This means tissue change, and includes all the physical and chemical processes by which the living body is maintained – as well as those by which the energy is made available for various forms of work. The constructive, chemical and physical processes by which food materials are adapted for the use of the body are collectively known as ANABOLISM. The destructive processes by which energy is produced by breaking down tissues into waste products is known as CATABOLISM. Basal metabolism is the term applied to the energy changes necessary for every day essential processes such as the beating of the heart, respiration, and maintenance of body warmth. This can be estimated, when a person is placed in a state of complete rest, by measuring the amounts of oxygen and carbon dioxide exchanged during breathing under certain standard conditions. (See also CALORIE.)