The simplest endocrine system consists of an endocrine gland that secretes a hormone, the hormone itself and a target tissue that responds to the hormone. Most endocrine systems are considerably more complex than this. A given hormone may be secreted by more than one endocrine gland, a hormone may affect many different tissues, several endocrine glands may be functionally connected by hormones within a system, and the result of the action of a hormone on its target tissue may influence the secretion of that hormone. The basic function of any endocrine system is still that of regulation. The endocrine systems also provide coordination of function in a temporal sense. This type of regulation is particularly apparent in reproductive endocrinology, in which normal function requires extremely specific patterns of change in the secretion of various hormones.
Physiological organization is brought about by higher centers of the brain, the hypothalamus, of the nervous system and by chemical substances in the circulatory fluids which are carried everywhere in the body, bringing about local changes, inequilibruim of physical and chemical conditions and affecting correlations of these changes in a bodywide way.
The functions of the body are regulated by the nervous system and by hormones. In general, the nervous system regulates the rapidly changing activities such as skeletal movements, smooth muscle contraction, and many glandular secretions. The hormonal system regulates the many metabolic functions of the body and the varying rates of chemical reactions. Hormones influence transport of substances through cell membranes and various aspects of cell metabolism and tissue growth. In some instances there are specific interrelationship between nervous stimuli and hormonal secretion. There are also many interactions between hormones, so that a disturbance in one endocrine gland can interfere with activities of other hormones. Hormones are specific in action and for specific cells respond.
In general, hormones are secreted into extracellular fluids and have effect on distant organs. In local hormones some physiologically active substances are released from specific sites in tissue.