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Endocrine system
Endocrine system: Body organs whose only known role is to
produce chemicals that regulate bodily functions are called
glands. Although the parts of the endocrine (EN-duh-krin)
system of glands are located all over the body, they are
connected into a single system by the bloodstream. Regulatory
chemicals produced by endocrine glands are known as hormones.
Size and location None of the endocrine glands are very large;
most are the size of a walnut or smaller, although the
pancreas is considerably larger. However, the part of the
pancreas that is an endocrine gland consists of the tiny
islets of Langerhans.
In some cases the location of an endocrine gland seems to
be significant, but other glands are thought to be situated
simply where they evolved. The pituitary is apparently lodged
in the brain because many of its functions are directly
controlled by brain structures.
Role determination The functions of the various glands have
been determined in various ways. Sometimes a gland is removed
in a human or an animal. The changes that result can then be
observed and attributed to the loss of the gland. Sometimes
a gland produces too much of its product, often because it
is enlarged or because there is a tumor on it. Again, the
changes reveal the action of the gland products (many glands
have more than one product).
Endocrine glands and their functions Adrenals Help regulate
blood pressure, blood sugar, and the sex drive; partially
control metabolism
Ovaries Produce hormones that regulate pregnancy, induce
female secondary sexual characteristics, and help control
calcium uptake into bones
Pancreas Controls blood sugar; the pancreas is also part of
the digestive system, acting as an exocrine gland that releases
into the small intestine chemicals that break down fats,
carbohydrates, and proteins
Parathyroids Regulate the release of stored calcium from
bones
Pineal Responds to light and helps regulate reproduction
Pituitary Under control of the brain, this "master gland"
produces hormones that control many other glands; produces
growth hormone
Testes Produce sperm and secondary sexual characteristics in
males
Thyroid Regulates metabolism, growth, and calcium uptake by
bones
Conditions that affect the endocrine system While nearly all
of the disorders of the endocrine system come from too
little or too much production of a particular hormone,
sometimes the effect from one hormone interferes with the
operation of other hormones. Notably, the pituitary regulates
the thyroid, testes or ovaries, and to some extent, the
adrenals. Too little or much of any one hormone can cause
significant effects in almost all parts of the body. A prime
example is diabetes mellitus, caused by too little insulin
action (although not always by too little insulin). Most
other diseases of this type are discussed in the entry
Hormone disorders.
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