Perfusion of the heart muscle is more active than in skeletal muscle and other avian tissues. As in mammals, perfusion (the passage of fluid through the lymphatic system or blood vessels to an organ or a tissue) occurs during diastole (the normal rhythmical dilatation of the heart during which the chambers fill with blood). The reduction in oxygen supply and increase in myocardial oxygen demand results in an increase in coronary blood flow. These factors come into play in birds that fly at high altitudes.