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Normal pulse pressure

Normal stroke volume

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Augmented stroke volume

Blood volume in aorta

Augmented stroke volume

Blood volume in aorta

FIGURE 19 The influence of a change in stroke volume on pressures recorded in the aorta. The slope of the line relating pressure and volume defines compliance. With a normal stroke volume, aortic pressure fluctuates between 120 mm Hg and 80 mm Hg. When stroke volume increases, pulse pressure increases as well, as indicated by the shaded area.

where SV = stroke volume and C = arterial compliance. In Fig. 19, arterial volume is plotted as a function of arterial pressure, and the diagonal line indicates vascular compliance. At the end of ejection the arterial system is distended with the stroke volume and pressure rises to 120 mm Hg. Over the ensuing diastole, volume decreases and pressure falls to 85 mm Hg, giving a pulse pressure of 35 mm Hg. If stroke volume is increased as shown by the shaded area, a greater pulse pressure will be achieved. Note that this increase

FIGURE 20 Aortas from individuals of three age groups (20-24, 36-42, and 71-78 years) were studied by filling them to various pressures with fluid. At each pressure, the volume was noted. The slope of each line relating volume to pressure describes the compliance for each group. Note that compliance is much higher in the youngest group. In older individuals, compliance is less, especially at higher aortic pressures. (Modified from Hallock P, Benson JC, J Clin Invest 1937; 16:596.)

FIGURE 20 Aortas from individuals of three age groups (20-24, 36-42, and 71-78 years) were studied by filling them to various pressures with fluid. At each pressure, the volume was noted. The slope of each line relating volume to pressure describes the compliance for each group. Note that compliance is much higher in the youngest group. In older individuals, compliance is less, especially at higher aortic pressures. (Modified from Hallock P, Benson JC, J Clin Invest 1937; 16:596.)

in stroke volume has resulted in an increase in both the mean pressure and the pulse pressure, even though compliance remained constant.

Changes in capacitance also affect pulse pressure. Figure 20 illustrates the volume-pressure relationships for aortas that were removed from individuals who had died at various ages. The slope of the line relating volume and pressure indicates compliance and is relatively high for vessels taken from younger individuals. With age, compliance decreases because of a change in the composition of the aortic wall. The effect of a loss of arterial compliance, such as that associated with aging, is shown in Fig. 21. In this figure, a person with normal arterial compliance is compared to a person with low compliance. If heart rate, ejection time, and resistance are the same for both situations, it follows that for any given stroke volume the pulse pressure will be greater in the individual with the lower compliance. Note that in this example a change in compliance does not affect mean arterial pressure, just the pulse pressure. The normal pulse pressure for a 60-year-old person averages 60 mm Hg, whereas that for a 20-year-old person averages only 40 mm Hg. Because the physical work done by the heart is the volume pumped times the systolic pressure, this puts an added burden on the elderly heart, even though the cardiac output and the mean arterial pressure may be the same as that in the young individual.

Distensibility of the arterial tree also allows for a more efficient use of the energy generated by the heart in the maintenance of blood flow. If blood vessels were rigid, blood flow and pressures would be very high during systole, but both would fall to zero during diastole. Such a flow pattern would require excessive work by the heart because it would have to pump against an extremely high pressure to force the entire cardiac output through the circulation during only a small fraction of the cardiac cycle. Much of the energy expended by the heart during ejection is stored in the distended walls of the arterial system. This stored energy serves to force blood into the periphery during the remainder of the cardiac cycle. A good analogy is the bagpipe player who blows into the bag in short spurts. The energy from those spurts of air is stored in a compliant bag so that air exits through the pipes in a steady stream, giving an uninterrupted tone even while the player is taking a breath.

The pressure pulse, like the blood, is transmitted down the aorta to the peripheral vessels. The vertical lines in Fig. 18 reveal that the pressure pulse takes about 0.25 sec to travel 40 cm down the aorta. The pressure profile becomes distorted as it traverses the aorta. Interestingly, reflected waves in the aorta cause the pulse pressure to actually increase in the distal

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