296 Chapter 15adrenal cortex Outer por-
tion of an adrenal gland.
adrenal medulla Inner por-
tion of an adrenal gland.
glucocorticoids Adrenal
cortex hormones that are
secreted at times of physi-
ological stress. The main
one is cortisol.
gluconeogenesis Synthe-
sis of glucose in the liver.
mineralocorticoids
Adrenal cortex hormones
(such as aldosterone) that
adjust blood levels of min-
eral salts.
adrenal glands and stress responses
n The adrenal glands make hormones that help regulate
blood levels of glucose, influence blood pressure, and
regulate blood circulation.
n Links to inflammation 9.4, Nutrient processing 11.8,
urine formation 12.4, Stress responses 13.6the adrenal cortex produces glucocorticoids
and mineralocorticoids
We have two adrenal glands, one on top of each kidney.
The outer part of each gland is the adrenal cortex (Fig-
ure 15.9). There, cells secrete two major types of steroid
hormones, the glucocorticoids and mineralocorticoids.
Glucocorticoids raise the blood level of glucose.
For instance, the body’s main glucocorticoid, cortisol, is
secreted when the body is stressed and glucose is in such
demand that its blood level drops to a low set point. That
level is an alarm signal and starts a stress response, which
a negative feedback mechanism later cuts off. Among
other effects, cortisol promotes the breakdown of muscle
proteins and stimulates the liver to take up amino acids,
from which liver cells synthesize glucose in a process
called gluconeogenesis. Cortisol also reduces how much
glucose tissues such as skeletal muscle take up from the
blood. This effect is sometimes called “glucose sparing.”
Glucose sparing is extremely important in homeostasis,
for it helps ensure that the blood will carry enough glu-
cose to supply the brain, which usually cannot use other
molecules for fuel. Cortisol also promotes the breakdown
of fats to fatty acids that can be used for energy.
Figure 15.9 diagrams the negative
feedback loop for cortisol. When the
blood level of cortisol rises above a
set point, the hypothalamus makes
less of the releasing hormone CRH.
The anterior pituitary responds by
secreting less ACTH, and the adre-
nal cortex secretes less cortisol. In a
healthy person, daily cortisol secre-
tion is highest when blood glucose is
lowest, usually in the early morning.
Chronic severe hypoglycemia, or
low blood sugar, can develop when
the adrenal cortex makes too little
cortisol. Then, mechanisms that
spare glucose and make new sup-
plies in the liver don’t work properly.
Glucocorticoids also reduce inflammation. The adre-
nal cortex pumps out more of these chemicals at times of
unusual physical stress such as a painful injury, severe
illness, or a strong allergic reaction. The extra cortisol and
other signaling molecules helps speed recovery. That is
why doctors prescribe cortisol-like drugs such as cortisonefor patients with asthma or serious inflammatory disor-
ders. Cortisone is the active ingredient in many over-the-
counter products for treating skin irritations.
Unfortunately, long-term use of heavy doses of gluco-
corticoids has serious side effects, including suppressing
the immune system. Long-term negative psychosocial
stress (such as from financial or relationship problems) has
the same effect.
For the most part, mineralocorticoids adjust the con-
centrations of mineral salts, such as potassium and sodium,
in extracellular fluid. The most abundant mineralocorticoid
is aldosterone. Section 12.4 explained that aldosterone acts on
the distal tubules of kidney nephrons, stimulating them to
reabsorb sodium ions and excrete potassium ions. The reab-
sorption of sodium in turn promotes reabsorption of water
from the tubules as urine is forming. Various circumstances
can cause the release of aldosterone. Common triggers
include falling blood pressure or falling blood levels of
sodium—which reduces blood volume because water moves
out of the bloodstream by osmosis.
In a fetus and early in puberty, the adrenal cortex also
makes large amounts of sex hormones. The main ones are
androgens (male sex hormones), but female sex hormones
(estrogens and progesterone) also are produced. In adults,
the reproductive organs generate most sex hormones.hormones from the adrenal medulla
help regulate blood circulation
The adrenal medulla is the inner part of the adrenal gland
shown in Figure 15.9. It contains neurons that release two
substances, epinephrine and norepinephrine. Both act
as neurotransmitters when they are secreted by neurons
elsewhere in the body. When the adrenal medulla secretes
them, however, their hormonelike effects help regulate
blood circulation and carbohydrate use when the body is
stressed or excited. For example, they increase the heart
rate, dilate arterioles in some areas and constrict them in
others, and dilate bronchioles. Thus the heart beats faster
and harder, more blood is shunted to heart and muscle
cells from other regions, and more oxygen flows to energy-
demanding cells throughout the body. These are aspects of
the fight–flight response noted in Chapter 13.
The operation of the adrenal medulla provides another
example of negative feedback control. For example, when
the hypothalamus sends the necessary signal (by way of
sympathetic nerves) to the adrenal medulla, the neuron
axons will start to release norepinephrine into the syn-
apse between the axon endings and the target cells. Soon,
norepi nephrine molecules collect in the synapse, setting
the stage for a localized negative feedback mechanism.
As the accumulating norepinephrine binds to receptors
on the axon endings, the release of norepinephrine soon
shuts down.15.7
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).