Cardiovascular Physiology
375
of blood ejected from each ventricle during systole is called
the
stroke volume (SV).
During the fi rst part of diastole, the ventricles begin to
relax, and the aortic and pulmonary valves close. (Physiologists
and clinical cardiologists do not all agree on the dividing line
between systole and diastole; as presented here, the dividing
line is the point at which ventricular contraction stops and
the pulmonary and aortic valves close.) At this time the AV
valves are also closed; thus, no blood is entering or leaving the
ventricles. Ventricular volume is not changing, therefore, and
this period is called
isovolumetric ventricular relaxation.
Note, then, that the only times during the cardiac cycle that
all valves are closed are the periods of isovolumetric ventricu-
lar contraction and relaxation.
Next, the AV valves open, and
ventricular fi
lling
occurs as blood fl ows in from the atria. Atrial contraction
occurs at the end of diastole, after most of the ventricular fi ll-
ing has taken place. This is an important point: The ventricle
receives blood throughout most of diastole, not just when the
atrium contracts. Indeed, in a person at rest, approximately 80
percent of ventricular fi lling occurs before atrial contraction.
This completes the basic orientation. We can now ana-
lyze, using
Figure 12–19
, the pressure and volume changes
that occur in the left atrium, left ventricle, and aorta during
the cardiac cycle. Events on the right side of the heart are
identical except for the absolute pressures.
Mid-Diastole to Late Diastole
Our analysis of events in the left atrium and ventricle and the
aorta begins at the far left of Figure 12–19 with the events
of mid- to late diastole. The highlighted numbers that follow
correspond to the numbered events shown in that fi
gure.
1
The left atrium and ventricle are both relaxed, but
atrial pressure is very slightly higher than ventricular
pressure.
2
The AV valve is forced open by this pressure difference,
and blood entering the atrium from the pulmonary
veins continues on into the ventricle.
To reemphasize a point made earlier: All the valves of the heart
offer very little resistance when they are open, so only very
small pressure differences across them are required to produce
relatively large fl ows.
3
Note that at this time—indeed, throughout all of
diastole—the aortic valve is closed because the aortic
pressure is higher than the ventricular pressure.
4
Throughout diastole, the aortic pressure is slowly
falling because blood is moving out of the arteries and
through the vascular system.
5
In contrast, ventricular pressure is rising slightly
because blood is entering the relaxed ventricle from the
atrium, thereby expanding the ventricular volume.
6
Near the end of diastole, the SA node discharges, as
signifi ed by the P wave of the ECG.
7
Contraction of the atrium causes a rise in atrial
pressure.
8
The elevated atrial pressure forces a small additional
volume of blood into the ventricle, sometimes referred
to as the “atrial kick.”
9
This brings us to the end of ventricular diastole, so the
amount of blood in the ventricle at this time is called
the
end-diastolic volume (EDV).
Systole
Thus far, the ventricle has been relaxed as it fi lls with blood.
But immediately following the atrial contraction, the ventri-
cles begin to contract.
10
From the AV node, the wave of depolarization passes
into and throughout the ventricular tissue—as signifi ed
by the QRS complex of the ECG—and this triggers
ventricular contraction.
11
As the ventricle contracts, ventricular pressure rises very
rapidly, and almost immediately this pressure exceeds
the atrial pressure.
12
This change in pressure gradient forces the AV valve to
close, thus preventing the backfl ow of blood into the
atrium.
13
Because the aortic pressure still exceeds the ventricular
pressure at this time, the aortic valve remains closed,
and the ventricle cannot empty despite its contraction.
For a brief time, then, all valves are closed during this
phase of isovolumetric ventricular contraction.
14
This brief phase ends when the rapidly rising ventricular
pressure exceeds aortic pressure.
15
The pressure gradient now forces the aortic valve to
open, and ventricular ejection begins.
16
The ventricular volume curve shows that ejection is
rapid at fi rst and then tapers off.
17
The amount of blood remaining after ejection is called
the
end-systolic volume (ESV).
Note that the ventricle does not empty completely. The
amount of blood that does exit during each cycle is the dif-
ference between what it contained at the end of diastole and
what remains at the end of systole. Thus:
Stroke volume = End-diastolic volume – End-systolic volume
SV
EDV
ESV
As Figure 12–19 shows, typical values for an adult at rest are
stroke volume = 70 ml, end-diastolic volume = 135 ml, and
end-systolic volume = 65 ml.
18
As blood fl ows into the aorta, the aortic pressure
rises along with the ventricular pressure. Throughout
ejection, only very small pressure differences exist
between ventricle and aorta because the aortic valve
opening offers little resistance to fl ow.
19
Note that peak ventricular and aortic pressures are
reached before the end of ventricular ejection; that
is, these pressures start to fall during the last part
of systole despite continued ventricular contraction.
This is because the strength of ventricular contraction
diminishes during the last part of systole.
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