HUMAN BIOLOGY

hoW do the hypothalamus and pituitary

glands interact in the production and

release of hormones?

• The hypothalamus produces hormones that are stored and
  released by the posterior pituitary or that regulate activity
  of the anterior pituitary.


• The posterior pituitary stores and releases ADH and oxytocin.


• The anterior lobe of the pituitary produces and releases ACTH,
  TSH, FSH, LH, PRL, and GH.

taKe-home message

hypothalamus Portion

of the forebrain that con-

trols processes related to

homeostasis and has endo-

crine functions.

pituitary gland Endocrine

gland that interacts with the

hypothalamus to control

many physiological func-

tions, including the activity

of some other glands.

ADH acts on cells of kidney nephrons and collecting
ducts. As Chapter 12 described, it promotes the reabsorp-
tion of water when the body must conserve water. The
hypothalamus also releases ADH into the bloodstream
when blood pressure falls below a set point. ADH causes
the arterioles in some tissues to narrow, so blood pressure
rises. This is why ADH is sometimes called vasopressin.
Oxytocin affects reproduction. In a pregnant woman,
for example, it triggers muscle contractions in the uterus
during labor and causes milk to be released when a mother
nurses her infant. In sexually active people, both male and
female, oxytocin apparently is a chemical trigger for feel-
ings of satisfaction after sexual contact. Studies suggest
that oxytocin is a “cuddle hormone” that helps stimulate
affectionate behavior.

The anterior pituitary lobe makes hormones

Unlike the posterior pituitary lobe, the anterior pitui tary
lobe produces and secretes six hormones:

Corticotropin ACTH

Thyrotropin TSH

Follicle-stimulating hormone FSH

Luteinizing hormone LH

Prolactin PRL

Growth hormone (somatotropin) GH (or STH)

Anterior pituitary hormones have widespread effects.
ACTH and TSH regulate the secretion of hormones
from the adrenal glands and thyroid gland, respectively.
FSH and LH influence reproduction, as described in
Chapter 16. Prolactin is best known for stimulating and
sustaining the production of breast milk, after other hor-
mones have primed the tissues. There also is evidence
that it promotes the synthesis of the male sex hormone
testosterone.
Growth hormone (GH) affects most body tissues. It stim-
ulates the processes by which cells divide and make new
proteins, and so has a major influence on growth. GH is
also important as a “metabolic hormone.” It stimulates cells
to take up amino acids and promotes the breakdown and
release of fat stored in adipose tissues when cells require
more fatty acids. GH also adjusts the rate at which cells
take up glucose. In this way it helps to maintain proper
blood sugar levels.
The hypothalamus regulates the anterior lobe by
secreting hormones that enter blood capillaries in the
pituitary stalk (Figure 15.4). The bloodstream carries
those hormones to another capillary bed in the anterior
lobe. There the hormones leave the blood and act on
their target cells. Most of these hormones are releasers
that spur target cells to secrete their own hormones. For
example, GnRH (gonadotropin-releasing hormone) trig-
gers the secretion of FSH and LH. These hormones are
called gonado tropins because they affect the functioning
of cells in the gonads, or reproductive organs. TRH (for
thyrotropin-releasing hormone) stimulates the release

C The bloodstream

delivers hormones to a

capillary bed in the

anterior lobe of pituitary.

B The hormones

are picked up by a

capillary bed at the

base of the hypothalamus.

adrenal

glands

testes in males,

ovaries in females

ACTH TSH

thyroid

gland

mammary

glands

most cells

(growth-

promoting

effects)

A Cell bodies of

different secretory

neurons in the

hypothalamus

secrete releasing

and inhibiting

hormones.

E Hormones

from anterior

lobe cells

enter small

blood vessels

that lead

to the

general

circulation.

D Molecules of the

releasing or inhibiting

hormone diffuse out of

capillaries and act on

endocrine cells in the

anterior lobe.

FSHLH PRL GH (STH)

Figure 15.4 Animated! The anterior pituitary lobe both

makes and releases hormones. Hormones from the

hypothalamus control this activity. (© Cengage Learning)

of TSH. Other hypothalamic hor-

mones are inhibitors. They block

secretions from cells in the anterior

pituitary. One of them, called soma-

tostatin, inhibits the secretion of

growth hormone and thyrotropin.

the endOCrIne system 291

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