Physiology describes the vital functions of living organisms and their organs, cells and molecules. Medical physiology takes a global view of human body, but in doing so requires an integrated understanding of events at the level of molecules, cells, and organs. Physiology is concerned with the function of organisms at many stages of organisation, from subselluler level to the intact organism. In the healty human, many variables are maintained within narrow limits. The list of controlled variables includes body temperature, blood pressure, the ionic composition of the body's various fluid compartments, the blood glucose levels, and the oxygen and carbondioxide concents of the blood.This ability to maintain the relative constancy of such critical variables, even face of substantial environmental changes, is known as homeostasis. One of the central goals of physiological research is the elucidation of homeostatic mechanisms.