Hormones Act on Target Cells
LEARNING OBJECTIVES
1.
Define
endocrine glands, hormones, hormone
receptors, and target cells.
2.
Describe
how steroid hormones act.
3.
Describe
how nonsteroid hormones act.
T
o maintain a steady state, the body must reg-
ulate many parameters, such as heart rate,
breathing, blood glucose, fluid balance, and so
on. The nervous system regulates many of these
on a moment-by-moment basis and is ready to rapidly re-
spond to changes within seconds. Besides the nervous system,
the
endocrine system
regulates many parameters chemi-
cally. Let’s look at the components of the endocrine system.
The Endocrine System Has
Many Components
The endocrine system consists of several components (see
Figure 9.1a):
Endocrine glands are glands
that secrete a chemical signal
directly
into
the
bloodstream
rather than through a duct or
tube. For example, the salivary
gland is an exocrine gland; it
secretes saliva through a duct.
In contrast, the pituitary, thyroid, parathyroid, adrenal,
and pineal glands are endocrine glands because they
secrete chemicals into the bloodstream.
T
endocrine
system
A system of
glands and hormone-
secreting cells that
regulate body func-
tions through chemical
messages (hormones).
C o m p o n en ts o f th e en d o crin e sy stem • Figure 9.1
This figure shows the major endocrine glands (red), organs that contain endocrine cells (blue), and some
nearby organs for reference (a), and indicates the functional components of the endocrine system (b).
a. Location of endocrine glands
Thyroid
gland
Trachea
Parathyroid
Glands
(behind thyroid
glands)
Pineal gland
Parathyroid
Glands
Skin ■
Lung ■
Liver -
Hypothalamus
Pituitary
Gland
i
,h
__________
b. How endocrine
components work.
Endocrine glands or
Endocrine cells
Thyroid Gland
Trachea
Thymus
Hormone
Blood
V
____________
A
— Heart
Target cells
1
1
L
A
t
(cell activity gets changed
by hormone)
— Stomach
Adrenal
Glands
Pancreas ■
Small
Intestine
Scrotum
Kidney
,
Uterus
^ 1
|>
1
Ovary
Female
Testes
254
Male
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