s ta g e
In the fight-or-flight stage,
the sympathetic nervous system (Chapter 7) kicks in and
stimulates heart, lungs, blood vessels, and the adrenal me-
dulla (see Figure 9.20):
Sympathetic stimulation of the heart and blood vessels
increases heart rate, blood pressure, and blood flow,
especially to skeletal muscles.
Sympathetic stimulation of the lungs dilates airways
and increases the rate of breathing.
releases epinephrine and norepinephrine.
These coordinated responses increase the availability
of oxygen and fuel to working muscles. However, this ini-
tial stage cannot be sustained for long.
R esistance s ta g e
The resistance stage takes over
when the fight-or-flight stage is almost complete. In this
stage, the body’s energy reserves are tapped to provide en-
ergy to working muscles or other tissues, such as sites of
injury. The resistance stage involves the hypothalamus, pi-
tuitary gland, adrenal cortex, and thyroid gland. Depend-
ing on the type and severity of the stress, the resistance
stage may be successful in combating the stressor and re-
turning the body to normal. However, when an individual
cannot defeat a powerful stressor, such as severe trauma,
the body becomes exhausted.
E xhaustion s ta g e
The exhaustion stage occurs when
the body’s energy resources become depleted, muscles
waste away, and the immune system is suppressed.
Several Organs and Hormones
Are Involved in the Stress Response
The sympathetic nervous system and the hypothalamus
direct the stress response by stimulating other organs and
glands, such as the adrenal glands, the pituitary gland, the
thyroid gland, the liver, and skeletal muscle (see Figure
9.20). Let’s discuss them at each stage of the stress response.
In the fight-or-flight stage, the sympathetic nervous
system is activated and releases norepinephrine, which:
Stimulates the heart, which increases heart rate, blood
pressure, and blood flow.
Constricts blood vessels to visceral organs, such as the
digestive system and skin. This reduces digestion and
diverts blood flow to working skeletal muscles, the
heart, and the lungs.
Stimulates sweat glands in the skin, which leads to
Dilates the airways of the lungs and increases the rate
Stimulates release of epinephrine and norepinephrine
from the adrenal medulla, enhancing the effects of the
sympathetic nervous system and causing the liver and
adipose tissue to begin mobilizing energy reserves. In
the liver, glycogen breaks down into glucose, which is
released into the blood. In adipose tissue, triglycerides
break down into fatty acids, which are released into
In the resistance stage, the hypothalamus stimulates the
pituitary gland, the thyroid gland, the adrenal glands, the
liver, and muscle, and adipose tissues (see Figure 9.18).
The hypothalamus secretes CRH, GHRH, and TRH, which
act on the anterior pituitary lobe (see Table 9.1). The an-
terior pituitary secretes TSH, hGH, and ACTH. TSH acts
on the thyroid gland to release thyroid hormones, which
increase metabolism. hGH acts on the liver and other tis-
sues through IGFs to sustain the breakdown of glycogen
and triglycerides. ACTH stimulates the adrenal cortex
to release cortisol, the major stress hormone. Cortisol
acts on the liver, adipose tissue, and skeletal muscle to
maintain the mobilization of energy reserves and reduce
inflammation. So, the resistance stage mobilizes energy
sources (glucose, fatty acids, amino acids) that tissues can
use to make ATP. Because the resistance stage relies on
chemical messages, it takes longer to initiate, but it can
be sustained for a long period of time to combat a stressor.
Exhaustion occurs due to prolonged exposure to corti-
sol, which, among other effects, depletes the body’s energy
resources, wastes away muscles, and suppresses the im-
mune system. Prolonged stress or frequent stresses may
weaken the body and make it susceptible to other diseases.
However, the exact role of stress in diseases is not known.
are the stages of the stress response?
are the components of the endocrine
system and their target organs involved in each
stage of the stress response?
The Endocrine System Coordinates the Stress Response 277