THE KIDNEY 5^ 



normal man, for example, 91 per cent of the PAH con- 

 tained in the renal arterial plasma is removed and ex- 

 creted into the urine in a single passage of the blood 

 through the kidneys. (That this figure is not 100 per cent 

 is partly attributable to the fact that some blood passes 

 from the renal artery to the renal vein by way of non- 

 excretory channels in the renal capsule, fat and other 

 adventitious tissue.) The 'extraction ratio' of PAH has 

 been accurately determined many times in man by the 

 simultaneous analysis of arterial and renal venous blood, 

 the latter collected by a long, flexible catheter passed 

 into the renal vein by way of an arm vein and thence 

 down the vena cava. This procedure must be carried out 

 under strict surgical conditions, but in the hands of ex- 

 perienced physicians presents neghgible dangers. 



The collection of urine and blood in fishes and other 

 cold-blooded animals offers httle more difficulty than in 

 man. Most fishes, other than the elasmobranchs, and all 

 the Amphibia have a urinary bladder of sorts, sometimes 

 quite large. Li such forms the bladder can be emptied 

 by catheterization and the luinary papilla closed with 

 a purse-string ligature; or a retention catheter to which 

 a small rubber bag is attached can be fastened in the 

 papilla and the bag emptied at convenient intervals, a 

 technique first used in fishes by E. Herter in 1891. 

 Where no urinary bladder is present, as in the elasmo- 

 branchs, urine can be collected from the urogenital sinus 

 by retention catheter and rubber bag. Blood can be col- 

 lected in a hypodermic syringe without injuring the fish 

 by puncturing the dorsal aorta from the ventral side of 

 the tail while the fish is held in a V-shaped trough at an 

 angle of 45°, with the fish's head imder water. With 

 practice, all operations can be completed in a few min- 

 utes and with no serious physiological disturbance to the 

 animal. 



Quantitative observations on the filtration rate and the 

 renal blood flow, as well as on detailed tubular trans- 



