THK KXCKHT10N OF L'KINE 547 



would have to be filtered through the capsule, and (51 liters of water returned to the 

 blood from the uriniferous tubules. This amount of water would be derived from 67 

 liters of plasma (see table on page 551). Since the bloodflow through the kidneys is 

 very great, at least 500 liters per day, only about 13 per cent of the fluid contained in 

 the blood passing through the glomerulus would pass by nitration through the capsule of 

 Bowman. The fact that such a large amount of fluid would have to be reabsorbed from 

 the uriniferous tubules is a possible a priori criticism of the theory, but Cushney points 

 out that the amount each tubule would have to absorb per hour would be very small 

 (in his experiment on a cat amounting to less than 0.014 c.c. per hour). 



According to the modern view, there are therefore two fundamentally 

 different processes occurring in the kidney; filtration in the capsule and 

 selective reabsorption in the tubules. It is important to consider some 

 of the evidence which is considered to indicate that both of these pro- 

 cesses occur. 



The Filtration Process. The filtration of the protein-free blood fluid 

 through the renal capsule, like that through any other membrane, depends 

 on several factors. (1) There must be a difference in the pressure between 

 the blood and the urinary filtrate. In the laboratory the pressure used in 

 filtering is usually supplied by gravity, but in the case of the filtration of 

 the urine through the capsule the force is furnished by the pressure of blood 

 in the glomerular vessels. (2) The character of the filter determines what 

 substances shall pass. The renal capsule is a membrane normally im- 

 pervious to the proteins of the blood, but pervious to the other constitu- 

 ents. Under certain conditions it loses this character. (3) The char- 

 acter of the fluid determines how readily it will filter through the mem- 

 brane. If the fluid contains a substance which can not pass through the 

 filter and which exerts an osmotic pressure in opposition to the filtering 

 force, the rate of filtration as well as the amount filtered, will be reduced. 



If the capsule acts as a filter it should be possible to alter the rate of 

 urine excretion by varying any of the above mentioned factors, and 

 experimentally this is true. The factors can be varied in several ways. If 

 the blood pressure is raised by tying off several of the branches of the aorta, 

 the urine is appreciably increased, or if the blood pressure is lowered, as can 

 be done by cutting the spinal cord the amount of urine is decreased. In the 

 artificially perfused kidney, the fluid exuding from the ureter increases 

 as the pressure of the perfusion fluid is raised, and decreases as 

 the pressure is decreased, and Richards and Plant 17 have recently shown 

 that this urine formation may run parallel with the pressure without 

 their being any alteration in blood flow. This strongly supports the 

 view that filtration is the function of the glomerulus. Apparently, excre- 

 tion can continue only as long as the colloids of the plasma are not notably 

 increased, for, as the osmotic pressure due to indiffusible colloids rises, the 

 pressure in the capillaries is no longer able to overcome it. The same point 

 has been shown by Starling and his pupils, who found that the excretion of 



