506
UNIT THREE
Hormones of the Adrenal Cortex
The cells of the adrenal cortex produce more than thirty
different steroids, including several hormones (cortico-
steroids). Unlike the adrenal medullary hormones, without
which a person can survive, some of those released by the
cortex are vital. In the absence of adrenal cortical secretions,
a person usually dies within a week without extensive elec-
trolyte therapy. The most important adrenal cortical hor-
mones are aldosterone, cortisol, and certain sex hormones.
Aldosterone
Cells in the outer zone (zona glomerulosa) of the adrenal
cortex synthesize
aldosterone.
This hormone is called a
mineralocorticoid
because it helps regulate the concentration
of mineral electrolytes, such as sodium and potassium ions.
More speciF
cally, aldosterone causes the kidney to conserve
sodium ions and to excrete potassium ions. The cells that
secrete aldosterone respond directly to changes in the com-
position of blood plasma. However, whereas an increase in
plasma potassium strongly stimulates these cells, a decrease
in plasma sodium only slightly stimulates them. Control of
aldosterone secretion is indirectly linked to plasma sodium
level by the
renin-angiotensin system.
Groups of specialized kidney cells (juxtaglomerular cells)
are able to respond to changes in blood pressure and the
plasma sodium ion concentration. If the level of either of
these factors decreases, the cells release an enzyme called
renin
(re
nin)
.
Renin reacts with a blood protein called
angiotensinogen (
an
je-o-ten-sin
o-jen) to release a pep-
tide called
angiotensin I.
Another enzyme (angiotensin-
converting enzyme, or ACE) primarily in lung blood vessels
catalyzes a reaction that converts angiotensin I into another
form,
angiotensin II,
carried in the bloodstream
(f g. 13.31)
.
When angiotensin II reaches the adrenal cortex, it stimulates
the release of aldosterone. ACTH is necessary for aldoster-
one secretion to respond to this and other stimuli.
Aldosterone, in conserving sodium ions, indirectly
retains water by osmosis. This helps maintain blood sodium
The effects of the adrenal medullary hormones generally
resemble those that result when sympathetic nerve F bers
stimulate their effectors: increased heart rate and force of car-
diac muscle contraction, elevated blood pressure, increased
breathing rate, and decreased digestive activity (see table
11.10). The hormonal effects last up to ten times longer
than the neurotransmitter effects because the hormones are
slowly removed from the tissues.
The ratio of the two hormones in the adrenal medullary
secretion varies with different physiological conditions, but
usually it is about 80% epinephrine and 20% norepineph-
rine. Although these hormones’ effects are generally similar,
certain effector cells respond differently, due to the relative
numbers of alpha and beta receptors in their membranes.
Both hormones can stimulate both classes of receptors,
although norepinephrine has a greater effect on alpha recep-
tors.
Table 13.10
compares some of the differences in the
effects of these hormones.
Impulses arriving on sympathetic nerve F bers stimulate
the adrenal medulla to release its hormones at the same time
sympathetic impulses stimulate other effectors. As a rule, these
impulses originate in the hypothalamus in response to stress.
Thus, the medullary secretions function together with the sym-
pathetic division of the autonomic nervous system in preparing
the body for energy-expending action—“F ght or fl ight.”
RECONNECT
To Chapter 11, Sympathetic Division, pages 424–426.
PRACTICE
31
Describe the location and structure of the adrenal glands.
32
Name the hormones the adrenal medulla secretes.
33
What general eF
ects do hormones secreted by the adrenal
medulla produce?
34
What usually stimulates release of hormones from the adrenal
medulla?
TABLE
13.10
|
Comparative Ef
ects oF Epinephrine and Norepinephrine
Structure or ±unction Af
ected
Epinephrine
Norepinephrine
Heart
Rate increases
±orce of contraction increases
Rate increases
±orce of contraction increases
Blood vessels
Vessels in skeletal muscle vasodilate, decreasing resistance to
blood ²
ow
Blood ²
ow to skeletal muscles increases, resulting from
constriction of blood vessels in skin and viscera
Systemic blood pressure
Some increase due to increased cardiac output
Great increase due to vasoconstriction, counteracted in
muscle blood vessels during exercise
Airways
Dilated
Some dilation
Reticular formation of brain
Activated
Little eF
ect
Liver
Promotes breakdown of glycogen to glucose, increasing blood
sugar level
Little eF
ect on blood glucose level
Metabolic rate
Increases
Increases
previous page 536 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 538 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off