These epicardial arteries then give off smaller branches that dive into the myocardium and become the microvascular resistance vessels that regulate coronary blood flow and its distribution. These vessels then give rise to a dense capillary network so that each cardiac myocyte is has several capillaries running parallel to the muscle fiber. Good autoregulation between 60 and 200 mmHg perfusion pressure helps to maintain normal coronary blood flow whenever coronary perfusion pressure changes due to changes in aortic pressure. Adenosine is an important mediator of active hyperemia and autoregulation. It serves as a metabolic coupler between oxygen consumption and coronary blood flow.
Nitric oxide is also an important regulator of coronary blood flow.
Activation of sympathetic nerves innervating the coronary vasculature causes only transient vasoconstriction mediated by a 1 -adrenoceptors . Progressive ischemic coronary artery disease results in significant vessel growth and collateralization within the myocardium. Collateralization increases myocardial blood supply by increasing the number of parallel vessels thereby reducing vascular resistance within the myocardium. Because of extravascular compression, the endocardium is more susceptible to ischemia especially at lower perfusion pressures.