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The endocrine system, along with the
nervous system, functions in the regulation of body
activities. The nervous system acts through electrical
impulses and neurotransmitters to cause muscle contraction
and glandular secretion. The effect is of short duration,
measured in seconds, and localized. The endocrine system
acts through chemical messengers called hormones that
influence growth, development, and metabolic activities.
The action of the endocrine
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system is measured in minutes, hours, or weeks and is more generalized
than the action of the nervous system.
There are two major categories of glands in the body - exocrine
and endocrine.
Exocrine Glands
Exocrine glands have ducts that carry their secretory product
to a surface. These glands include the sweat, sebaceous,
and mammary glands and, the glands that secrete digestive
enzymes.
Endocrine Glands
The endocrine glands do not have ducts to carry their product
to a surface. They are called ductless glands. The word endocrine
is derived from the Greek terms "endo," meaning within, and
"krine," meaning to separate or secrete. The secretory products
of endocrine glands are called hormones and are secreted directly
into the blood and then carried throughout the body where
they influence only those cells that have receptor sites for
that hormone.
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