404 A MANUAL OF PHYSIOLOGY 



amount of work by the kidney. For the osmotic pressure 

 of urine is several times as great as that of the plasma of 

 the blood. Blood-plasma freezes at 0*55 to 0*65 C. (on 

 the average, say, o'6 C.). The osmotic pressure corre- 

 sponding to o'6 C. is 5,662 millimetres of mercury (p. 361), 

 or, in round numbers, 75 metres of water. Human urine has 

 been found to freeze at 1*38 to 2*11 C. (say, on the 

 average, r8 C.), and for highly concentrated urines the 

 depression of the freezing-point may be considerably greater. 

 The osmotic pressure corresponding to r8 C. is 16,986 

 millimetres of mercury or 225 metres of water. This exceeds 

 the osmotic pressure of the plasma by 150 metres of water. 

 In separating a kilogramme of urine from the blood the 

 kidney accordingly does work approximately equivalent to 

 raising a weight of a kilogramme to the height of 150 metres, 

 i.e., 150 kilogramme -metres. It is evident that the excess 

 of the blood-pressure in the glomeruli over the pressure of 

 the urine in the tubules, which, even if we neglect the latter 

 altogether, since there is only slight resistance to the flow of 

 urine towards the bladder, cannot at most be greater than 100 

 millimetres of mercury, or 1*35 metres of water, will account 

 for only an insignificant part of this work. The rest must be 

 done at the expense of the energy of the food-materials taken up 

 by, and transformed in, the cells concerned with the secretion 

 of the urine. But we do not know in what way these cells, 

 by applying this energy, perform the remarkable feat of 

 permanently maintaining a difference of fifteen atmospheres 

 in the osmotic pressure of the liquids in contact with their 

 attached and free surfaces. 



But, secondly, there is positive proof that the ' rodded ' 

 epithelium of the tubules, which no one supposes to be 

 abandoned more to mere physical influences than the 

 epithelium of the salivary glands, plays a part in the 

 secretion of some of the urinary constituents. For Bowman 

 saw crystals of uric acid in the epithelium of the convoluted 

 tubules of birds. Heidenhain found that urate of soda and 

 indigo - carmine injected into the blood of a rabbit are 

 excreted by the epithelium of the convoluted tubules and 

 the ascending part of Henle's loop. And Nussbaum's experi- 



