THE SECRETION OF URINE 1287 



normal conditions a constant production of a fluid free from protein, 

 but having the same crystalloid concentration as the blood-plasma. 

 With any rise of general blood pressure the amount of this transudate 

 is increased ; with any fall it is diminished. The small qualitative 

 changes which are constantly occurring in the blood as the result of the 

 taking of food or the activity of different organs, probably produce but 

 little effect on the amount of glomerular fluid. Only indirectly, as the 

 result of these events on the general blood pressure, or possibly in con- 

 sequence of the production of substances having a vaso-dilator effect 

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

 glomeruli be affected. These structures therefore have the twofold 

 function of regulating the total amount of circulating fluid and of 

 providing an indifferent 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 con- 

 ditions, 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 modifications in its passage towards the ureter. It receives urea, 

 uric acid, phosphates, 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 

 intestine. 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 



