616
UNIT 3
Regulation and Integration of the Body
16
ships of the hypothalamus, the “director” of the stress response,
with adrenal hormones.
Homeostatic Imbalance
16.9
A deficiency in adrenal medulla hormones is not a problem be-
cause these hormones merely intensify activities set into motion
by the sympathetic nervous system neurons. Unlike glucocorti-
coids and mineralocorticoids, adrenal catecholamines are not
essential for life.
On the other hand, hypersecretion of catecholamines, some-
times arising from a medullary chromaffin cell tumor called a
pheochromocytoma
(fe-o-kro
0
mo-si-to
9
mah), produces symp-
toms of uncontrolled sympathetic nervous system activity—
hyperglycemia
(elevated blood glucose), increased metabolic
rate, rapid heartbeat and palpitations, hypertension, intense
nervousness, and sweating.
Check Your Understanding
12.
List the three classes of hormones released from the adrenal
cortex and for each briefly state its major effect(s).
For answers, see Appendix H.
When a short-term stressor activates the body to fight-or-
flight status, the sympathetic nervous system is mobilized.
Blood vessels constrict and the heart beats faster (together rais-
ing the blood pressure), and blood is diverted from temporarily
nonessential organs to the heart and skeletal muscles. Blood
glucose levels rise, and preganglionic sympathetic nerve end-
ings weaving through the adrenal medulla signal for release of
catecholamines, which reinforce and prolong the fight-or-flight
response.
Unequal amounts of the two hormones are stored and re-
leased. Approximately 80% is epinephrine and 20% norepine-
phrine. With a few exceptions, the two hormones exert the
same effects (see Table 14.2, p. 534). Epinephrine is the more
potent stimulator of metabolic activities, bronchial dilation,
and increased blood flow to skeletal muscles and the heart, but
norepinephrine has a greater influence on peripheral vasocon-
striction and blood pressure. Epinephrine is used clinically as a
heart stimulant and to dilate the bronchioles during acute asth-
matic attacks.
Unlike hormones from the adrenal cortex, which promote
long-lasting body responses to stressors, catecholamines cause
fairly brief responses.
Figure 16.17
depicts the interrelation-
Table 16.3
Adrenal Gland Hormones: Summary of Regulation and Effects
HORMONE
REGULATION OF RELEASE
TARGET ORGAN AND EFFECTS
EFFECTS OF HYPERSECRETION
h
AND HYPOSECRETION
g
Adrenocortical Hormones
Mineralocorticoids (chiefly
aldosterone)
Stimulated
by renin-angiotensin-
aldosterone mechanism (activated
by decreasing blood volume or
blood pressure), elevated blood
K
1
levels, and ACTH (minor
influence)
Inhibited
by increased blood
volume and pressure, and
decreased blood K
1
levels
Kidneys: increase blood levels of
Na
1
and decrease blood levels
of K
1
; since water reabsorption
accompanies sodium retention,
blood volume and blood pressure
rise
h
Aldosteronism
g
Addison's disease
Glucocorticoids (chiefly
cortisol)
Stimulated
by ACTH
Inhibited
by feedback inhibition
exerted by cortisol
Body cells: promote
gluconeogenesis and
hyperglycemia; mobilize fats for
energy metabolism; stimulate
protein catabolism; assist body
to resist stressors; depress
inflammatory and immune
responses
h
Cushing's syndrome
g
Addison's disease
Gonadocorticoids (chiefly
androgens, converted to
testosterone or estrogens
after release)
Stimulated
by ACTH; mechanism
of inhibition incompletely
understood, but feedback
inhibition not seen
Insignificant effects in males;
contributes to female libido;
development of pubic and axillary
hair in females; source of estrogen
after menopause
h
Masculinization of females
(adrenogenital syndrome)
g
No effects known
Adrenal Medullary Hormones
Catecholamines (epinephrine
and norepinephrine)
Stimulated
by preganglionic
fibers of the sympathetic nervous
system
Sympathetic nervous system target
organs: effects mimic sympathetic
nervous system activation;
increase heart rate and metabolic
rate; increase blood pressure by
promoting vasoconstriction
h
Prolonged fight-or-flight
response; hypertension
g
Unimportant
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