Page 21, O2 Uptake, Exercise & Cor. Circ., Dr. D. Penney

Coronary Circulation (continued..)

A major source of flow resistance is the contraction of the heart itself. Tension, thus pressure, which is generated in the wall tends to collapse arteries and veins. This factor ordinarily accounts for 1/4 to 1/3 of coronary vascular resistance. Thus at resting heart rate only 15% of coronary flow occurs during systole in the left ventricle, the remainder occurring during diastole (Figure 3.08). However at very high heart rates, 50% of flow occurs during systole, since diastole shortens far more than systole as heart rate increases. Coronary flow in the right ventricle with its far lower pressure is affected to a much smaller extent by compressive forces.

Therefore, flow through the coronary circulation is phasic and rather peculiar, not steady. Flow is not dependent on the difference between the arterial and venous pressures, but rather depends on the difference between the arterial and ventricular luminal pressures. Incomplete relaxation of the ventricle as in myocardial infarction, failure, etc. causes luminal pressure to remain high, which as well as slowing filling, also impedes coronary flow.