TECHNICAL NOTES AND BIBLIOGRAPHY 245 



feature, their function is now secondarily diverted to 

 nonexcretory purposes. 



For the history of the development of modem renal 

 physiology the reader must be referred to technical 

 sources {50, 52, 53, 56, 57, 62} and to references ap- 

 pended to Chapter X. The comparative anatomy of the 

 kidney is discussed in Chapter VIII. 



It has been noted that the energy for glomerular fil- 

 tration is suppMed by the heart and transmitted to the 

 glomeruh through the pressure of the arterial blood. We 

 may therefore think of the glomeruli as playing a passive 

 role in the process of filtration, and no increased burden 

 of work devolves upon them when the concentration of 

 any substance in the plasma is increased to any level 

 whatsoever. But the many processes of tubular reabsorp- 

 tion and excretion are, with few exceptions, 'active' 

 processes: the tubule cells must remove the substance 

 from a low concentration in one medium (either urine 

 or blood) and transport it to the other medium against 

 a concentration gradient, an operation that will not pro- 

 ceed spontaneously but requires the local expenditure 

 of energy by the tubule cells. This energy is made avail- 

 able within the cells by the metabolism of suitable fuel 

 stuffs and fed into the 'transport mechanism' by elab- 

 orate enzyme systems, and these circumstances impose 

 upon every reabsorptive or excretory process certain 

 quantitative hmitations that take the form of maximal 

 rates of transport. In many cases these maximal rates can 

 be quite accurately measiu-ed by presenting to the tu- 

 bules an excess of any substance by artificially raising its 

 concentration in the plasma, and they prove to be fairly 

 reproducible in successive examinations in any one ani- 

 mal. In general, saturation of one reabsorptive system 

 (for example glucose) does not interfere vdth the other 

 reabsorptive systems (sodium, potassium, phosphate, sul- 

 fate, vitamin C, and so on) , showing that each transport 

 system operates more or less independently of all others. 



