PHYSIOLOGY OP TO-DAY 



In regard to the function or functions of the tubules, 

 Opinions have also been divided. Some have held that the 

 cells of the tubules were reabsorptive; that is, substances 

 passed from the lumen of the tube back through the cells 

 to the lymph and blood stream; others have postulated the 

 reverse process, a passage of substances from the blood and 

 lymph into the lumen of the tubule — a process which is 

 designated as secretion; and still others have assumed the 

 tubules to be both reabsorptive and secretory in function. 

 It has now been generally accepted by workers in this 

 field that tubular reabsorption takes place; in fact, tubular 

 reabsorption of some substances at least follows as a 

 necessary corollary of glomerular filtration. 



In regard to secretion by the renal tubule, however, 

 there has been a long and active controversy for many 

 years as to whether or not it occurs at all. Recently, 

 experiments on the kidneys of two marine fish — lowly 

 animals and apparently evolved for no other purpose than 

 for physiological experimentation — in which only tubules 

 and no glomeruli occur, have proven definitely that the 

 renal tubule can secrete. How much tubular secretion 

 occurs in the mammal and man, as well as in the different 

 classes of vertebrates, must be determined by further 

 work. Such research is in active progress and leads inevit- 

 ably into the field of comparative physiology. It is possible 

 that this line of work will also tell us something about the 

 functions of the microscopically distinct parts of a single 

 renal tubule. 



It may well be asked if the above controversy about the 

 mechanism by which the kidney eliminates urine is not 

 purely academic, having no practical importance for 

 mankind. Such is most certainly not the case. The kidneys 

 are the organs of the body upon which falls the greatest 

 r61e in keeping the composition of the blood plasma — the 

 internal environment of the organism — at that degree of 



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