Chapter #39 The endocrine and Reproductive Systems

A gland is an organ that makes and releases a secretion. Exocrine
glands release their secretions through ducts directly to the organs
that use them. Endocrine glands release hormones into the bloodstream.
The endocrine system is made up of endocrine glands.
Hormones are chemicals made in one part of the body that
affect cells elsewhere in the body. Hormones bind to target cells.
Target cells are specific chemical receptors on cells.

There are two types of hormones: steroid hormones and
nonsteroid hormones. Steroid hormones can cross cell membranes
of target cells, enter the nucleus, and turn genes on or off.
Nonsteroid hormones cannot cross cell membranes. Compounds
called secondary messengers carry the messages of nonsteroid
hormones inside target cells.

All cells, except for red blood cells, produce hormonelike substances
called prostaglandins. Prostaglandins usually affect only
nearby cells and tissues. They are known as “local hormones.”
The endocrine system is controlled by feedback mechanisms
that help maintain homeostasis. For example, the level of a
hormone in the blood may be the feedback that signals a gland to
make more or less of the hormone. Two hormones with opposite
effects may work together to maintain homeostasis. This is called
complementary hormone action.

39–2 Human Endocrine Glands
There are several endocrine glands scattered throughout the body.
• The pituitary gland secretes nine hormones that regulate body
functions and control the actions of other endocrine glands.
• Hormones from the hypothalamus control the secretions of
the pituitary gland.
• Hormones from the thyroid gland regulate metabolism.
• Hormones from the thyroid gland and parathyroid glands
maintain blood calcium levels.
• The adrenal glands make hormones that help the body
prepare for and deal with stress.
• The pancreas is both an exocrine gland and an endocrine
gland. Hormones produced in the pancreas help keep levels
of glucose in the blood stable.
• Reproductive glands, or gonads, make gametes and secrete
sex hormones. The female gonads, ovaries, produce eggs.
The male gonads, testes, produce sperm.

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