Flow is tightly coupled to oxygen demand. This is necessary because the heart has a very high basal oxygen consumption (8-10 ml. O2/min/100 g.), and the highest A-VO2 difference of a major organ (10-13 ml./100 ml). In non-diseased coronary vessels, whenever cardiac activity and oxygen consumption increases, there is an increase in coronary blood flow (active hyperemia) that is nearly proportionate to the increase in oxygen consumption.
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 a1-adrenoceptors. This brief (and small) vasoconstrictor response is followed by vasodialation caused by enhanced production of vasodialator metabolites (active hyperemia) due to increased mechanical and metabolic activity of the heart resulting from b1-adrenoceptor activation of the myocardium. Therefore, sympathetic activation to the heart results in coronary vasodialaton and increased coronary flow due to increased metabolic activity (increased heart rate, contractility), despite direct vasoconstrictor effects of sympathetic activation on the coronaries. This is termed “functional sympatholysis”.
Venous drainage refers to the route by which blood leaves an organ. After flowing through capillaries of the myocardium, about 20 % of the coronary blood empties directly from small veins into the right ventricle. The other 80% returns to the right atrium by the following route: the great cardiac vein collects blood from the anterior aspect of the heart and travels alongside the anterior interventricular artery. It carries blood from the apex of the heart toward the atrioventricular sulcus. The middle cardiac vein, found in the posterior sulcus, collects blood from the posterior aspect of the heart. It too carries blood from the apex and upward. The coronary sinus collects blood from these and smaller cardiac veins. It passes across the posterior aspect of the heart in the coronary sulcus and empties blood into the right atrium.
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