L. J. Poo

THE RELATION BETWEEN THE SERUM UREA CONCENTRATION AND THE PROTEIN CONSUMPTION OF NORMAL INDIVIDUALS

When a patient with renal disease is found to have a concentration of urea in his blood that is higher than usual we would like to be able to derive from the degree of increase in concentration some idea as to the degree of decrease in the urea-excreting function of his kidneys. In the individual patient this cannot safely be done. True, when we plot the blood urea concentrations of a large number of patients against their urea clearances, a general relation does emerge (1, 2). However, in any one of such a group of patients the prediction from concentration to clearance may fall dangerously far from the truth. This is inevitable because the concentration of urea in the body is determined not only by how much urea runs out of the system through the kidneys but also by how much urea runs into it from the liver. Whenmore runs out than runs in, the concentration falls to a lower level which is maintained as long as the discrepancy persists. When more runs in than runs out the concentration level rises and remains high as long as the excess of in-flow over out-flow continues. Unlike salt, the concentration of urea is confined within no narrow zone of variation by any regulatory mechanism. Urea is a substance to which the body is chemically and physically indifferent. It is an end product of protein metabolism and participates in no chemical reactions. Except in the kidney it has no osmotic effect because it is distributed evenly through the water of all organs and tissues. So we need not be surprised when we find, as we do, that there is a wide scatter in the urea concentrations in the blood of normal individuals (3, 4). In spite of wholly normal renal function we shall continue to find this high variability until we make measurements on normal subjects who are taking the same amounts of protein in their food; and in our patients, whose