The term metabolism in its broad sense may be defined as the sum of all the chemical changes which occur in the living organism.  Defined in this way, it includes practically all of physiology and biochemistry, and much of the present volume could be included under this heading. In a more limited sense, how ever, the term is used to designate the processes whereby the foodstuffs taken into the organism are utilized to form the constituents of the body, to yield the heat necessary for maintaining a constant body temperature, and to furnish the energy necessary for maintaining the cellular and other manifold activities which characterize the living organism.

To discuss the metabolic activities of the body one may follow the course of given constituent trough the changes which it undergoes from its ingestion to its elimination. Thus the metabolism of the foodstuffs which serve as sources of energy and as basic materials of the synthesis of cellular protoplasm are considered in this light in Chaps 2, 3, and 4. One may also follow the course of individual substances such of water, sodium, potassium, calcium, iodide, or other metabolites, as is done in subsequent chapters where they are considered separately or in connection with the organs with which they are most concern. The present chapter is concerned primarily with the total energy exchange of the body as a whole and with certain aspects of metabolism not considered elsewhere.

The animal organism may be looked upon as a complex chemical machine, each unit of which is coordinated in its activity with her the remainder of the organism. Characteristic of the living organism, as opposed to nonliving chemical system, are the multitude of reactions occurring simultaneously at body temperature.