Page 18, O2 Uptake, Exercise & Cor. Circ., Dr. D. Penney

Coronary Circulation (continued..):

At rest, only about 1/3 of capillaries are perfused at any given time. During exercise, or any condition which increases myocardial energy utilization, coronary vasodilation takes place by mechanisms to be discussed below, increasing blood flow. Part and parcel of the increase in the caliber of small arteries, arterioles, etc. is capillary recruitment which increases the percent of capillaries being perfused. This serves to further decrease intercapillary and mean diffusion distances. With maximal recruitment, intercapillary and diffusion distances may be decreased to 6.5 and 3.3 um, respectively.

Contractile state or inotropicity, heart rate, and wall tension or stress are three major determinants of myocardial oxygen demand, hence coronary flow. An increase in one or more of these parameters increases the heart's need for oxygen. Wall tension is related to diastolic dimension and ventricular pressure through the Law of Laplace, T = P x R. Wall stress is simply wall tension normalized to wall thickness. Afterload, external work, electrical activation and depolarization, and basal metabolic requirements are minor determinants of oxygen utilization.