Regulation of arterial pressure by pressure diuresis. A higher output of salt and water in response to increased arterial pressure reduces blood volume. Blood volume is reduced until pressure returns to its normal level. The curve on the left shows the relationship in a person with normal blood pressure. The curve on the right shows the same relationship in an individual who is hypertensive. Note that the hypertensive individual has an elevated arterial pressure at a normal output of salt and water. (Modified from Guyton AC, Hall JE. Medical Physiology. 10th Ed. Philadelphia: WB Saunders, 2000, p. 203.)
ume and, ultimately, blood volume. As discussed earlier in this chapter and in Chapter 15, a decrease in blood volume reduces stroke volume by lowering the end-diastolic filling of the ventricles. Decreased stroke volume lowers cardiac output and arterial pressure. Pressure diuresis persists until it lowers blood volume and cardiac output sufficiently to return mean arterial pressure to a set level. A decrease in mean arterial pressure has the opposite effect on salt and water excretion. Reduced pressure diuresis increases blood volume and cardiac output until mean arterial pressure is returned to a set level.
Pressure diuresis is a slow but persistent mechanism for regulating arterial pressure. Because it persists in altering salt and water excretion and blood volume as long as arterial pressure is above or below a set level, it will eventually return pressure to that level. In hypertensive patients, the curve shown in Figure 18.6 is shifted to the right, so that salt and water excretion are normal at a higher arterial pressure. If this were not the case, pressure diuresis would inexorably bring arterial pressure back to normal.
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