12 - Endocrine systems

Abstract

This chapter discusses the characteristics and functioning of endocrine systems. Hormones, or endocrine secretions, have traditionally been defined as chemical substances secreted directly into the bloodstream from a specialized tissue and producing effects on distant organs. They are a pathway of communication alternative to the nervous system, differing from it in being slower, but at the same time more suitable for widespread integrated activity in a number of effector organs or cells. Endocrine systems are suitable for mediating changes in activity of many organs together, particularly when the responses are required in minutes, or hours, or days, rather than in fractions of a second. In general, hormones increase or decrease the activity of target organs or cells; they rarely initiate activities. Although many hormones are secreted at a low basal rate, few maintain a constant level of production as their secretion is adjusted in response to nervous stimuli or to changing conditions around the secreting glands. Hormones are messenger substances, passing information from one cell to another, usually through the circulatory system, and often over great distances. A number of hormones are protein or glycoprotein molecules that are able to travel easily in the bloodstream without the need for carrier molecules..