thermogenesis The production of heat within tissues to raise body temperature. It occurs especially in birds and mammals, animals that maintain their temperature within a narrow range (i.e. endotherms), but is also found in some ‘cold-blooded’ vertebrates and invertebrates. There are two types of thermogenesis, of which the more familiar is shivering. This involves repeated rapid contractions of antagonistic sets of skeletal muscles, which produce little net movement so that most of the chemical energy (in the form of ATP) is converted to heat rather than mechanical work. Nonshivering thermogenesis takes place in fat cells (adipose tissue) and involves the breakdown of stored fat to generate heat in situ instead of its being transported to the liver for conversion to ATP. This process is activated by the sympathetic nervous system and is accomplished in two ways: nonproductive cyclical active transport of ions across the fat-cell plasma membrane and uncoupling of electron transport from ATP synthesis within the fat-cell mitochondria. The net result is release of energy from existing ATP and oxidation of fat to produce heat instead of ATP. Certain mammals have deposits of a special adipose tissue called brown fat that is adapted to provide the body with bursts of intense heat. Stimulation of brown fat oxidation enables rapid warming during the arousal of hibernating animals, for example. Brown fat deposits are also present in human babies and other neonate mammals, to help protect them against hypothermia.
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