Critical power (CP) is a fundamental concept describing fatigue and exhaustion. The main physiological determinant of CP is the ability to utilise oxygen. This in turn is dependent primarily on diffusion distance. During exercise, many different tissue systems must increase their metabolic demand. It is argued that each tissue system, such as cardiac, respiratory and leg muscles, has their own CP. Cardiac muscle has the greatest CP relative to its maximum power because it has the shortest diffusion distances. Respiratory muscle also has a substantially higher relative CP than leg muscle. The higher relative CPs of cardiac and respiratory muscle are due in part to the homeostatic functions these tissues provide. This built in protective design can be disrupted in certain conditions such as hypoxia. During high intensity exercise, fatigue and ensuing exhaustion will occur if any contributing physiological system functions above its CP.