C h a p t e r 1 7
Control of Cardiovascular Function
391
During this time, which is called the
isovolumetric
relaxation period,
ventricular volume remains the same
but ventricular pressure drops until it becomes less than
atrial pressure (see Fig. 17-15B). As this occurs, the AV
valves open, and the blood that has been accumulat-
ing in the atria during systole flows into the ventricles.
Most of ventricular filling occurs during the first third of
diastole, which is called the
rapid filling period.
During
the middle third of diastole, inflow into the ventricles
is almost at a standstill. The last third of diastole is
marked by atrial contraction, which gives an additional
thrust to ventricular filling. When audible, the third
heart sound is heard at the end of the rapid filling period
due to vibrations that are caused by the abrupt cessa-
tion of ventricular distention and by the deceleration
of blood entering the ventricles. A third heart sound is
sometimes heard in children with thin chest walls and in
persons with a distended or noncompliant ventricle. A
fourth heart sound, when present, occurs during the last
third of diastole as the atria contract (see Fig. 17-15C).
During diastole, the ventricles increase their volume
to approximately 120 mL (i.e., the
end-diastolic vol-
ume
), and at the end of systole, approximately 50 mL
of blood (i.e., the
end-systolic volume
) remains in the
ventricles. The difference between the end-diastolic and
end-systolic volumes (approximately 70 mL) is called the
100
120
80
60
40
20
100
80
40
ECG
Hear
sounds
Ventricular
volume
(mL)
Time (sec)
Diastole
Systole
1st
2nd 3rd
4th
R
Q
P
T
P
R
Q
Aortic
valve
opens
Aortic valve closes
Aortic pressure
Left ventricular pressure
Atrial pressure
Atrial
contraction
Isovolumetric
contraction
Isovolumetric
relaxation
Ventricular
ejection
Ventricular
filling
Atrial
contraction
A
B
C
Left atrium
Right atrium
Left ventricle
Right ventricle
Pressure
(mm Hg)
FIGURE 17-15.
(Top) Events in the left side of the heart, showing changes in aortic pressure, left
ventricular pressure, atrial pressure, left ventricular volume, the electrocardiogram (ECG), and heart
sounds during the cardiac cycle. (Bottom) Position of the atrioventricular valves during
(A)
the
isovolumetric contraction and ejection phases of ventricular systole,
(B)
the isovolumetric relaxation
and ventricular filling phases during early diastole, and
(C)
atrial contraction.
