Clinical signs of volume depletion. Note that laboratory test results for volume expansion and contraction are similar. Serum sodium (Na) concentration may be increased or decreased in either volume expansion or contraction, depending on the cause and intake of free water (see Chapter 1). Acid-base disturbances, such as metabolic alkalosis, and hypokalemia are common in both conditions. The similarity of the laboratory test results of volume depletion and expansion results from the fact that the "effective" arterial volume is depleted in both states despite dramatic expansion of the extracellular fluid volume in one.

w from a reduction in mean arterial pressure (MAP). Some disorders decrease cardiac output, such as congestive heart failure owing to myocardial dysfunction; others decrease systemic vascular resistance, such as high-output cardiac failure, atriovenous fistulas, and cirrhosis. Because MAP is the product of systemic vascular resistance and cardiac output, all causes lead to the same result. As shown in Figures 2-3 and 2-4, small changes in MAP lead to large changes in urinary Na excretion. Although edema-tous disorders usually are characterized as resulting from contraction of the effective arterial volume, the MAP, as a determinant of renal perfusion pressure, may be the crucial variable (Figs. 2-26 and 2-28 provide supportive data). The mechanisms of edema in nephrotic syndrome are more complex and are discussed in Figures 2-36 to 2-39.

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