Studies in the excretion of urea

Abstract

1. The theories of kidney function are discussed.2. The above theories are examined and applied to the excretion of urea.3. Experimental work directed towards determining the mode of urea excretion is reviewed.4. The xanthhydrol method - involving precipitation of urea as insoluble crystals of dixanthyl urea - is described.5. Following review of the literature dealing with the part played by urea in elasmobranch physiology, the xanthhydrol method is applied to ascertain the method of urea excretion adopted in the kidney of these fish. It is suggested that the normal excretion occurs through the glomerular capsule. Any urgent necessity for urea elimination is met by secondary tubular secretion of urea.6. The method is also applied in the kidney of the frog, rat, mouse, guinea-pig, cat, dog, monkey, and man. In these the excretion is found to be solely glomerular.7. The discussion concerns the comparative value of the different methods used; and the mode of excretion of urea as elicited by the above experiments. It is suggested that in all instances the glomerular/ 44. glomerular capsule is primarily concerned in the excretion of urea. The apparent discrepancy between the kidney of the elasmobranch and that of the frog and the mammals is attributed to the different part played by urea in the physiology of these animals. In the mammals and the frog urea is utilised as a means of nitrogenous elimination and is excreted by the glomerulus only; in the elasmobranch urea, in addition to its excretory function, is vital for the maintenance of osmotic balance and to this is attributed the secondary tubular secretion, which, under certain circumstances, is added to the primary glomerular excretion.