2. the sum of the physical and chemical processes by which living organized substance is built up and maintained (
anabolism), and by which large molecules are broken down into smaller molecules to make energy available to the organism (
catabolism). Essentially these processes are concerned with the disposition of the nutrients absorbed into the blood following digestion.
There are two phases of metabolism: the anabolic and the catabolic phases. The anabolic, or constructive, phase is concerned with the conversion of simpler compounds derived from the nutrients into living, organized substances that the body cells can use. In the catabolic, or destructive, phase these organized substances are reconverted into simpler compounds, with the release of energy necessary for the proper functioning of the body cells.
The rate of metabolism can be increased by exercise; by elevated body temperature, as in a high fever, which can more than double the metabolic rate; by hormonal activity, such as that of thyroxine, insulin, and epinephrine; and by specific dynamic action that occurs following the ingestion of a meal.
The basal metabolic rate refers to the lowest rate obtained while an individual is at complete physical and mental rest. Metabolic rate usually is expressed in terms of the amount of heat liberated during the chemical reactions of metabolism. About 25 per cent of all energy from nutrients is utilized by the body to carry on its normal function; the remainder becomes heat.