The heart requires a lot of energy, not just to contract but also to maintain ion gradients. The myocardium performs only aerobic (oxygen-dependent) metabolism and not anaerobic (oxygen-independent) metabolism. This means that the myocardium requires large amounts of oxygen.
The oxygen is supplied by the coronary circulation. When the myocardium works harder it requires more energy and therefore more blood, so the coronary circulation must increase.
The myocardium generates most of its energy during rest from breaking down free fatty acids in the blood. During activity the myocardium switches to using mostly lactate instead.
The energy the myocardium uses is directly related to the amount of myocardial muscle strain or tension. It is harder for the muscle to contract when it is under strain, so the muscle needs more energy to contract.
The strain depends on multiple factors:
- The afterload
- The contractility
- The heart rate
When either of these increase the energy demand of the myocardium increases.
19. Cardiac output. Measurement, normal standards and physiological variations
21. Ventricular wall tension and the Laplace relationship