200 



THE ARMY FOR HOME DEFENCE 



study it (Chap. XXII.) we will see that the colloids in it have the 

 power of binding a large amount of water and certain solutes. 

 Serum globulin, for instance, binds a considerable amount of 

 chloride. That is, the threshold refers not so much to a differential 

 sill in the kidney as to the differential binding of water, organic and 

 inorganic solutes by the hydrophilic colloids and disperse lipoid 

 particles in the blood. 



(2) The concentration of the substances in the urine differs 

 markedly from their concentration in the plasma. Not only is 

 this so, but similar substances, e.g., potassium and sodium, undergo 

 alterations in concentration to a different extent. The following 

 table illustrates this point (Table XXIX.). 



TABLE XXIX 



We ought actually to have in the first column of concentrations, 

 not the gross amount of these substances in the blood, but a much 

 higher series of figures, viz. the concentrations of the free salts in 

 free water. We know that the quantity of water free in the blood 

 is remarkably small, not over 10 per cent., and probably some- 

 w'here about 5 per cent., but we have lio reliable figures for the 

 distribution of the solutes. Even if this correction were made it 

 would account only for a few differences in concentration, and 

 would reduce the figures in the last column quite considerably, but 

 would not clear all our difficulties away. 



The bone of contention between the two modern schools is the 

 function of the tubule. One group holds that the tubular epithc' 

 Hum absorbs water and threshold salts from the fluid passing down 

 the lumen. The other group holds that salts are excreted into the 

 lumen by its lining epithelium. Much of the evidence produced is 



