TIIKOIUKS OF KIDNEY ACTION 199 



This, of course, would only give the minimum work of the 

 kidney, even if we knew the concentration and degree of dissocia- 

 tion of each and every urinary constituent. It may be advisable 

 again to call the student's attention to the fact that the energy 

 used in effecting any change is independent of the means by which 

 that change is effected. The work done, as calculated from the 

 Hill-Uonnan formula above, is simply the minimum necessary to 

 cause the change in the molecular concentration. It is independent 

 of any process and commits one to no theory (see Chap. III.). 



Rhorer has calculated the work done by the kidney in concen- 

 trating urea and sodium chloride, and from his figures Cushny 

 considers that, as the concentration of urea causes the consumption 

 of about 0-7 Cat., and similarly about 0-5 Cal. are used in con- 

 centrating sodium chloride (per litre of urine), it would not be an 

 overestimate to state that the production of a litre of urine entails 

 the expenditure of at least 1-2 Cals. This value, however, is but 

 a fraction of the chemical energy used as determined by the 

 oxygen consumption (5 Cals.). We have seen above that for each 

 volume of urine excreted, the kidney consumes about an equal 

 volume of oxygen. There is thus a discrepancy between the total 

 energy absorbed and the apparent work done. In other words, 

 the efficiency of the kidney 



actual work done (in Cals.) 1-2 



= , ,. V, , , X 100 = ^ X 100 = 24 per cent., 



energy used (m Cals.) 5 



a value closely approximating to the value found for the efficiency 

 of muscle. 



Some of the apparently " wasted " energy goes to keeping the 

 machine warm and serves other domestic purposes. Some, again, 

 may be used in maintaining the permanent low surface tension on 

 the cell-lumen interface in the tubules (Macallum). 



Theory of Mechanism. There are two series of facts which are 

 very difficult to explain. 



(1) Some substances occurring in blood and urine have threshold 

 values, i.e., they are not excreted till their concentration in the 

 blood reaches a certain value, e.g., water, chlorides, uric acid, 

 glucose, etc. Others, like urea, creatinin, galactose, etc., have so 

 low a threshold value that they may be classed with foreign 

 solutes as non-threshold substances. Now water and chlorides 

 are more diffusible than urea and creatinin, and yet the latter 

 seem to be readily eliminated from the blood whenever they are 

 present irrespective of concentration. In the two chapters on 

 disperse systems we discussed the question of free and bound water. 

 Blood plasma is a complex disperse system, and when we come to 



