The heart uses a variety of fuels-mainly fatty acids but also glucose, lactate, and ketone bodies. Metabolism of heart muscle differs from that of skeletal muscle in three important respects.

1. The variation in work output is far less than that seen in skeletal muscle. That is, the heart must work steadily and continuously in order to keep the organisms alive.

2. The heart is a completely aerobic tissue, whereas skeletal muscle can function anaerobically for limited periods. Mitochondria are much more densely packed in heart than in other cells, making up nearly half the volume of a heart cell.

3. The heart contains negligible energy reserves as glycogen or lipid, although there is a small amount of creatine phosphate.

The supply of both oxygen and fuels from the blood to the heart must be continuous to meet its unending energy demands.