respectively (see Figures
12–6 and 12–7). These valves are also referred to as the semi-
lunar valves, due to the half-moon shape of the cusps. These
valves permit blood to ﬂ
ow into the arteries during ventricular
contraction but prevent blood from moving in the opposite
direction during ventricular relaxation. Like the AV valves,
they act in a purely passive manner. Whether they are open
or closed depends upon the pressure differences across them.
Vessels supplying the heart with oxygenated blood originate
from behind the cusps of the aortic valve.
Another important point concerning the heart valves
is that, when open, they offer very little resistance to ﬂ ow.
Consequently, very small pressure differences across them suf-
ﬁ ce to produce large ﬂ ows. In disease states, however, a valve
may become narrowed so that it offers a high resistance to
ﬂ ow even when open. In such a state, the contracting cardiac
chamber must produce an unusually high pressure to cause
ﬂ ow across the valve.
There are no valves at the entrances of the superior and
inferior venae cavae (plural of vena cava) into the right atrium,
and of the pulmonary veins into the left atrium. However,
atrial contraction pumps very little blood back into the veins
because atrial contraction constricts their sites of entry into the
atria, greatly increasing the resistance to backﬂ ow. (Actually, a
little blood is ejected back into the veins, and this accounts for
the venous pulse that can often be seen in the neck veins when
the atria are contracting.)
summarizes the path of blood ﬂ ow through
the entire cardiovascular system.
The cardiac muscle cells of the myocardium are arranged in
layers that are tightly bound together and completely encircle
the blood-ﬁ lled chambers. When the walls of a chamber con-
tract, they come together like a squeezing ﬁ st and exert pres-
sure on the blood they enclose.
Like smooth and skeletal muscle, cardiac muscle is an
excitable tissue that converts chemical energy in the form of
ATP into force generation. Action potentials propagate along
cell membranes, calcium enters the cytosol, and the cycling of
Valve almost completely closed
Valve partly open
Left AV (bicuspid)
Right AV (tricuspid)
Valves of the heart. (a) Superior view of the heart with the atria removed. (b) Photographs of the pulmonary valve from the pulmonary trunk
looking down into the right ventricle. In the ﬁ rst photo, the valve is in the process of opening as blood ﬂ ows through it from the right ventricle
into the pulmonary trunk. In the second photo, the valve is in the process of closing, the cusps being forced together when the pressure of the
blood in the pulmonary trunk is greater than the pressure in the right ventricle.
From R. Carola, J. P. Harley, and C. R. Noback,
Human Anatomy and Physiology,
McGraw-Hill, New York, 1990 (photos by Dr. Wallace McAlpine).