The Heart - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 103

semilunar valves, when the ventricle pressure exceeds the aorta or pulmonary artery pres-
sure, these valves open. When the ventricles relax, the pressure gradient reverses and the
valves close to prevent the backflow of blood into the heart. Looking at the physiology of
the heart valves, there are muscles attached to the AV valves (the papillary muscles).
These muscles do not aid in valve opening, but they actually function to maintain the
valves in a tightly closed position. During ventricle contraction, the papillary muscles con-
tract to prevent the AV valves from opening in the reverse direction, into the atria.
Without this preventative mechanism, blood would have a preference to flow back into
the atria instead of into the aorta or pulmonary arteries, because the pressure in the atria
is far lower than either of these vessels. For a complete discussion on the motion of the
heart and the heart valve function, see Sections 4.4 and 4.5 , respectively.
The work required for the heart to function is directly related to the pressure-volume
relationship of the two ventricles. There is a very small component of energy needed to
accelerate blood, but this is much less than the total work of the heart and is typically
ignored when calculating heart work. In the pressure-volume relationship for the left ventri-
cle, there are typically four phases that are discussed ( Figure 4.4 ; this is shown for the left
ventricle). The first phase (filling phase) is when the left ventricle is at rest and the left
atrium is contracting, filling the left ventricle with blood. During this phase, the blood vol-
ume in the left ventricle increases from approximately 45 mL (end-systolic volume) to
approximately 115 mL (end-diastolic volume). The pressure in the left ventricle increases
from approximately 1 mmHg to approximately 5 mmHg during this time period. The sec-
ond phase (isovolumic contraction) is the period during which the volume of the left ventri-
cle does not change but the pressure increases rapidly. There is no change in left ventricular
volume because both the mitral and the aortic valves are closed. However, the left ventricu-
lar myocytes are contracting, which effectively increases the pressure to approximately
80 mmHg. This is the pressure in the aorta prior to valve opening (it is assumed that blood
is incompressible and therefore there is no volume change). During the third phase (ejection
phase), the pressure of the ventricle increases to approximately 120 mmHg, due to further
contraction of the left ventricle myocytes and then reduces back to approximately
80 mmHg. The volume of the left ventricle also reduces because blood is being ejected into

FIGURE 4.4 The pressure-volume
relationship for the left ventricle, which
describes the work that the left ventricle
conducts. The work is the area within
the pressure-volume curve. The work
the heart conducts can drastically
change under disease conditions.

150

125 Phase 3

100

75

50 Phase 4 WORK

Phase 2

25

0

Phase 1

0 25 50 75 100 125 150

Left ventricular volume (mL)