Figure 192

Increases in cardiac output, urinary output, and arterial pressure caused by increased blood volume in dogs whose nervous pressure control mechanisms had been blocked. This figure shows return of arterial pressure to normal after about an hour of fluid loss into the urine. (Courtesy Dr. William Dobbs.)

in cardiac output to about double normal and increase in mean arterial pressure to 205 mm Hg, 115 mm Hg above its resting level. Shown by the middle curve is the effect of this increased arterial pressure on urine output, which increased 12-fold. Along with this tremendous loss of fluid in the urine, both the cardiac output and the arterial pressure returned to normal during the subsequent hour. Thus, one sees an extreme capability of the kidneys to eliminate fluid volume from the body in response to high arterial pressure and in so doing to return the arterial pressure back to normal.

Graphical Analysis of Pressure Control by the Renal-Body Fluid Mechanism, Demonstrating an "Infinite Feedback Gain" Feature. Figure 19-3 shows a graphical method that can be used for analyzing arterial pressure control by the renal-body fluid system. This analysis is based on two separate curves that intersect each other: (1) the renal output curve for water and salt in response to rising arterial pressure, which is the same renal output curve as that shown in Figure 19-1, and (2) the curve (or line) that represents the net water and salt intake.

Over a long period, the water and salt output must equal the intake. Furthermore, the only place on the graph in Figure 19-3 at which output equals intake is s a rm o c t u a. t u o o e

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