Hormones are chemicals that are secreted by endocrine glands. Unlike exocrine glands (see chapter 5), endocrine glands have no ducts, but release their secretions directly into the blood system, which carries them throughout the body. However, hormones only affect the specific target organs that recognize them. For example, although it is carried to virtually every cell in the body, follicle stimulating hormone (FSH), released from the anterior pituitary gland, only acts on the follicle cells of the ovaries causing them to develop.
A nerve impulse travels rapidly and produces an almost instantaneous response but one that lasts only briefly. In contrast, hormones act more slowly and their effects may be long lasting. Target cells respond to minute quantities of hormones and the concentration in the blood is always extremely low. However, target cells are sensitive to subtle changes in hormone concentration and the endocrine system regulates processes by changing the rate of hormone secretion.
Diagram 16.1: The main endocrine organs of the body
The main endocrine glands in the body are the pituitary, pineal, thyroid, parathyroid, and adrenal glands, the pancreas, ovaries and testes. Their positions in the body are shown in diagram 16.1.