Snapshot A 40-year-old man is training for a marathon. During a particularly steep hill, he experiences sudden chest pain. As he rests, the chest pain disappears. Later, his physician informs him that his angina is caused by increased myocardial oxygen demand. Introduction Distribution of cardiac output (CO) liver > kidney > muscles > brain liver receives the highest percentage of CO because kidneys are much smaller than the liver, kidneys have the highest percentage of CO per gram of tissue Heart unlike other organs, the heart receives its blood supply during diastole 90% of blood flow through coronary arteries is during diastole during diastole, the blood vessels are open and under low pressure during systole, ventricles contract and the subendocardial coronary vessels are compressed, inhibiting myocardial perfusion as the heart rate increases, the amount of time spent in diastole is relatively decreased myocardial oxygen demand is determined by ↑ contractility ↑ afterload ↑ heart rate ↑ diameter of the ventricle (or wall tension) the heart extracts approximately 100% of oxygen in coronary circulation largest arteriovenous oxygen difference an increase in oxygen demand is supported by an increase in coronary blood flow not an increase in extraction of oxygen from blood like in other organs