Essentials of Anatomy and Physiology

The Endocrine System 237

tion of both hormones is stimulated by sympathetic
impulses from the hypothalamus, and their functions
duplicate and prolong those of the sympathetic divi-
sion of the autonomic nervous system (mimeticmeans
“to mimic”).

Epinephrine and Norepinephrine

Epinephrine (Adrenalin) and norepinephrine (nor-
adrenalin) are both secreted in stress situations and
help prepare the body for “fight or flight.” Norepi-
nephrineis secreted in small amounts, and its most
significant function is to cause vasoconstriction in the
skin, viscera, and skeletal muscles (that is, throughout
the body), which raises blood pressure.
Epinephrine, secreted in larger amounts, increases
the heart rate and force of contraction and stimulates
vasoconstriction in skin and viscera and vasodilation in
skeletal muscles. It also dilates the bronchioles,

decreases peristalsis, stimulates the liver to change

glycogen to glucose, increases the use of fats for

energy, and increases the rate of cell respiration. Many

of these effects do indeed seem to be an echo of sym-

pathetic responses, don’t they? Responding to stress is

so important that the body acts redundantly (that is,

exceeds what is necessary, or repeats itself) and has

both a nervous mechanism and a hormonal mecha-

nism. Epinephrine is actually more effective than sym-

pathetic stimulation, however, because the hormone

increases energy production and cardiac output to a

greater extent. The hormones of the adrenal medulla

are summarized in Table 10–6, and their functions are

shown in Fig. 10–9.

ADRENAL CORTEX

The adrenal cortexsecretes three types of steroid

hormones: mineralocorticoids, glucocorticoids, and

Table 10–5 HORMONES OF THE PANCREAS

Regulation

Hormone Functions of Secretion

Glucagon

(alpha cells)

Insulin (beta cells)

Somatostatin

(delta cells)

• Increases conversion of glycogen to glucose in the liver


• Increases the use of excess amino acids and of fats for energy


• Increases glucose transport into cells and the use of glucose for
  energy production


• Increases the conversion of excess glucose to glycogen in the liver
  and muscles


• Increases amino acid and fatty acid transport into cells, and their
  use in synthesis reactions


• Decreases secretion of insulin and glucagon


• Slows absorption of nutrients

Hypoglycemia

Hyperglycemia

Rising levels of insulin

and glucagon

Table 10–6 HORMONES OF THE ADRENAL MEDULLA

Regulation

Hormone Function(s) of Secretion

Norepinephrine

Epinephrine

• Causes vasoconstriction in skin, viscera, and skeletal muscles


• Increases heart rate and force of contraction


• Dilates bronchioles


• Decreases peristalsis


• Increases conversion of glycogen to glucose in the liver


• Causes vasodilation in skeletal muscles


• Causes vasoconstriction in skin and viscera


• Increases use of fats for energy


• Increases the rate of cell respiration

Sympathetic impulses

from the hypo-

thalamus in stress

situations