CHAPTER 4 • CARDIAC FUNCTION
65
Atrial pressures transiently increase during
this phase owing to continued venous return
and possibly to bulging of AV valves back
into the atrial chambers, which results in a
“c wave” noted in the atria and their proximal
veins (e.g., in the jugular vein).
Phase 3. Rapid Ejection: Aortic
and Pulmonic Valves Open; AV
Valves Remain Closed
When the intraventricular pressures exceed
the pressures within the aorta and pulmonary
artery, the aortic and pulmonic valves open and
blood is ejected out of the ventricles. Ejection
occurs because the total energy of the blood
within the ventricle exceeds the total energy
of blood within the aorta. The total energy of
the blood is the sum of the pressure energy
and the kinetic energy; the latter is related to
the square of the velocity of the blood flow. In
other words, ejection occurs because an energy
gradient is present (mostly owing to pressure
energy) that propels blood into the aorta and
pulmonary artery. During this phase, ventric-
ular pressure normally exceeds outflow tract
pressure by only a few millimeters of mercury
(mm Hg). Although blood flow across the
valves is high, the relatively large valve open-
ing (i.e., providing low resistance) requires
only a few mm Hg of a pressure gradient to
propel flow across the valve. Maximal outflow
velocity is reached early in the ejection phase,
and maximal (systolic) aortic and pulmonary
artery pressures are achieved, which are typi-
cally about 120 and 25 mm Hg in the aorta
and pulmonary artery, respectively.
While blood is being ejected and ventric-
ular volumes decrease, the atria continue to
fill with blood from their respective venous
inflow tracts. Although atrial volumes are
increasing, atrial pressures initially decrease
(X
descent) as the base of the atria is pulled
downward, expanding the atrial chambers.
No heart sounds are ordinarily heard dur-
ing ejection.
The opening of healthy valves
is silent.
The presence of a sound during
ejection (i.e., ejection murmurs) indicates
valve disease or intracardiac shunts (see
Chapter 9).
Phase 4. Reduced Ejection:
Aortic and Pulmonic Valves
Open; AV Valves Remain Closed
Approximately 150 to 200 milliseconds after
the QRS, ventricular repolarization (T wave)
occurs. This causes ventricular active tension
to decrease (i.e., muscle relaxation occurs) and
the rate of ejection (ventricular emptying) to
fall. Ventricular pressure falls slightly below
outflow tract pressure; however, outward flow
still occurs owing to kinetic (or inertial) energy
of the blood that helps to propel the blood into
the aorta and pulmonary artery. Atrial pres-
sures gradually rise during this phase owing to
continued venous return into the atrial cham-
bers. The end of this phase concludes systole.
Phase 5. Isovolumetric
Relaxation: All Valves Closed
As the ventricles continue to relax and intra-
ventricular pressures fall, a point is reached at
which the total energy of blood within the ven-
tricles is less than the energy of blood in the
outflow tracts. When this total energy gradient
reversal occurs, the aortic and pulmonic valves
to abruptly close. At this point, systole ends and
diastole begins. Valve closure causes the Sec-
ond Heart Sound (S2), which is physiologically
and audibly split because the aortic valve closes
before the pulmonic valve. Normally, little or
no blood flows backward into the ventricles as
these valves close. Valve closure is associated
with a characteristic notch (incisura) in the
aortic and pulmonary artery pressure tracings.
Unlike in the ventricles, where pressure rapidly
falls, the decline in aortic and pulmonary artery
pressures is not abrupt because of potential
energy stored in their elastic walls and because
systemic and pulmonic vascular resistances
impede the flow of blood into distributing arter-
ies of the systemic and pulmonary circulations.
Ventricular volumes remain constant (iso-
volumetric) during this phase because all valves
are closed. The residual volume of blood that
remains in a ventricle after ejection is called the
end-systolic volume (ESV). For the left ven-
tricle, this is approximately 50 mL of blood.
The difference between the EDV (120 mL)
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