164 ENERGY CHANGES INVOLVED IN SECRETION 



to 0*042 kilogram-metres, and multiplying by this factor, we 

 obtain 1214 x 0-042 = 50-9 kilogram- metres as the work done by 

 the kidneys in secreting the urea against osmotic pressure. The 

 work done in similarly secreting the sodium chloride is less than 

 10 kilogram- metres, as can be shown by a similar calculation, and 

 these two form the chief amount of the work done against osmotic 

 pressure, because the amount of the other constituents is com- 

 paratively low. 



The estimate of 100 kilogram-metres would therefore be 

 certainly above the amount of total work done by the kidneys 

 in the twenty-four hours, and it must be pointed out that this 

 amount is by no means large. Expressed as heat it would only, 

 if it were all taken as heat from the urine secreted, lower the 

 temperature between 1 and 2 C. 



The osmotic pressure of a secretion expressed as a hydrostatic 

 pressure may give a very high value ; thus Dreser found in the 

 morning urine of man a lowering of the freezing point amounting 

 to 2' 3 C., which corresponds to an osmotic pressure of 282 metres 

 of water, or over 30 atmospheres of pressure. 



In the urine of other animals still higher osmotic pressures 

 are obtained ; thus in the cat an osmotic pressure of 49,800 grm. 

 per square centimetre was calculated by Dreser, and the statement 

 is made that if the work of concentration were carried out by 

 the cells of the kidney tubules, these results would imply that 

 these cells can exert a force six times greater than the absolute 

 force of human muscle (8000 grm. per square centimetre). 



Such a statement and such a view as to the action of the cells 

 of the tubules is, however, a highly absurd one. The kidney cells 

 do carry out the work of concentration, but we have no evidence 

 that they exert or resist the least possible pressure in the process. 

 Although the osmotic pressure is so high, the amount of energy 

 change, as is shown by the calculation given above, is comparatively 

 very small, and the work of the kidney cell consists in supplying 

 this small amount of energy, from energy in another form, by 

 transformation of a corresponding small amount of the energy 

 which it takes up as nutrient matter from the plasma. That 

 anything approximating to the osmotic pressure of the separated 

 urine develops in this process of energy transformation, or indeed 

 that there is any pressure developed whatever, we possess not 

 the smallest fraction of experimental evidence. All that is known 



