The microcirculation of the coronary vasculature functions as a penetrating vascular network running from epicardium to endocardium with direct communications of both arterioles and venules with the ventricular chambers. The degree of capillary flow and utilization reflect the fact that the heart is a constantly functioning muscle. The cyclical compression of this vascular system also sets it apart from the conditions in the remainder of the systemic circulation. There are considerable species differences in the number of collateral vessels. Response to stress, such as hypoxia or arterial narrowing, involves conversion of collateral microvessels into communicating arteries, a reaction which can be seen to occur within days of the initial stimulus.