The pituitary gland has two structural and
functional divisions: the anterior lobe and the pos-
terior lobe. The anterior pituitary lobe directs the
activities of the thyroid, parathyroid, adrenal, and
sex glands (gonads). The posterior pituitary lobe
stores and secretes two hormones it receives from
the hypothalamus: ANTIDIURETIC HORMONE (ADH),
sometimes called vasopressin, and OXYTOCIN. The
release of ADH directs the KIDNEYS to withhold
more water, increasing blood volume and thus
blood pressure. Oxytocin stimulates contractions
of the UTERUSduring CHILDBIRTH, and the mother’s
letdown REFLEXwhen BREASTFEEDING. Oxytocin also
appears to play a role in sexual arousal in both
women and men.
Recent research suggests oxytocin interacts
with the limbic system, the intersection of neuro-
logic and biochemical response to emotional
stress. Levels of oxytocin in the bloodstream rise
when stress hormone levels rise, leading
researchers to speculate that oxytocin presents a
counterbalance to the fight-or-flight response the
stress hormones evoke by helping calm the body
and restore homeostasis.
Stress management: the adrenal glands The
adrenal glands drape across the tops of the kid-
neys. They produce an array of hormones that
increase body functions in response to physiologic
stress, such as heart rate and blood pressure. The
two portions of the adrenal gland—the cortex
(outer portion) and the medulla (inner portion)—
have unique functions. The adrenal cortex, rind-
like in appearance, secretes steroid hormones that
it synthesizes from cholesterol. The primary adre-
nal cortex hormones are ALDOSTERONE and
CORTISOL. Aldosterone regulates the fluid balance
in the blood by directing the absorption of water
and sodium in the kidneys. This is a fundamental
component of a blood pressure–regulation mecha-
nism called the RENIN-angiotensin-aldosterone
(RAA) system. Cortisol has numerous effects
related to metabolic functions throughout the
body. The adrenal cortex also produces small
amounts of estrogen, PROGESTERONE, andTESTOS-
TERONEin men and women alike.
The adrenal medulla, the core of the adrenal
gland, secretes EPINEPHRINE, NOREPINEPHRINE (also
called adrenaline and noradrenaline), and
DOPAMINE. These hormones initiate rapid increases
in heart rate, respiration, and blood pressure,
along with changes blood distribution, to allow
the body to enter the classic fight-or-flight mode—
the stress response. These hormones also provide
the “adrenaline rush” that appeals to people who
enjoy high-risk activities. Chemically these three
substances are catecholamines. In the bloodstream
the catecholamines are hormones regulating cell
activity. In the interstitial (between-cell) fluid,
they function as neurotransmitters, facilitating
electrical impulses between nerves. The midbrain,
including the hypothalamus, controls the adrenal
medulla’s secretory activity.DOPAMINE “TRANSPLANT” FOR PARKINSON’S DISEASE
In the 1980s neurologists experimented with
transplanting adrenal medullary tissue into the
brains of people who had PARKINSON’S DISEASE, a
progressively degenerative condition that results
from depleted DOPAMINE in the BRAIN. Doctors
hoped the adrenal medullary tissue would take
root in the brain and continue to produce NOREPI-
NEPHRINE, a precursor HORMONE to dopamine,
which other brain chemicals would convert to
much-needed dopamine. The risks of the proce-
dure far outweighed the possible benefits, how-
ever, and failed to produce consistent results.
Doctors today have largely abandoned the
method.Metabolic homeostasis: the thyroid gland
Spread across the throat like an elongated butter-
fly, the thyroid gland secretes the hormones THY-
ROXINE(T 4 ) and TRIIODOTHYRONINE(T 3 ), which direct
the rate at which cells consume energy. These
hormones regulate numerous body functions,
notably heart rate, digestive rate, and thermoregu-
lation (body temperature, particularly response to
cold). The thyroid’s two lobes perform the same
functions. The thyroid gland also secretes CALCI-
TONIN, which acts to decrease calcium levels in the
blood.
Thyroid hormones are essential for life. People
who have HYPOTHYROIDISM (underactive thyroid
gland) or who have had their thyroid glands
destroyed or surgically removed must take lifelong
thyroid hormone supplement or replacement
therapy. Congenital thyroid deficiency, once called
cretinism, in which the thyroid gland is missing orThe Endocrine System 99