Disorders of renal concentrating ability.

The ability of the kidneys to excrete a concentrated urine is impaired together with other renal functions whenever the kidneys are progressively scarred and the amount of functioning renal parenchyma is diminished. Concentrating capacity and glomerular filtration rate are therefore reduced roughly in parallel in many common diseases which cause widespread scarring of the kidneys. The present discussion, however, will be concerned with some rarer disorders of the kidney in humans that are characterized by disproportionate, sometimes specific, involvement of the concentrating function. In some instances these patients exemplify principles of renal physiology well established in dogs and rats. In other cases they represent unique experiments of nature which have illuminated our understanding of the concentrating process in man. One way in which the kidney of man differs from that of the dog and the rat is in the fraction of the total number of nephrons that dip long loops of Henle into the renal medulla and therefore contribute to the elaboration of a concentrated urine. Whereas all of the dog's nephrons have long loops, and one third of the rat's glomeruli filter fluid which will find its way to the papillary tip, the papilla of a man's kidney contains thin loops from only about 10 to 14 per cent of its nephrons.' Those few nephrons that do contribute to the renal pyramid and hence to the formation of a concentrated urine, originate chiefly in glomeruli in the juxtamedullary portion of the cortex. For these reasons, when destruction of the kidney is confined to the medulla in man, concentrating ability may be severely impaired while glomerular filtration rate is only slightly lowered. Diseases limited chiefly to the cortex (like acute glomerulonephritis) are likely at first to spare concentrating ability and the quotient of TCH20 and GFR in such patients is usually normal. On the other hand, when the juxtamedullary portion of the kidney is specifically involved, and, of course, when there is selective * Professor of iMedicine.