Regulation of coronary blood flow.

A very schematic summary of possible factors that affect coronary vascular resistance is illustrated in Fig. 11. These factors are mechanical, myogenic, metabolic, and neural. Mechanical influence is passive and is essentially determined by the extravascular compression which is important only during systole and particularly in the subendocardial layers of the left ventricular myocardium. With respect to the myogenic factor it is controversial whether it plays a role in adjustments of coronary vascular tone. Metabolic processes are possibly integrated at the level of the myocardial cell PO2 which is the resultant of oxygen supply and need, which in turn depend on the myocardial contractile activity as well as other biochemical processes. A decrease in myocardial PO2 gives rise to the release of the vasodilator metabolite adenosine. Nevertheless, other chemical factors (H+, K+, osmolarity) known to be released by the heart (which by themselves are poor and transient coronary vasodilators and whose release does not correspond with changes in coronary resistance) may play a role by modulating the coronary sensitivity to adenosine and other factors.