THE SECRETION OF URINE 1151 



the renal vessels, will the amount of the urine turned out by the glomeruli 

 bs affected. These structures therefore have the twofold function of 

 regulating the total amount of circulating fluid and of providing an in- 

 different fluid, which will, so to speak, flush the kidney tubules and carry 

 down any constituents excreted in a concentrated form by the cells of these 

 tubules. The constant production of a glomerular transudate might result, 

 especially in terrestrial animals, in the loss to the organism of water, or, 

 under certain nutritive conditions, of substances indispensable as normal 

 constituents of the serum, such as sodium chloride, which could not be made 

 good at the expense of the food. It is for this reason that an absorptive 

 mechanism sensitive to and reflecting the nutritive condition of the whole 

 body, especially as concerns water and salts, should be present in the tubules. 

 As the result of the complementary processes of absorption and secretion in 

 the tubules, the unchanging glomerular filtrate undergoes great modifica- 

 tions in its passage towards the ureter. It receives urea, uric acid, phos- 

 phates, and under certain conditions water, from the cells of the convoluted 

 tubules. It gives up salts, especially sodium chloride, and generally water 

 to the same or other cells of the tubules. So that finally, instead of a fluid 

 isotonic with the blood and containing only about 0-1 per cent, urea, we 

 have a fluid of deep yellow colour, with a molecular concentration four or six 

 times greater than that of the blood, and containing between 2 and 3 per 

 cent. urea. We have at the present time no means of judging the relative 

 amounts of fluid furnished respectively by the glomeruli and the tubules to 

 the fully formed urine. It is probable that, under ordinary circumstances, 

 the processes of secretion and absorption of fluid go on pari passu in the 

 urinary tubules just as they do in the mucous membrane of the small in- 

 testine. The demonstration of secretory powers in the cells of the con- 

 voluted tubules relieves us from the necessity of the assumption made 

 by Ludwig as to the quantity of fluid normally turned out through the 

 glomeruli. On the hypothesis that the sole function of the tubules was one 

 of absorption, and that all the urinary constituents were derived from the 

 glomerular transudate, thirty litres of fluid would have to be filtered through 

 the glomeruli in order to excrete the 30 grm. urea which is the daily output 

 of a man. Of these thirty litres, twenty-eight litres would have to be 

 reabsorbed in the tubules. Since the amount of blood flowing through the 

 two kidneys in a man probably varies between 1600 and 1800 litres in the 

 twenty-four hours, there would be no difficulty in the production of such an 

 amount as thirty litres, which would only represent a concentration in the 

 blood in its passage through the glomeruli of under 2 per cent. The secre- 

 tion and reabsorption of such large quantities of fluid seem, however, a 

 clumsy way of arriving at a urine, whose composition should be adopted 

 to the needs of the animal ; and as we have seen, the occurrence of an actual 

 secretion of urea by the cells of the tubules removes the necessity for assum- 

 ing any such wasteful proceeding. It is probable that the actual amount of 

 the glomerular filtrate in the twenty- four hours may not exceed to any large 

 extent the actual amount of urine formed by the whole kidney in this time. 



