Figure 15

Arterial pressure and sodium excretion. In principle, sodium balance can be regulated by altering sodium intake or excretion by the kidney. However, intake is dependent on dietary preferences and usually is excessive because of the abundant salt content of most foods. Therefore, regulation of sodium balance is achieved primarily by altering urinary sodium excretion. It is therefore of major significance that, for any given set of conditions and neurohumoral environment, acute elevations in arterial pressure produce natriuresis, whereas reductions in arterial pressure cause antinatriuresis [9]. This phenomenon of pressure natriuresis serves a critical role linking arterial pressure to sodium balance. Representative relationships between arterial pressure and sodium excretion under conditions of normal, high, and low sodium intake are shown. When renal function is normal and responsive to sodium regulatory mechanisms, steady state sodium excretion rates are adjusted to match the intakes. These adjustments occur with minimal alterations in arterial pressure, as exemplified by going from point 1 on curve A to point 2 on curve B. Similarly, reductions in sodium intake stimulate sodium-retaining mechanisms that prevent serious losses, as exemplified by point 3 on curve C. When the regulatory mechanisms are operating appropriately, the kidneys have a large capability to rapidly adjust the slope of the pressure natriuresis relationship. In doing so, the kidneys readily handle sodium challenges with minimal long-term changes in extracellular fluid (ECF) volume or arterial pressure. In contrast, when the kidney cannot readjust its pressure natriuresis curve or when it inadequately resets the relationship, the results are sodium retention, expansion of ECF volume, and increased arterial pressure. Failure to appropriately reset the pressure natriuresis is illustrated by point 4 on curve A and point 5 on curve C. When this occurs the increased arterial pressure directly influences sodium excretion, allowing balance between intake and excretion to be reestablished but at higher arterial pressures. (Adapted from Navar [3].)

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