ENERGY CHANGES INVOLVED IN SECRETION 269 



finally get on making all these substitutions in the above equation, 

 for the value of the work done expressed in small calories: 



W = 620 x 0-5 x 1-7-0434 = 1214 calories. 



This amount of energy may be expressed as mechanical work 

 by remembering that the small calory is approximately equivalent 

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

 obtain 1214x0-042=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 compara- 

 tively low. 



The estimate of 100 kilogram-metres would therefore be cer- 

 tainly above the amount of total work done by the kidneys against 

 osmotic pressure 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 of that excretion 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 grms. 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 grms. 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 oaciculation 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 



