Countercurrent Exchange of Heat in the Dog Kidney

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Abstract
The mechanisms of renal heat clearance were studied in anesthetized dogs by recording renal venous and cortical temperatures during step changes of arterial blood temperature induced by infusion of 0.9% saline of room temperature into the renal artery. The venous thermodilution curves were well described by a three-exponential function with average rate constants: k1 = 10.4 min-1, k2 = 2.2 min-1, and k3 = 0.59 min-1, and intercepts at 34%, 44%, and 22%, respectively. The various components were interpreted as follows: The slow component mainly represents heat conduction from inner medullary and perirenal tissue as shown by extrarenal temperature recording and by calculating the heat distribution volume. Local cortical recordings showed that cortex is not a homogeneous compartment, and k2 reflects an average cortical heat clearance, k1 is much higher than blood flow per gram of tissue in any kidney region and results from heat exchange between intrarenal arteries and veins. The arteriovenous diffusion of heat explains the higher temperatures observed in the cortex and the renal medulla than in renal venous blood. The arteriovenous thermodilution technique is unsuited for local and total renal blood flow measurements, because of countercurrent diffusion and heat exchange witli perirenal tissue.