevated blood potassium;
decreased blood volume or blood pressure (acti-
vates the renin-angiotensin mechanism of the
kidneys). - Cortisol—(Fig. 10–11) increases use of fats and
amino acids for energy; decreases use of glucose
to conserve glucose for the brain; anti-inflamma-
tory effect: blocks effects of histamine and stabi-
lizes lysosomes to prevent excessive tissue
damage. Stimulus: ACTH from hypothalamus
during physiological stress.Ovaries—in pelvic cavity on either side of
uterus (see Fig. 10–1)- Estrogen—produced by follicle cells. Promotes
maturation of ovum; stimulates growth of blood
vessels in endometrium; stimulates development
of secondary sex characteristics: growth of duct
system of mammary glands, growth of uterus, fat
deposition. Promotes closure of epiphyses of
long bones; lowers blood levels of cholesterol
and triglycerides. Stimulus: FSH from anterior
pituitary. - Progesterone—produced by the corpus luteum.
Promotes storage of glycogen and further
growth of blood vessels in the endometrium;
promotes growth of secretory cells of mammary
glands. Stimulus: LH from anterior pituitary. - Inhibin—inhibits secretion of FSH.
Testes—in scrotum between the upper
thighs (see Fig. 10–1)- Testosterone—produced by interstitial cells.
Promotes maturation of sperm in testes; stimu-
lates development of secondary sex characteris-
tics: growth of reproductive organs, facial and
body hair, larynx, skeletal muscles; promotes clo-
sure of epiphyses of long bones. Stimulus: LH
from anterior pituitary. - Inhibin—produced by sustentacular cells.
Inhibits secretion of FSH to maintain a constant
rate of sperm production. Stimulus: increased
testosterone.
Other Hormones- Melatonin—secreted by the pineal gland during
darkness; brings on sleep. - Prostaglandins—synthesized by cells from the
phospholipids of their cell membranes; exert
their effects locally. Are involved in inflammation
and pain, reproduction, nutrient metabolism,
changes in blood vessels, blood clotting.
Mechanisms of Hormone Action (see Fig.
10–12)- A hormone affects cells that have receptors for it.
Receptors are proteins that may be part of the cell
The Endocrine System 247