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Blood flow4/28/2023 ![]() Progressive ischemic coronary artery disease results in the growth of new vessels (termed angiogenesis) and collateralization within the myocardium. ![]() However, if parasympathetic activation of the heart results in a significant decrease in myocardial oxygen demand due to a reduction in heart rate, then intrinsic metabolic mechanisms will increase coronary vascular resistance by constricting the vessels. Parasympathetic stimulation of the heart (i.e., vagal nerve activation) elicits modest coronary vasodilation (due to the direct effects of released acetylcholine on the coronaries). This is termed "functional sympatholysis." Nitric oxide is also an important regulator of coronary blood flow.Īctivation of sympathetic nerves innervating the coronary vasculature causes only transient vasoconstriction mediated by α 1-adrenoceptors. This brief (and small) vasoconstrictor response is followed by vasodilation caused by enhanced production of vasodilator metabolites ( active hyperemia) due to increased mechanical and metabolic activity of the heart resulting from β 1-adrenoceptor activation of the myocardium. Therefore, sympathetic activation to the heart results in coronary vasodilation and increased coronary flow due to increased metabolic activity (increased heart rate, contractility) despite direct vasoconstrictor effects of sympathetic activation on the coronaries. 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.Īdenosine is an important mediator of active hyperemia and autoregulation. It serves as a metabolic coupler between oxygen consumption and coronary blood flow. The following summarizes important features of coronary blood flow:įlow is tightly coupled to oxygen demand. This is necessary because the heart has a very high basal oxygen consumption (8-10 ml O 2/min/100g) and the highest A-VO 2 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. This anatomic distribution is important because these cardiac regions are assessed by 12-lead ECGs to help localize ischemic or infarcted regions, which can be loosely correlated with specific coronary vessels however, because of vessel heterogeneity, actual vessel involvement in ischemic conditions needs to be verified by coronary angiograms or other imaging techniques. There are also anterior cardiac veins and thesbesian veins drain directly into the cardiac chambers.Īlthough there is considerable heterogeneity among people, the following table indicates the regions of the heart that are generally supplied by the different coronary arteries. Capillary blood flow enters venules that join together to form cardiac veins that drain into the coronary sinus located on the posterior side of the heart, which drains into the right atrium. A high capillary-to-cardiomyocyte ratio and short diffusion distances ensure adequate oxygen delivery to the myocytes and removal of metabolic waste products from the cells (e.g., CO 2 and H +). The arterioles branch into numerous capillaries that lie adjacent to the cardiac myocytes. As in all vascular beds, it is the small arteries and arterioles in the microcirculation that are the primary sites of vascular resistance, and therefore the primary site for regulation of blood flow. When the vessels are not diseased, they have a low vascular resistance relative to their more distal and smaller branches that comprise the microvascular network. These vessels distribute blood flow to different regions of the heart muscle. The left and right coronary arteries and their branches lie on the surface of the heart, and therefore are sometimes referred to as the epicardial coronary vessels. ![]() ![]() The left and right coronary arteries originate at the base of the aorta from openings called the coronary ostia located behind the aortic valve leaflets. The major vessels of the coronary circulation are the left main coronary that divides into left anterior descending and circumflex branches, and the right main coronary artery.
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