158 THE ARMY FOR HOME DEFENCE 



of urine secreted, the kidney consumes about an equal volume 

 of oxygen. Assuming that the oxygen all goes to the oxidation of 

 glucose, then each cubic centimetre of oxygen will cause the 

 liberation of 0-05 cals. of energy from 0-015 grams of glucose. 

 That is, the intake of energy for the output of about a litre of 

 urine will be about 50 cals. There is thus a discrepancy between 

 the total energy absorbed and the apparent work done. Meltzer 

 and others account for this on the principle of the " factor of 

 safety." 



Macallum dissents from this explanation. He adduces evidence 

 of a permanent low surface tension on the lumen-cell interface 

 and ascribes the excess of energy used to the continuous main- 

 tenance of this surface energy gradient. On the other hand the 

 difficulties to be overcome in measuring the fall in oxygen potential 

 in the blood passing through the renal vessels are very great, and 

 it may be that although the results are relatively accurate their 

 absolute values may be somewhat high. Barcroft and Brodie 

 estimate that about ^ of the total oxygen intake goes to the 

 kidney. 



These workers have attempted, by means of estimations of 

 the oxygen intake of the renal cells, to tell when the kidney acts 

 as a passive filter and when the renal cells take an active part in 

 secretion. During filtration, they say, no osmotic work is done, 

 and therefore no energy is required and no oxygen used. Actual 

 renal activity will entail expenditure of energy and consumption 

 of oxygen. 



A solution made up so that in composition it approximated to 

 blood minus the colloids was injected into a blood vessel of a cat. 

 Immediately there was an increase in the amount of urine secreted 

 but no increase in the amount of oxygen consumed proof 

 positive of the non-participation of the renal cells. Now let us 

 see what has happened. The introduction of the saline fluid causes : 



(1) A temporary increase in the volume of blood corresponding 

 to the amount of the fluid injected. 



(2) An increase in general blood pressure and therefore an 

 increased pressure in the renal arterioles. 



(3) An increase in the rate of the blood flow through the kidney 

 vessels. 



(4) A decrease in the concentration of the corpuscles of the 

 blood. This results in a decreased oxygen carrying power and a 

 decreased viscosity. 



(5) A dilution of the colloids of the blood. 



