RELATIONSHIPS BETWEEN UREA EXCRETION, RENAL BLOOD FLOW, RENAL OXYGEN CONSUMPTION, AND DIURESIS. THE MECHANISM OF UREA EXCRETION

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Abstract
Simultaneous studies of renal blood flow, of the removal of oxygen and urea from the renal blood by the kidneys, and of the excretion of urea and water were made on normal unanesthetized dogs with kidneys explanted by Rhoads'' technic, so that blood from the renal vein could be obtained by needle puncture through the skin. The renal blood flow was calculated from the rate of urea excretion and the decrease in blood urea content which occurred as the blood passed the kidneys. The results apparently afford the first data on the proportion of urea removed from the blood, the blood flow, and the O consumption in the kidneys, obtained without operation or anesthesia. The kidneys usually removed 6-12% of the urea from the blood perfusing them. This % was independent of the blood urea conc. within such wide ranges as 8-140 mgm. % urea N. It is this constancy in % urea removal which makes urea clearance independent of the blood urea conc. The renal blood flow was 2-10 cc. per gm. kidney per min. The normal spontaneous variations in the urea clearance paralleled chiefly variations in renal blood flow, the effect of variations in the % of urea removed from the blood being less important. In occasional exps., however, the removal of urea from the blood by the kidney was momentarily reversed, and urea previously concentrated in the kidney diffused back into the blood (apparently only for a few min. each time), possibly because trauma during the needling of the renal vein caused a reflex paralysis of the tubular cells, which lost their relative impermeability to diffusion of urea from the tubular lamina back into the blood. The occurrence suggests that such reabsorption of unusual proportions of urea may, under unusual physiologic or even pathologic conditions, become a cause of diminished urea clearance, even though the usual cause is retarded renal blood flow. Removal of one kidney was followed by an increase (in most cases apparently reading a max. in 1 mo.) in the blood flow (av. 68%), O consumption (av. 81%), and urea clearance (av. 43%) of the remaining kidney. No relation of renal blood flow or renal O consumption could be found to the work of the kidney in excreting either water or urea at greatly varying rates. Apparently neither the excretory work nor the processes directly connected with it control the O consumption of the kidney, which must be governed by the energy requirements of the non-excretory processes in the organ. This conclusion is explicable by the fact that, as calculated by Brodie, Barcroft, and others, the thermo-dynamic work ordinarily done by the kidney in excretion is less than 1% of the energy furnished by the respiration of the organ. Renal O consumption and blood flow tend to run parallel in their variations. Presumably both respond to similar metabolic demands of the kidney. The blood in the renal vein is usually more than 85% oxygenated, indicating that the tissues in the kidney are kept under higher O tension than in most other organs.