Co-ordination of body functions in animals is effected among other thins by hormones. Hormones are chemical compounds produced by the ductiess glands. Hormones are chemical messengers. They are discharged in the blood from where they reach different parts of the body. In general, a hormone will go to a particular organ and influence its functions. The organ which a hormone influences is called the target organ of that hormone.

In a way, a hormone acts as a trigger or switch. Endocrine glands are directly or indirectly controlled by the nervous system. The latter receives information from the outer world in the form of stimuli. Hormone released from a ductless gland (endocrine gland) may remain effective for a short while. Secretion of hormone depends upon whether an endocrine gland has received any message from the internal and external sources or factors. Even the target organs can have a measure of control on the endocrine gland which is termed negative feedback mechanism, as it tells the endocrine gland not to secrete the hormone in far excess amount.