448
U N I T 5
Circulatory Function
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.
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
Blood flow
Systole
Diastole
To heart
muscle
(myocardium)
To heart
muscle
(myocardium)
Coronary
artery
Aortic valve
cusps
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
Myocardial metabolism
and need for blood flow
Endothelial
cell
Release of vasoactive
mediators
Vasoconstricting
factors
Vasodilating
factors
Vasoconstriction
Vasodilation
U N D E R S T A N D I N G
Myocardial Blood
