Renal localization of albumin and its function in the concentrating mechanism

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Abstract
Renal plasma albumin spaces determined with 131I-labeled human albumin, after TCA precipitation to remove free 131I, averaged 15. 3, 21. 3, and 37.1 in normal cortex, medulla, and papilla, respectively. In the papilla, albumin-131I equilibrated rapidly and was not altered by occluded or increased lymph flow. Fluorescein-labeled albumin given intravenously was detected only in renal vessels and the intensity of fluorescence increased markedly from the cortex to the papilla. These data indicate that albumin is chiefly intravascular and that an increasing gradient for albumin exists from the cortex to the papillary tip. It is suggested that this gradient is achieved by the countercurrent exchange of water from descending to ascending limbs of the vasa recta driven by hydrostatic pressure which diminishes blood flow, especially to the inner medulla, and prevents solute washout; and that the increased oncotic pressure of vasa recta blood removes water of tubular origin from the medullary interstitium and thereby helps to establish the high osmotic gradient essential for urine concentration.