8 Physiology of the Kidney 



aglomeriilar tubule is capable of excreting all the more im- 

 portant constituents normally present in fish urine: magnes- 

 ium, sulfate, chloride, potassium, ammonia, creatinine, crea- 

 tine, uric acid, the foreign substances, iodides, nitrites, thio- 

 sulphates and sulphocyanates, and the dyes, indigocarmin, 

 neutral red and phenol red. There has subsequently been 

 added to this list the organic iodine compounds, diodrast and 

 hippuran.® By another technique, utilizing in vitro cultures 

 of kidney tissue, Chambers and Kempton^ and Cameron and 

 Chambers^ have afforded a direct visual demonstration of 

 tubular excretion in the mesonephric tubule of the chick and 

 the metanephric tubule of man. Good evidence of the tubu- 

 lar excretion of urea in the frog has been advanced by Mar- 

 shalP^ and independently by Hober.^^ 



The data now available on man leave no doubt that tubu- 

 lar excretion plays a very important part in the excretion of 

 phenol red," diodrast and hippuran,^'' ^' iopax, neoiopax, 

 skiodan*^ and creatinine,^^ but even prior to these demonstra- 

 tions the possibility of tubular excretion in man had to be 

 accepted on inference. Cytologically, the aglomerular tubule 

 appears to be roughly homologous with the proximal segment 

 of the human nephron and, without implying a perfect par- 

 allel in function, the demonstration that the aglomerular 

 tubule can excrete a large variety of substances made it neces- 

 sary to assume, in the absence of evidence to the contrary, 

 that the human tubule might also be able to excrete them. 

 Since, by the filtration-reabsorption mechanism of the neph- 

 ron, the glomerular filtrate undergoes a variable degree of 

 concentration by the reabsorption of water, it is impossible 

 to determine directly from the composition of blood and 

 urine whether the excretion of any one of several substances 

 involves filtration alone, or filtration plus either tubular re- 



