1288 PHYSIOLOGY 



30 grin, 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 

 adapted to the needs of the animal ; and as we have seen, the occur- 

 rence of an actual secretion of urea by the cells of the tubules removes 

 the necessity for assuming 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. 



