LEARNING OBJECTIVE: Identify endocrine glands and the hormone(s) they produce, and determine the effect each hormone has on the body.

Homeostasis, the self-balancing of the body's internal environment, is achieved and maintained by the endocrine system and the nervous systems. These systems work alone and together to perform similar functions in the body: communication, integration, and control. Their communication capabilities provide the means for controlling and integrating the many different functions performed by organs, tissues, and cells. The endocrine system, however, performs these functions by different mechanisms than the nervous system.

The endocrine system sends messages by way of chemical messengers called hormones. Minute amounts of these hormones are secreted from endocrine gland cells into the blood and distributed by the circulatory system. Endocrine glands secrete hormones directly into the blood, because they have no duct system. The glands of this system are often called ductless glands. Cells that are affected by the hormone are referred to as target organ cells.

Today, many hormones can be extracted from the glands of animals or produced synthetically. Medical officers may prescribe these naturally derived or synthetic hormones for patients who are deficient in them or who might otherwise benefit from their use. For example, oxytocin (the hormone which stimulates uterine contractions during pregnancy) has been synthesized and is used during the delivery process for women who are deficient in this hormone.

The hormone-producing glands include the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, and gonads (the testes and ovaries) (fig. 1-47).

HYPOTHALAMUS
The hypothalamus, a structure in the brain, synthesizes chemicals that are secreted to the pituitary

CRANIAL NERVE FUNCTION(S)

Table 1-3.-Cranial Nerves

gland to stimulate the release of its hormones and to help regulate body temperature (fig. 1-47).