The blood and oxygen needs of the heart are considerable. At rest, the left ventricle uses twenty times as much oxygen as skeletal muscle and on effort its consumption is five times greater again. The level of extraction of oxygen from coronary blood by the myocardium is very high at rest and thus can only increase by limited amounts on effort. Coronary venous blood is the most desaturated in the body. Furthermore, the anaerobic capacity of the heart is limited. As a result, the increased oxygen needs of the heart on effort can be dealt with only an increase in coronary flow. There is in fact a balance between myocardial oxygen supply and needs.