Normal coronary blood flow at rest is about 250 mL (70 mL/100 g tissue/min), which is approximately 5% of the resting cardiac output (5 L). During exercise, coronary blood flow can increase three to six times. The right coronary artery supplies a larger portion of the heart in about 50% of the population. The left coronary artery is dominant in only 20% of people, while the remaining 30% have nearly equal blood flow in both coronary arteries.
Coronary Blood Flow in Different Phases of the Cardiac Cycle
The blood flow in the right and left coronary arteries varies during different phases of the cardiac cycle. For the left coronary artery, blood flow is higher during diastole than systole. During systole, the contraction of the heart muscle compresses the left coronary artery, reducing blood flow. This compression is relieved during diastole, allowing for increased blood flow. In the diagram below, blood flow is zero during isovolumic contraction. In contrast, the right coronary artery receives blood flow during both the systolic and diastolic phases.
Regulation of Coronary Circulation
Chemical Regulation of coronary circulation
Coronary blood flow is highly sensitive to changes in the concentration of metabolites associated with tissue activity. The accumulation of various metabolites in the myocardial interstitial fluid causes vasodilation. Chemical regulation of coronary blood flow is significantly more critical than neural regulation. As heart rate and contractility increase, local chemicals such as H+, CO2, lactic acid, ADP, and AMP accumulate, leading to the dilation of coronary blood vessels.
Neuronal Regulation of coronary circulation
The coronary arteries are innervated by both sympathetic and parasympathetic autonomic nerves.
Sympathetic Nervous System : The coronary arteries have both alpha and beta adrenergic receptors, with beta receptors predominating.
Beta receptor stimulation causes vasodilation, while alpha receptor stimulation causes vasoconstriction. Overall, sympathetic stimulation results in vasodilation of the coronary vessels.
Sympathetic stimulation has both direct and indirect effects:
Direct Effect : Stimulation of beta receptors in the coronary vessels leading to vasodilation.
Indirect Effect: Sympathetic stimulation of the heart increases heart rate and contractility, raising the metabolic requirements of the heart. This leads to a decrease in local PO2, an increase in PCO2, a rise in hydrogen ion concentration, and the accumulation of various metabolites, which have a potent vasodilatory effect on the coronary vessels.
Parasympathetic Nervous System:
There are relatively few parasympathetic (vagal) fibers supplying the coronary arteries. The direct effect of these fibers is coronary vasodilation.
However, the indirect effect is more significant. Vagal stimulation slows the heart rate and decreases cardiac contractility, reducing the formation of metabolites in the cardiac muscle and leading to vasoconstriction. Since this indirect effect is stronger than the direct effect, the overall result of parasympathetic stimulation is coronary vasoconstriction.