Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

Myocardial Blood

U N D E R S T A N D I N G

Blood flow in the coronary vessels that supply the myocardium is influenced by (1) the aortic pressure, (2) autoregulatory mechanisms, and (3) compression of the intramyocardial vessels by the contracting heart muscle.

Blood flow

Aortic Pressure. The two main coronary arteries that supply blood flow to the myocardium arise in the sinuses behind the two cusps of the aortic valve. Because of their loca- tion, the pressure and flow of blood in the coronary arteries reflects that of the aorta. During systole, when the aortic valve is open, the veloc- ity of blood flow and position of the valve cusps cause the blood to move rapidly past the coronary artery inlets, and during diastole, when the aortic valve is closed, blood flow and the aortic pressure are transmitted directly into the coronary arteries. 1 Autoregulatory Mechanisms. The heart normally extracts 60% to 80% of the oxygen in the blood delivered to it, leaving little in reserve. Accordingly, oxygen deliv- ery during periods of increased metabolic demand depends on auto- regulatory mechanisms that regu- late blood flow through a change in vessel tone and diameter. During increased metabolic demand, vasodi- lation produces an increase in blood flow; during decreased demand, vasoconstriction or return of vessel tone to normal produces a reduction in flow. The mechanisms that link the metabolic activity of the heart to changes in vessel tone result from vasoactive mediators released from myocardial cells and the vascular endothelium. 2

Coronary artery

Aortic valve cusps

To heart muscle (myocardium)

To heart muscle (myocardium)

Systole

Diastole

Myocardial metabolism and need for blood flow

Endothelial cell

Release of vasoactive mediators

Vasoconstricting factors

Vasodilating factors

Vasoconstriction

Vasodilation