iQii] William J. Gies 301 



may distribute itself uniformly throughout the blood and the urinary 

 membrane, or it may be present in either a greater or a less concentra- 

 tion in the urinary membrane than in the blood. Just vvhat will happen 

 is dependent upon the nature of the dissolved substance and the physical 

 and chemical composition of the blood and the urinary membrane at the 

 time. Of greatest importance are such facts as the presence and 

 absence of lipoids, the character of the colloids concerned, and the State 

 of these colloids as determined by the presence of acids, alkalies, salts, 

 or various non-electrolytes. In other words, the laws of partition and 

 the laws of adsorption again come into play. These differences in the 

 distribution of a dissolved substance between the blood and the urinary 

 membrane are rendered strikingly apparent when dyes are used as the 

 dissolved substances. (Page 201.) 



But this distribution of a dissolved substance between the blood and 

 the urinary membrane represents in the end only a static affair, and 

 the secretion of dissolved substances in the urine is a dynamic one. It 

 requires no special comment to see now why only through the con- 

 tinuous secretion of water from the kidney can a continuous Separation 

 of dissolved substance from the urinary membrane (secretion) be 

 rendered possible. The presence of water in Bowman's capsule and in 

 the uriniferous tubules introduces the third phase into our secretory 

 System and breaks down continuously the equilibrium that is trying to 

 become established between the dissolved substance in the blood and 

 the dissolved substance in the urinary membrane. (Page 202.) 



The attempt to establish an equilibrium between the dissolved sub- 

 stances in the urinary membrane and the dissolved substances in the 

 urine (originally only water) as it passes down the uriniferous tubules 

 makes for a diffusion of dissolved substances out of the urinary mem- 

 brane, and so tends to destroy, all the time that water is being secreted 

 by the kidney, the equilibrium which is trying to be established between 

 the dissolved substances in the blood and the dissolved substances in the 

 urinary membrane. When now we recall the physico-chemical fact that 

 when any dissolved substance is offered simultaneously to a liquid col- 

 loid, a solid colloid, and water (as is the case in the kidney), an unequal 

 distribution of the dissolved substance between the three phases is the 

 rule, then we will have no difficulty in understanding why a difference in 

 quantitative composition between the blood, kidney tissue, and urine, 

 so far as dissolved substances are concerned, is also the rule. Where- 

 fore a " selective " secretion is to be expected rather than to be 

 wondered at. (Page 202.) 



