658 THE SECRETION OF URINE. 



An ingenious attempt has been made by Liebermann 1 to explain the 

 chemical mechanism by which the cells of the tubules effect this change in 

 reaction. This author has described a class of bodies which may be extracted 

 from the mucous membrane of the stomach or from the kidney, and which 

 consist of compounds of lecithin and proteid. These he designates lecith- 

 albumins. These substances are acid in nature, and are capable of combining 

 with alkalies. Liebermann imagines that, as the alkaline salts of the blood 

 plasma pass through the epithelial cells of the kidney, they are split up by these 

 acid insoluble lecith-albumins, which combine with a portion of the bases, so 

 that the remainder of the fluid which reaches the lumen of the tubule contains 

 acid salts or free acid. Of course this process would come to an end as soon as 

 the acid affinities of the lecith-albumins in the cells were satisfied, and in this 

 way one might explain the speedy appearance of an alkaline reaction when 

 large quantities of urine are secreted. Under normal circumstances, however, 

 Liebermann assumes that the carbon dioxide, which is the normal product of 

 tissue metabolism in the kidney, splits up the compound formed in the cells 

 into free lecith-albumin and alkaline carbonates, these latter being then 

 removed by the venous blood stream. As supporting evidence for this hypo- 

 thesis, LieJ3ermann states that the kidney tissue, like lecith-albumin itself, if 

 treated with soda solution, and then washed repeatedly with water to remove 

 excess of the latter, becomes strongly alkaline. If now the alkaline tissue be 

 suspended in water, through which a stream of CO., is passed, and be then 

 again washed thoroughly, it will be found to be strongly acid, having given up 

 all its soda to the carbon dioxide. 



Conclusions. It is evident that the experimental facts at our pre- 

 sent disposal do not allow of a definite decision as to the exact manner 

 in which the secretion of urine is effected. It will be convenient, 

 therefore, to summarise the two modes of interpretation, either of which 

 may be applied to the known facts. 



According to the Bowman-Heiclenhain. hypothesis, the secretion of 

 urine is due to the activity of two. sets of cells. The flattened epithelial 

 cells covering the glomeruli take up from the blood, circulating through 

 the glomerular capillaries, water and salts, and transfer these substances 

 to the beginning of the urinary tubule. Their activity is chiefly depend- 

 ent on the activity of the blood flow through the capillaries. But they 

 may be also excited to active secretion by the presence of certain of the 

 urinary constituents in the blood, such as water and salts, or possibly by 

 diuretics, such as caffein. On the other hand, the roddecl cells, lining the 

 convoluted tubules and the ascending loop of Henle, secrete specific 

 urinary constituents, such as urea and uric acid, together with a certain 

 amount of water. They also secrete certain abnormal constituents of the 

 blood, such as indigo -car mine. Their activity is chiefly determined by 

 the amount of urea or uric acid in the blood. 



If, on the other hand, we accept Ludwig's hypothesis, we must 

 introduce into it certain modifications, necessitated by later inquiries, 

 and assume that in the secretion of urine, as in so many other of the 

 bodily functions, there is a mixture of what we may term physical and 

 physiological processes. It seems probable that in the glomeruli the 

 process is largely if not exclusively physical ; that is to say, we have here 

 a transudation of the watery and crystalloid constituents (including 

 urea) of the blood plasma. The extent and nature of this transudation 

 are determined 



1. By the pressure in the glomerular capillaries. 



1 Arch.f. d. ges. PUysioL, Bonn, 1894, Bd. liv. S. 585. 



