• The parathyroid glands secrete parathyroid hormone,
which mobilizes calcium from bone, increases calcium
reabsorption in the kidneys, and stimulates calcitriol
secretion by the kidneys. Calcitriol stimulates calcium
absorption by the small intestine. Parathyroid hormone
and calcitonin act in opposite ways to control blood
calcium levels.
• The pancreas, as seen here, secretes glucagon and
insulin. Both glucagon and insulin act mainly on the
liver and skeletal muscle; they have opposite effects on
blood glucose. In response to low blood sugar levels
(hypoglycemia), glucagon mobilizes energy sources to
raise low blood glucose by stimulating the breakdown
of liver glycogen into glucose, adipose tissue triglyc-
erides into fatty acids, and muscle proteins into amino
acids. In response to high blood sugar levels (hypergly-
cemia), insulin lowers high blood glucose by stimulating
all cells to take up glucose from the blood, and it has
the opposite effect of glucagon on liver, adipose tissue,
and skeletal muscle.
The pancreas • Figure 9.12a
Pancreas
q
Endocrine Glands Regulate
Other Key Body Functions 269
• The adrenal cortex secretes a mineralocorticoid called
aldosterone, which affects sodium reabsorption and po-
tassium excretion in the kidneys. Aldosterone secretion,
which is controlled by the renin-angiotensin-aldoste-
rone pathway, influences blood volume and pressure.
• As shown, the adrenal cortex also secretes a glucocor-
ticoid called cortisol, which mobilizes energy sources
from skeletal muscle, the liver, and adipose tissue. Corti-
sol also reduces inflammation. Its secretion is controlled
by hypothalamic CRH secretion
Control of glucocorticoid
and pituitary ACTH secretion.
secretion • Figure 9.16
Corticotropin releasing
'hormone (CRH)
Cqrticotropji
(ACTH)
The adrenal medulla secretes
epinephrine and norepinephrine, which
increase heart rate, blood pressure, and
blood flow and mobilize energy sources.
The ovaries secrete estrogen and progester-
one. Secretions are controlled by the hy-
pothalamus via gonadotropin-releasing
hormone (GnRH) and the pituitary gland
via follicle-stimulating hormone (FSH) and
luteinizing hormone (LH). These hormones
control the menstrual cycle, initiate ovulation,
and maintain pregnancy.
• The testes secrete testosterone under
control of the hypothalamus (GnRH)
and pituitary gland (LH, FSH). Testos-
terone supports sperm develop-
ment and maintains male sexual
characteristics.
Cortisol
• Other body tissues (for
example, heart, kidney, intes-
tine) secrete different hormones that have a variety of functions
(for example, control blood volume, increase red blood cell
production, stimulate insulin secretion).
4
The Endocrine System Coordinates
the Stress Response 275
• As shown here, the stress response has three stages: fight-or-
flight, resistance, and exhaustion.
The stress response • Figure 9.20
Stressors
stimulate
Hyp9thalamus
N erve
impulses
S
gr
Sympathetic
centers in
spinal cord
Sympathetic nerves
f
Adrenal
medulla
(
Epinephrine
and
Cortisol Insulin-likeGrowthThyroid hormones
Factors (IGFs)
(T3 and T4)
T
,
' I '
' I '
norepinephrine
stress responses Stress responses mobilize energy resources:
Supplement and prolong
“fight-or-flight” responses
Summary 281
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