Body Functions. Body functions are the physiological or psychological functions of body systems. The body's functions are ultimately its cells' functions. Survival is the body's most important business.Survival depends on the body's maintaining or restoring homeostasis, a state of relative constancy, of its internal environment.ts major activities or functions are responding to changes in the body's environment, exchanging materials between the environment and cells, metabolizing foods, and integrating all of the body's diverse activities. The body's ability to perform many of its functions changes gradually over the years.