^ Hormones Act on Target Cells 254
• The endocrine system consists of both endocrine glands
and endocrine cells of other organs. The endocrine system
regulates many physiological parameters chemically.
• Endocrine cells secrete hormones that bind to receptors
in target cells and cause some actions in the target cells to
evoke physiological responses.
• Hormones can be classified as steroid hormones or nonste-
roid hormones.
• Steroid hormones can go directly through the target cell
membrane, and stimulate specific genes to make new pro-
teins, which alter the activity of the target cell.
• As shown, nonsteroid hormones, usually must bind to a
hormone receptor on the surface of the cell membrane, and
elicit the formation of a "second messenger” inside the cell
(for example, cyclic AMP, calcium), which alters the target
cell's activity.
Nonsteroid hormones • Figure 9.3
0 0 0
Blood capillary
d U i U i b •
• Nonsteroid hormones generally act faster, but their effects
are shorter than those of steroid hormones.
Endocrine Glands Regulate Key Body
Functions 257
• The endocrine glands and organs containing endocrine cells
secrete approximately 30 different hormones that regu-
late and control many body functions, including chemical
composition and volume of blood, metabolism, contractions
of smooth and cardiac muscles, secretions of endocrine
glands, growth, development, and reproduction.
• Endocrine disorders involve either hyposecretion or hyper-
secretion of hormones. Causes of disorders can be either
primary (within the affected gland) or secondary (within a
gland that regulates the affected gland).
• The hypothalamus and pituitary gland regulate many
endocrine glands and body functions. The hypothalamus
secretes hormones (ADH, oxytocin) directly into the blood-
stream through the neurosecretory cells of the posterior
pituitary. Antidiuretic hormone (ADH) affects fluid reab-
sorption in the kidneys.
• The hypothalamus secretes releasing hormones (GnRH,
GHRH, thyrotropin-releasing hormone (TRH), CRH, PRH)
and inhibiting hormones (GHIH, PIH) into a common blood
supply, where they affect the anterior pituitary.
• The anterior pituitary secretes many hormones that affect
human growth.
• Other anterior pituitary hormones (TSH, ACTH, FSH, LH
PRL) control the secretions of the thyroid gland, the adrenal
glands, the ovaries/testes, and breast development and milk
■ As shown
here, the
thyroid gland
secretes thy-
roid hormones
ronine) that increase
basal metabolism, and
calcitonin, which decreas-
es blood calcium levels by
inhibiting the activity of a
type of bone cell called
oste oclasts that release
calcium into the blood.
Thyroid hormone levels • Figure 9.8
Hypothalamus senses low thyroid hormone
levels in the blood and releases TRH.
Follicular cells
release thyroid
hormones into
the blood.
The Endocrine System
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