Pressurevolume Relationships In The Ventricle

Let us examine the basis for Frank's pressure-volume plot. Ventricular muscle cells are organized into a hollow spheroid with the muscle fibers oriented parallel to the surface of that sphere. When the myocytes develop tension, the ventricular walls are caused to move inward, elevating the pressure of the blood in the lumen of the ventricle. Because of this geometry, tension in the individual ventricular myocytes is proportional to the pressure in the lumen. Similarly, the length of each myocyte will be proportional to the volume of blood contained within the lumen. These relationships are actually somewhat nonlinear because of the LaPlace relationship (as discussed later) but, for now, a simple linear relationship can be assumed. As a result of the preceding relationships, the length-tension curve that was presented for a cardiac muscle fiber in Chapter 7 can be relabeled, as shown in Fig. 4B. The vertical axis is changed from tension to pressure and the horizontal axis is changed from length to volume, but the shapes of the curves are unchanged. This approach yields two pressure-volume plots: one for the contracted state (the upper curve) and one for the relaxed state (the lower curve). As discussed in Chapter 10, the relationship between pressure and volume for a distensible container is described by its compliance. The more compliant a structure is, the more its volume will change for a given change in filling pressure. Figure 4 reveals that during diastole the heart is very compliant because a small increment in filling pressure will cause a large increment in ventricular volume. Note that during systole, the slope of the pressure-volume curve increases dramatically, making it much less compliant.

FIGURE 4 A schematic representation of the frog-heart preparation used by Otto Frank to demonstrate the relationship between the force of contraction and ventricular end-diastolic volume. Four different end-diastolic volumes were selected and the corresponding diastolic and systolic pressures generated by the heart are denoted as D1-D4 and S1-S4, respectively. (A) The ventricular pressures for a complete cardiac cycle are shown for the four volumes. (B) The diastolic (dashed line) and systolic (solid line) pressures from that heart are plotted against the balloon volume, which gives a pressure-volume plot for the ventricle in both systole (upper line) and diastole (lower line). The difference between the two lines at any volume is the actively developed pressure.

FIGURE 4 A schematic representation of the frog-heart preparation used by Otto Frank to demonstrate the relationship between the force of contraction and ventricular end-diastolic volume. Four different end-diastolic volumes were selected and the corresponding diastolic and systolic pressures generated by the heart are denoted as D1-D4 and S1-S4, respectively. (A) The ventricular pressures for a complete cardiac cycle are shown for the four volumes. (B) The diastolic (dashed line) and systolic (solid line) pressures from that heart are plotted against the balloon volume, which gives a pressure-volume plot for the ventricle in both systole (upper line) and diastole (lower line). The difference between the two lines at any volume is the actively developed pressure.

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