The
pituitary gland, about the size of a pea, hangs from a short stalk at the base of the
brain. It is sometimes called the "master gland" because it has a
legion of functions and appears to exert a controlling and coordinating influence
over other glands, which respond to the hormones it secretes. The pituitary is
divided into three lobes and secretes at least nine known hormones.
Hormones from the posterior
lobe (e.g. pituitary) act to stimulate smooth-muscle tissue, regulate water
balance, and help control kidney function. Dysfunction in this lobe can
initiate the "thirsty disease," diabetes insipidus, in which a patient may drink and void as
much as 10 gallons of water a day.
The
intermediate lobe of the pituitary gland secretes one known hormone, which
possibly helps regulate skin color.
The anterior
lobe of the pituitary supplies the hormones that spark normal growth and
development and that specifically stimulate other endocrine glands. One of
these, for example, influences body size. Too much of it can produce a
gigantic stature; too little, a dwarf. Other anterior pituitary hormones have
to do with the distribution and storage of body fat; imbalance here can cause
the gross obesity of the "fat lady."
The complicated interaction
of the pituitary with other glands should be kept in mind. Not only do
pituitary hormones stimulate other endocrine glands; their hormones also act
upon it. ACTH, for example, is a pituitary hormone. It stimulates the adrenal
glands. Furthermore, hormones produced by the ovaries act upon the pituitary
gland, and vice versa.
Gonads
The
testes produce sperm and the ovaries mature ova, but both the male and the
female gonads also function as endocrine glands. Their role in the reproductive
process will be discussed in a later chapter (page 000). To summarize briefly:
the testes produce the male sex hormone, testosterone, which promotes the
development of the secondary sex characteristics of the male. The ovaries elaborate
several female sex hormones. These are concerned not only with the development
of the secondary sex characteristics of the female but also with the rhythmic
regulation of the menstrual cycle and the whole process of readying the female
body for reproduction and childbirth.
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