484
UNIT THREE
and blood pressure. Endocrine hormones also play vital roles
in reproduction, development, and growth.
Small groups of specialized cells produce some hor-
mones. However, the larger endocrine glands—the pituitary
gland, thyroid gland, parathyroid glands, adrenal glands,
and pancreas—are the subject of this chapter
(fig. 13.3)
.
Subsequent chapters discuss several other hormone-secret-
ing glands and tissues.
13.3
HORMONE ACTION
Hormones are released into the extracellular spaces sur-
rounding endocrine cells. From there, they diffuse into the
bloodstream and are carried to all parts of the body.
Chemistry of Hormones
Chemically, most hormones are either steroids or steroidlike
substances—or they are nonsteroids, including amines, pep-
tides, proteins, or glycoproteins. Thus, hormones are organic
compounds. Hormones can stimulate changes in target cells
even in extremely low concentrations.
FIGURE 13.2
Chemical communication. (
a
) Neurons release
neurotransmitters into synapses, af
ecting postsynaptic cells. (
b
) Glands
release hormones into the bloodstream. Blood carries hormone
molecules throughout the body, but only target cells respond.
Hormone secretion
(a)
(b)
Thyroid
gland
Exocrine gland
(sweat gland)
Blood flow
Skin
Exocrine
cells
Duct
Endocrine
gland
Endocrine
cell
FIGURE 13.1
Types oF glands. (
a
) Endocrine glands release hormones
into the internal environment (body ±
uids). (
b
) Exocrine glands secrete
to the outside environment through ducts that lead to body surFaces.
TABLE
13.1
|
A Comparison Between the Nervous System and the Endocrine System
Nervous System
Endocrine System
Cells
Neurons
Glandular epithelium
Chemical signal
Neurotransmitter
Hormone
Speci²
city oF action
Receptors on postsynaptic cell
Receptors on target cell
Speed oF onset
Seconds
Seconds to hours
Duration oF action
Very brieF unless neuronal activity continues
May be brieF or may last For days even iF secretion ceases
Endocrine glands and their hormones help regulate
metabolic processes. They control the rates of certain chemi-
cal reactions; aid in transporting substances through mem-
branes; and help regulate water balance, electrolyte balance,
Target cells
(cells with hormone
receptors) respond
to hormone
Hormones have no
effect on other cells
(a)
(b)
Post-
synaptic
cell
responds
Neuron
transmits
nerve
impulse
Neurotransmitter
released into
synapse
Nerve impulse
Bloodstream
Glandular
cells
secrete
hormone into
bloodstream
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