374
Chapter 12
As Figure 12–18 illustrates, both systole and diastole can
be subdivided into two discrete periods. During the fi rst part
of systole, the ventricles are contracting but all valves in the
heart are closed, and so no blood can be ejected. This period
is termed
isovolumetric ventricular contraction
because the
ventricular volume is constant. The ventricular walls are devel-
oping tension and squeezing on the blood they enclose, raising
the ventricular blood pressure. However, because the volume
of blood in the ventricles is constant and because blood, like
water, is essentially incompressible, the ventricular muscle fi bers
cannot shorten. Thus, isovolumetric ventricular contraction
is analogous to an isometric skeletal muscle contraction: The
muscle develops tension, but it does not shorten.
Once the rising pressure in the ventricles exceeds that
in the aorta and pulmonary trunk, the aortic and pulmonary
valves open, and the
ventricular ejection
period of systole
occurs. Blood is forced into the aorta and pulmonary trunk as
the contracting ventricular muscle fi bers shorten. The volume
Systole
(a)
(b)
Blood flows out of ventricle
Ventricles
contract
Atria
relaxed
Ventricles
relaxed
Atria
relaxed
Ventricles
relaxed
Atria
relaxed
Ventricles
relaxed
Atria
contract
Ventricles
contract
Atria
relaxed
Closed
Closed
Closed
Open
Diastole
Blood flows into ventricles
Atrial contraction
AV valves:
Aortic and
pulmonary valves:
Closed
Closed
Open
Closed
Open
Closed
AV valves:
Aortic and
pulmonary valves:
Isovolumetric ventricular contraction
Isovolumetric ventricular relaxation
Ventricular ejection
Ventricular filling
Figure 12–18
Divisions of the cardiac cycle: (a) systole; (b) diastole. The phases of the cycle are identical in both halves of the heart. The direction in which
the pressure difference
favors
fl ow is denoted by an arrow; note, however, that fl ow will not actually occur if a valve prevents it.
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