499
CHAPTER THIRTEEN
Endocrine System
Oxytocin has no established function in males, although
it is present in the male posterior pituitary. There is evidence
that it may stimulate the movement of certain fl
uids in the
male reproductive tract during sexual activity.
Table 13.6
reviews the hormones of the pituitary gland.
PRACTICE
21
What is the function of ADH?
22
How is the secretion of ADH controlled?
23
What eF
ects does oxytocin produce in females?
13.6
THYROID GLAND
The
thyroid gland
(thi
roid gland), as
f gure 13.18
shows,
is a vascular structure that consists of two large lateral lobes
connected by a broad
isthmus
(is
mus). The thyroid lies just
below the larynx on either side and anterior to the trachea.
The gland is specialized to remove iodine from the blood.
Structure of the Gland
A capsule of connective tissue covers the thyroid gland, which
is made up of many secretory parts called
follicles.
The cavities
in these follicles are lined with a single layer of cuboidal epi-
thelial cells and are F lled with a clear viscous
colloid,
which
consists primarily of a glycoprotein called
thyroglobulin.
The
follicular cells produce and secrete hormones that may either
be stored in the colloid or released into nearby capillaries
(f g.
13.19)
. Other hormone-secreting cells, called
extrafollicu-
lar cells
(C cells), lie outside the follicles.
Oxytocin also has an antidiuretic action, but less so
than ADH. In addition, oxytocin can contract smooth mus-
cles in the uterine wall, playing a role in the later stages of
childbirth. The uterus becomes more sensitive to oxytocin’s
effects during pregnancy. Stretching of uterine and vaginal
tissues late in pregnancy, caused by the growing fetus, initi-
ates nerve impulses to the hypothalamus, which then signals
the posterior pituitary to release oxytocin, which, in turn,
stimulates the uterine contractions of labor.
In the breasts, oxytocin contracts certain cells near the
milk-producing glands and their ducts. In lactating breasts,
this action forces liquid from the milk glands into the milk
ducts and ejects the milk.
The mechanical stimulation of suckling initiates nerve
impulses that travel to the mother’s hypothalamus, which
responds by signaling the posterior pituitary to release oxy-
tocin, which, in turn, stimulates milk release. Thus, milk is
normally not ejected from the milk glands and ducts until
the baby suckles. The fact that milk is ejected from both
breasts in response to suckling is a reminder that all target
cells respond to a hormone.
TABLE
13.6
|
Hormones of the Pituitary Gland
Anterior Lobe
Hormone
Action
Source of Control
Growth hormone (GH)
Stimulates increase in size and rate of division of body
cells; enhances movement of amino acids through
membranes; promotes growth of long bones
Secretion inhibited by somatostatin (SS) and stimulated by growth
hormone-releasing hormone (GHRH) from the hypothalamus
Prolactin (PRL)
Sustains milk production after birth; ampli±
es eF
ect
of LH in males
Secretion inhibited by prolactin-release inhibiting hormone (PIH) and
may be stimulated by yet to be identi±
ed prolactin-releasing factor
(PR²) from the hypothalamus
Thyroid-stimulating hormone
(TSH)
Controls secretion of hormones from the thyroid
gland
Thyrotropin-releasing hormone (TRH) from the hypothalamus
Adrenocorticotropic hormone
(ACTH)
Controls secretion of certain hormones from the
adrenal cortex
Corticotropin-releasing hormone (CRH) from the hypothalamus
²ollicle-stimulating hormone
(²SH)
Development of egg-containing follicles in ovaries;
stimulates follicular cells to secrete estrogen; in
males, stimulates production of sperm cells
Gonadotropin-releasing hormone (GnRH) from the hypothalamus
Luteinizing hormone (LH)
Promotes secretion of sex hormones; releases egg
cell in females
Gonadotropin-releasing hormone (GnRH) from the hypothalamus
Posterior Lobe
Hormone
Action
Source of Control
Antidiuretic hormone (ADH)
Causes kidneys to reduce water excretion; in high
concentration, raises blood pressure
Hypothalamus in response to changes in blood water concentration and
blood volume
Oxytocin (OT)
Contracts muscles in uterine wall and those
associated with milk-secreting glands
Hypothalamus in response to stretch in uterine and vaginal walls and
stimulation of breasts
If the uterus is not su³
ciently contracting to expel a fully developed
fetus, oxytocin is sometimes given intravenously to stimulate uterine
contractions, inducing labor. Oxytocin is also administered to the
mother following childbirth to ensure that the uterine muscles con-
tract enough to squeeze broken blood vessels closed, minimizing the
danger of hemorrhage.
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