GLOMERULAR FILTRATION AND UREA EXCRETION IN RELATION TO URINE FLOW IN THE DOG

Abstract

The urea clearance in relation to urine flow was examined under a variety of conditions in 6 dogs (824 clearance periods), the simultaneous creatinine clearance being used in the majority of the periods in 5 dogs to measure the rate of glomerular filtration. The rate of a glomerular filtration is essentially constant and unrelated to the rate of urine flow in the ordinary exptl. range of the latter. It may, however, be depressed by dehydration, or elevated by the administration of large doses of water. At the highest urine flow obtainable (creatinine U/P ratio = 10) about 40% of the urea filtered is reabsorbed. As the urine flow decreases an increased fraction of the filtered urea is reabsorbed; the increase in the reabsorbed fraction being approximately related to the logarithm of the creatinine U/P ratio. Diuresis following a low urine flow is accompanied by a marked, transient exaltation of the urea clearance relative to the creatinine clearance, which may disappear before the peak of diuresis is reached. This exaltation of urea clearance is evoked by osmotic diuresis in a normal or pituitrinized dog, as well as by water diuresis in the normal. The concept of an augmentation limit with its corollary of standard and max. clearance does not appear to be applicable to excretion in the dog. A simple diffusion hypothesis, positing that urea escapes from the tubule distal to the point of water reabsorption and in consequence of the concentration gradient created by this reabsorption is alone inadequate to explain the observed relationships between the deficit in the urea clearance and the rate of urine formation.