Nature of Fluids Which Functionally Distend the Kidney

Abstract

The nature of the fluid draining from the kidney, after its artery was occluded, was investigated. Samples of systemic arterial blood, renal venous blood and urine were also analyzed. It was found that the fluid draining from the kidney after occlusion is a mixture of vascular blood and another fluid designated as "diluting fluid," each contributing half to the composite mixture. In volume the mixture is 26 per cent of the functionally distended kidney. With the assumption that the renal extracellular fluid can be considered a simple mixture of blood plasma and a cell-free fluid, the composition of the "diluting fluid" was deduced from the known compositions of vascular blood and total fluid draining. The ratios of its content in a given substance to that in systemic (or renal venous) plasma are: for Na and Ca, 1.0; K, 1.5; Cl, 1.2; PO4, 2.0; urea, 1.8; plasma protein, 0.3; albumin, 0.4; glucose, 0.4, and osmolarity, 1.2. The fluid bears little or no relation to urine, especially since the urine varied considerably between individual dogs whereas the "diluting fluid" was relatively constant in composition. It was also found that the hematocrit of the fluid draining after arterial occlusion progressively decreased as it flowed out, until the last portion contained only 5 per cent red cells. It is concluded that since renal lymph has approximately the same composition in protein, urea, glucose, and inulin as does "diluting fluid," the latter is, in all probability, renal interstitial fluid. Under the conditions of the experiment, it drains out of the kidney slowly relative to blood drainage. It is large in volume, particularly when compared with the capillaries that nourish it. Its high protein content explains the observation that the kidney is apparently naturally distended with a fluid disproportionately rich in plasma protein.

Examination of the Intrarenal Hematocrit Ratio with the Technique of Circulatory Stop-Flow in Dogs

“…The maximal dilution of the hematocrit to 56% of the renal vein value came about because of the favorable gradient of influx of fluid created by the positive value of 15 to 20 mm Hg in plasma colloid osmotic pressure. This is in agreement with the observations of Swann et al (10) in which the hematocrit of the fluid drained from the vessels of the kidney showed an average value of 27%, compared with 42.8% in the arterial and 43.7% in the venous blood. The protein in the "diluting fluid" was also reduced from an average of 4.6 g/100 ml in the control venous plasma to 2.0 in the drained fluid.…”

“…One of the important concerns about many techniques involving distribution of cells and plasma in the kidney is the fact that as soon as the kidney's circulation is interrupted (ligation of artery and veins preparatory to subsequent handling), capillary pressure decreases below that of the plasma colloid osmotic pressure, and redistribution of fluid from the interstitium into the capillary circulation can be anticipated, resulting in dilution of capillary blood and decrease of the hematocrit ratio. This is particularly relevant to techniques in which drainage of blood is the basis for the assessment of hematocrit (9,10).…”

“…To the other extreme is the method employing labeled cells (Cr 51 , P 32 ) and labeled albumin (T-1824, I 131 ), which yields the largest volume, averaging 27 ml/100 g kidney (1,(4)(5)(6)(7)(8) with the renal blood hematocrit ratio averaging 20%, slightly less than half of the systemic hematocrit ratio. Another technique, drainage of blood and subsequent washout, gives intermediate values for the renal hematocrit (9,10).…”

Composition and ‘functional distension’ of the rat's renal cortex

“…The following procedure was followed in order to obtain a 'kidney fluid' analogous to that described by Swann, Valdivia, Ormsby & Witt (1956). The left kidney was exposed as before, and the fatty tissue was dissected from the hilum, in order to define the renal vessels and ureter.…”

Ion and Water Transport in Individual Segments of the Nephron

“…This requires, of course, that the reported intratubular potassium concentradon of 110 mM/1 is correct, that none is compartmentalized at higher concentration at the expense of general cell water, and all is free to exert its full chemical activity. Another premise [which may be incorrect (48)] is that the ionic com position of interstitial fluid is accurately reflected in the concentra tion of ions in plasma. One must assume from these experiments that an active absorptive mechanism exists at the luminal membrane.…”