Coronary
Circulation
(Text Resource = Textbook of Physiology.
Volume 2., Howell, Fulton, Ruch, and Patton, W.B. Suanders Co.,
1989)
Vasculature
Coronary Blood Flow
The myocardium is supplied by two arteries that branch from the
base of the aorta. These arteries are the Right and Left Coronary
Arteries, (RCA and LCA). These arteries further branch into
circumflex, anterior and posterior arteries. The terminal end of
the arterioles feed into the capillary beds. The capillary beds
traverse through all of the myocardial tissue with the terminal
point at the endocardial tissue. Myocardial capillaries have a
density rate of 3300 per mm3, which allows the
myocardium has have the greatest rate of perfusion to surface area
of any organ in the body. Blood then collects and flows outward
towards the epicardium to epicardial veins that empty into the
right atrium. There are, however, some small vessels that drain
directly into the heart chamber without passing through the
epicardial veins.
Normal Blood Flow at Rest
Normal resting coronary blood flow ranges from 0.6 to 0.8
ml/min per gram of myocardium. Blood flow, per unit mass at the
right ventricle is 2/3 that of the left. The difference is
probably due to reduced tension development of the right
ventricle, therefore reducing oxygen demand. Likewise, the flow to
the atria is about 1/2 of the left ventricle. Coronary blood flow
is maintained at a constant rate through changes in coronary
arterial pressure. Therefore, there is a stable flow rate
throughout a relatively wide range in pressure change. See graphic
below.
Note that there is a relatively
constant blood flow throughout a wide range of change in coronary
arterial pressure. Also, note that the myocardial metabolism
alters the autoregulatory mechanism.
Coronary autoregulation allows for a sustained coronary flow
(perfusion) at a variance of coronary pressures. Once above or below
the autoregulatory ranges, coronary blood flow becomes dependent on
coronary pressure.
Coronary Tissue Pressures
Systolic Coronary Perfusion
The myocardial tissue is compressed and nearly stops arterial
blood inflow to the tissue. The intramyocardial pressure generated
during systole is greatest in the endocardium where the systolic
endocardial pressure approximates left systolic pressure. The
pressure gradient increases from the endocardial tissues to the
epicardial tissues. As a consequence, the epicardial tissues may
be perfused during systole whereas the endocardial tissues are
only perfused during diastole. Conversely, the venous outflow
from endo-to-epicardial tissues is accelerated during systole.
Diastolic Coronary Perfusion
The pressure gradient established during diastole is dependent
upon aortic pressure and right atria pressure. Perphaps, more
specifically, between the resting myocardial transmural pressure
and aortic pressure. The transmural pressure is the resistive
pressure to endocardial perfusion, where as the aortic pressure is
the driving pressure for endocardial perfusion. Endocardial tissue
will have the greatest transmural tension because of the
compressive stress of the resting heart tissue. Thus, the
endocardial tissue is most difficult, from a pressure gradient
perspective, to perfuse during systole and diastole. Under normal
physiological conditions, endocardial perfusion is easily met by a
sufficient aortic pressure gradient. Under pathological
conditions, with an increase in aortic or ventricular chamber
pressure, especially during diastole, endocardial perfusion can be
compromised.
Local Metabolic Regulation of Coronary Blood
Flow
Myocardial Oxygen Metabolism
Neural Control
Reflex Control
Exercise