A gland is an organ which produces a secretion, such as a hormone which may enter the bloodstream, a juice which enters the digestive tract or a substance such as sweat which enters the external environment. Other terms used in this connection are the adjectives endocrine and exocrine, endocrine referring to internal secretions or hormones, and exocrine to the production of juices to be delivered to the outside world. The tube of the intestine is regarded as the outside world, since it connects with it at either end.
The term hormone comes from the Greek hormao, meaning to urge on or excite. Hormones are released into the bloodstream from one tissue, and cause an effect in another. But the way in which they exert their effect must be distinguished from that of a metabolite - the ketone body, 3-hydroxybutyrate, for example. This substance is also produced in one organ (the liver) and causes an effect (uptake and oxidation) in another - e.g. skeletal muscle. But the essence of hormone action is that the hormone affects substances other than itself, typically by causing regulation of a metabolic pathway. Hormones consist of a variety of types of chemical: peptides, glycoproteins, steroids and other small molecules, mostly derivatives of amino acids. At their target tissue they act by binding to a specific receptor, itself a protein (Section 2.3). For peptide and protein hormones this receptor is usually an integral protein of the cell membrane. For steroid hormones and thyroid hormones it is within the cell; the hormone/ hormone receptor complex enters the nucleus (or is formed in the nucleus) and affects DNA transcription, and thus synthesis of specific proteins.
The endocrine and exocrine glands are not in themselves major consumers of energy relative to other tissues in the body. But clearly their products have important effects in regulating energy supply and storage in the body. Here the
major hormone-producing glands, and some exocrine tissues, relevant to energy metabolism will be considered. The location of the glands to be discussed is shown in Fig. 5.1.
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