Human Physiology, 14th edition (2016)

Endocrine Glands 339

The adrenal medulla is innervated by preganglionic sympa-

thetic axons and secretes its hormones whenever the sympathetic

nervous system is activated during “fight or flight” (chapter 9,

fig. 9.7). These sympathoadrenal effects are supported by the

metabolic actions of epinephrine and norepinephrine: a rise

in blood glucose due to stimulation of hepatic glycogenolysis

(breakdown of glycogen) and a rise in blood fatty acids due to

stimulation of lipolysis (breakdown of fat). The endocrine regu-

lation of metabolism is described more fully in chapter 19.

Functions of the Adrenal Medulla

The cells of the adrenal medulla secrete epinephrine and
norepinephrine in an approximate ratio of 4 to 1. The effects
of these catecholamine hormones are similar to those caused
by stimulation of the sympathetic nervous system, except that
the hormonal effect lasts about 10 times longer. The hormones
from the adrenal medulla increase the cardiac output and heart
rate, dilate coronary blood vessels, increase mental alertness,
increase the respiratory rate, and elevate the metabolic rate.

Figure 11.19 Simplified pathways for the synthesis of steroid hormones in the adrenal cortex. The adrenal cortex
produces steroids that regulate Na^1 and K^1 balance (mineralocorticoids), steroids that regulate glucose balance (glucocorticoids), and
small amounts of sex steroid hormones. (DHEA  5  dehydroepiandrosterone.)

Zona glomerulosa Zona fasciculata and zona reticularis

Mineralo-

corticoids

Glucocorticoids Sex steroids

Cholesterol

Pregnenolone

Progesterone

Deoxycorticosterone

Corticosterone

Aldosterone

Cholesterol

Pregnenolone

Progesterone

Deoxycorticosterone

Corticosterone

17-Hydroxypregnenolone

17-Hydroxyprogesterone

Deoxycortisol

Cortisol

Dehydroepiandrosterone

(DHEA)

Androstenedione

Other androgens

Figure 11.20 Activation of the pituitary-adrenal
axis by nonspecific stress. Negative feedback control of the
adrenal cortex (blue arrows) is also shown.

CRH

Anterior

pituitary

Higher

brain centers

Adrenal

cortex ACTH

Cortisol

Nonspecific stress

Negative

feedback

Sensor

Integrating center

Effector

Hypothalamus

Clinical Investigation CLUES

Rachel’s physician said her symptoms might be the

result of a pheochromocytoma, but he needed to test for

more likely possibilities.

• What is a pheochromocytoma, and how might it
  account for Rachel’s symptoms?

CLINICAL APPLICATION

A pheochromocytoma is a tumor, usually benign, of the

catecholamine-secreting cells of the adrenal medulla. This

tumor secretes excessive amounts of epinephrine and nor-

epinephrine, causing vasoconstriction, increased cardiac

rate and strength of contraction, hypertension, and other

symptoms. The high blood pressure can lead to heart dis-

ease and stroke, as well as kidney failure if not treated with

alpha and beta-adrenergic receptor blockers and then by

surgical removal of the affected adrenal gland.