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50 100 150

Ventricular volume (mL)

FIGURE 6 The effect of an increase in contractility on the ejection loop is shown by the dashed lines. Note that stroke volume increases because of a decrease in end-systolic volume.

50 100 150

Ventricular volume (mL)

FIGURE 6 The effect of an increase in contractility on the ejection loop is shown by the dashed lines. Note that stroke volume increases because of a decrease in end-systolic volume.

Venous Filling Pressure

The end-diastolic volume is determined by the venous filling pressure. Because the ventricle is very compliant, small changes in venous filling pressure have a major effect on end-diastolic volume and thus stroke volume. This relationship makes venous filling pressure a primary determinant of cardiac output, which will be discussed in further detail in Chapter 14, where the control of cardiac output is considered. Because of its pivotal role in controlling cardiac output, the central venous pressure is of great importance clinically and is often monitored closely in critically ill patients, particularly those who have lost blood volume. Figure 7 shows that an increase in venous pressure increases stroke volume entirely by changing end-diastolic volume and does not affect the end-systolic volume. Factors that increase venous filling pressure tend to increase cardiac output and those that decrease venous pressure decrease cardiac output.

Aortic Pressure

It is easy to see in Fig. 8 that the end-systolic volume is determined by the aortic pressure. As aortic pressure rises, equilibrium point C will occur at a larger end-systolic volume. Increasing end-systolic volume reduces stroke volume. Thus, a rise in aortic pressure tends to decrease stroke volume, whereas a falling aortic pressure tends to increase it. As was seen with contractility, changes in aortic pressure affect stroke volume by changing only the end-systolic volume.

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