ABSTRACT
INTRODUCTION The heart depends principally on aerobic metabolism as it can obtain a maximum of 7% of its adenosine triphospate (ATP) requirement through anaerobic glycolysis. In contrast to skeletal muscle, the cardiac muscle never stops working and pumps blood with approximately 60-70 beats per minute under resting conditions. Consequently, oxygen consumption normalized per gram of myocardium is 20-fold higher than that of skeletal muscle. One way the heart has adapted to the high oxygen demand is by achieving a high level of oxygen extraction, reaching as much as 70-80% of the arterially delivered oxygen, as compared to 30-40% in skeletal muscle. This high level of oxygen extraction is facilitated by a high capillary density: 3000 per mm2 compared to 400 per mm2 in skeletal muscle. Since the heart extracts already 70-80% of the delivered oxygen, it follows that a 4-5-fold increase in oxygen demand, as occurs during maximum exercise, can only be met by an increase in oxygen delivery, i.e. an increase in coronary blood flow (CBF). Indeed, despite the high resting flows in the heart (0.7-0.8 ml/min/g of myocardium, compared to 0.05-0.10 ml/min/g of skeletal muscle), coronary blood flow can still increase 4-5-fold during exercise.
