ABSTRACT
The endocrine system represents one of the two major, interconnected, control systems in the body, the other being the nervous system. The distribution of endocrine tissues in the body is shown in Fig. 1. The system involves the regulation of organ activity by chemicals (hormones) synthesized elsewhere in the body, and a hormone may be defined as a molecule which is synthesized in a tissue (endocrine tissue), secreted into the bloodstream, and acts upon a target tissue to regulate metabolic processes (e.g. the rate of metabolic reactions, transmembrane transport including secretion, protein synthesis, cell growth and division). Such effects can be short lived, lasting for just a few seconds, or more chronic, lasting for days or more. The magnitude of the response is concentration related, and the sensitivity of a tissue to a given hormone concentration is governed by the number of receptors in the target tissue. The precise mode of action of the hormone varies with its structure, but will invariably involve the interaction of the hormone with a specific receptor, often a large polypeptide, located on the target cell surface or in the cytoplasm. This interaction initiates an intracellular response in the target cell. The release of a hormone itself usually represents the last step in a cascade of reactions initiated in the brain (hypothalamus) in response to external stimuli such as pain, depression, smell, visual stimuli, etc., or to metabolic stimuli such as metabolite, electrolyte and hormone
concentration in the blood. In this way, the hypothalamus acts as a collector of information regarding a number of processes that control whole-body homeostasis (see Topic A2), and will act to restore homeostasis when this is disturbed. The hypothalamus signals to the endocrine tissue via the pituitary gland, and gives rise to the concept of the hypothalamic-pituitary-end-organ axes responsible for the control of thyroid, adrenal, sex gland, and probably also alimentary functions.
