246 The Endocrine System
Endocrine glands are ductless glands that
secrete hormones into the blood. Hormones
exert their effects on target organs or tis-
sues.
Chemistry of Hormones
- Amines—structural variations of the amino acid
tyrosine; thyroxine, epinephrine. - Proteins—chains of amino acids; peptides are short
chains. Insulin, GH, glucagon are proteins; ADH
and oxytocin are peptides. - Steroids—made from cholesterol; cortisol, aldos-
terone, estrogen, testosterone.
Regulation of Hormone Secretion
- Hormones are secreted when there is a need for
their effects. Each hormone has a specific stimulus
for secretion. - The secretion of most hormones is regulated by
negative feedback mechanisms: As the hormone
exerts its effects, the stimulus for secretion is
reversed, and secretion of the hormone decreases.
Pituitary Gland (Hypophysis)—hangs from
hypothalamus by the infundibulum; enclosed
by sella turcica of sphenoid bone (see Figs.
10–1 and 10–2)
- Posterior Pituitary (Neurohypophysis)—stores
hormones produced by the hypothalamus (Figs.
10–2 and 10–3 and Table 10–1).
- ADH—increases water reabsorption by the kid-
neys, decreases sweating, in large amounts causes
vasoconstriction. Result: decreases urinary out-
put and increases blood volume; increases BP.
Stimulus: nerve impulses from hypothalamus
when body water decreases. - Oxytocin—stimulates contraction of myomet-
rium of uterus during labor and release of milk
from mammary glands. Stimulus: nerve impulses
from hypothalamus as cervix is stretched or as
infant sucks on nipple.
- Anterior Pituitary (Adenohypophysis)—secretions
are regulated by releasing hormones from the
hypothalamus (Fig. 10–3 and Table 10–2).
- GH—through intermediary molecules, IGFs,
GH increases amino acid transport into cells
and increases protein synthesis; increases rate
of mitosis; increases use of fats for energy
(Fig. 10–4). Stimulus: GHRH from the hypo-
thalamus.- TSH—increases secretion of thyroxine and T 3
by the thyroid. Stimulus: TRH from the hypo-
thalamus. - ACTH—increases secretion of cortisol by the
adrenal cortex. Stimulus: CRH from the hypo-
thalamus. - Prolactin—initiates and maintains milk produc-
tion by the mammary glands. Stimulus: PRH
from the hypothalamus. - FSH—In women:initiates development of ova in
ovarian follicles and secretion of estrogen by fol-
licle cells.
In men: initiates sperm development in the testes.
Stimulus: GnRH from the hypothalamus. - LH—In women:stimulates ovulation, transforms
mature follicle into corpus luteum and stimulates
secretion of progesterone.
In men:stimulates secretion of testosterone by
the testes. Stimulus: GnRH from the hypothala-
mus.
Thyroid Gland—on front and sides of tra-
chea below the larynx (see Fig. 10–1 and
Table 10–3)- Thyroxine (T 4 ) and T 3 —(Fig. 10–5) produced by
thyroid follicles. Increase use of all food types for
energy and increase protein synthesis. Necessary
for normal physical, mental, and sexual develop-
ment. Stimulus: TSH from the anterior pituitary. - Calcitonin—produced by parafollicular cells.
Decreases reabsorption of calcium from bones
and lowers blood calcium level. Stimulus: hyper-
calcemia.
Parathyroid Glands—four; two on posterior
of each lobe of thyroid (see Figs. 10–6 and
10–7 and Table 10–4)- PTH—increases reabsorption of calcium and
phosphate from bones to the blood; increases
absorption of calcium and phosphate by the
small intestine; increases reabsorption of calcium
and excretion of phosphate by the kidneys, and
activates vitamin D. Result: raises blood calcium
and lowers blood phosphate levels. Stimulus:
hypocalcemia. Inhibitor: hypercalcemia.
STUDY OUTLINE