SECRETION OF WATER, 



641 



depending only on the blood pressure in the glomerular capillaries 

 and the permeability of the filtering membrane. 



2. That this dilute urine is concentrated in the tubules by giving up 

 its water to the surrounding lymph, in consequence of differences of 

 concentration between the glomerular filtrate and the lymph. 



3. That all the urinary constituents are turned out of the blood 

 through the glomeruli (i.e. with the water) in dilute solution. 



In discussing the experimental data bearing on these propositions, 

 we shall find that only in the first part of the theory are the 

 experimental facts consonant with Ludwig's hypothesis, and that it is 

 impossible to explain the formation of normal urine without assuming 

 the active intervention (i.e. the performance of work) by certain of the 

 living elements of the Iddney in the process. In this case, as in so 

 many others in physiology, the "how" of the cellular activity is 

 at present absolutely unknown to us, although we may confidently 

 expect, with the advance of the science, to be able to trace the 

 manner in which the cell utilises the energy of its food for this special 

 purpose. 



Secretion of water. One of the strongest facts in favour of Ludwig's 

 hypothesis is the indubitable 

 connection which exists be- 

 tween the circulation through 

 the kidney and the amount 

 of urine, i.e. of water, secreted. 

 It is evident that a mechanical 

 filtration or separation of the 

 watery and crystalloid consti- 

 tuents of the blood in the 

 glomeruli must depend on two 

 factors 



1. The difference . of pres- 

 sure between the blood in the 

 glomerular capillaries and the 

 urine in Bowman's capsule. 

 Since under normal circum- 

 stances there is a free outflow 

 of urine from the capsule by 

 means of the tubules, we may 

 regard its pressure as practi- 

 cally nil, so that the only 

 changeable factor in the pro- 

 cess will be the blood pressure 

 in the capillaries. 



2. The rapidity of the blood flow through the glomeruli must also 

 have some influence on the filtration, as this process will go on the 

 more readily the more often the fluid presented to the filter is renewed. 

 As a rule, the changes that raise the pressure in the capillaries also 

 increase the velocity of the blood through them, so that it becomes 

 difficult to dissociate the part played by each factor in influencing the 

 urinary secretion. 



If we consider the manner in which changes in the glomerular 

 circulation are brought about, we see that it may be affected by changes 

 either in the general blood pressure or in the calibre of the smaller 

 VOL. i. 41 



FIG. 59. Roy's oncometer. (For explanation of 

 lettering, see next figure. ) 



