The Heart - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 109

blood volume enters the ventricles, hence their function as a primer pump. Note that the dis-
cussion was for the entire heart, but Figure 4.7 shows the left side of the heart.
The pressure in the atria remains fairly constant (and low) during the entire cardiac
cycl e. However, there are three major changes that occur within the atrial pressure wave-
form, and they are denoted as the a, c, and v waves. The a wave is associated with atrial
contraction and occurs immediately after the P wave of the ECG. During the a wave, both
atria experience an increase in pressure of about 6 to 7 mmHg, with the left atrium
experiencing a slightly higher pressure increase than the right atrium. The c wave corre-
sponds to the beginning of ventricular contraction and occurs immediately after the QRS
complex of the ECG. This is caused primarily by the ventricular pressure acting on the AV
valves. Also, at the beginning of systole, there is a small amount of blood backflow into
the atria because the valves have not yet closed. Combined, these two changes induce an
increase in atrial pressure. The v wave is a steady increase in atrial pressure that occurs
during ventricular contraction, which is caused by venous blood entering the atria. When
the AV valves reopen, this increased pressures aids in blood movement directly into the
ventricles without atrial contraction.
Figure 4.7 depicts the cardiac cycle for the left ventricle, the left atrium, and the aorta.
The aortic pressure curve is what is measured when a patient has his or her blood pressure
taken, or more accurately, the maximum aortic pressure and the pressure at which the aor-
tic valve opens. At the point that the aortic valve opens, the pressure in the aorta increases
due to blood being forced into the vascular system from the left ventricle. At peak systole,
the blood pressure in the aorta reaches approximately 120 mmHg under normal condi-
tions. At the point that the aortic valve closes, the pressure in the aorta is approximately
100 mmHg. At the time of valve closure, the pressure increases by approximately 5 to
10 mmHg, because there is an immediate cessation of backflow of blood into the left ven-
tricle. The backflow of blood occurs because the left ventricular pressure has dropped
below the aortic pressure. The slight dip in the aortic pressure is termed the dicrotic notch.
The following rise in aortic pressure is referred to as the dicrotic wave. Then due to the
viscoelastic recoil of the aorta, there is a slow, but continual, decrease in aortic pressure
during diastole. At the end of diastole and the isovolumic contraction phase, the aortic
pressure is approximately 80 mmHg. Once the left ventricular pressure surpasses this, the
aortic valve opens and the pressure increases again to approximately 120 mmHg.
An additional interesting point is the sounds that the heart makes during the cardiac
cycle. A physician can listen to these sounds via a stethoscope. These sounds are caused by
first the closure of the AV valves and then the closure of the semilunar valves. The sounds
are generated by the valve vibrations during the closing process. As mentioned previously,
the AV valves can bulge into the atrium and the semilunar valves can bulge into the ventri-
cles immediately after closure. This is caused by an increase and a reversal in the pressure
gradient across the leaflets. With respect to the AV valves, the papillary muscles and the
tendons that attach to the valves (the chordate tendineae) experience recoil, inducing valve
leaflet vibration. The semilunar valves themselves are highly elastic and experience recoil
due to the pressure difference. This elastic recoil generates an audible sound.
As we know, the heart beats approximately 72 beats/minute for an average human.
Compared with other animals, one can see that heart rate is inversely proportional to
body mass ( Figure 4.8 ). There are many models that can be used to describe this