UNIFORMITIES AND COMPARISONS AMONG COMPONENTS 387 



titles of recognized dimensions ; each has its connotations. What 

 usefulness has each in describing phenomena of the sorts that are 

 dealt with in this investigation 1 



Metabolic rate is ordinarily the number of molecules or quanta 

 appearing or disappearing in a unit of living material per unit of 

 time (L^L"^T"^, or sL^^T'^). Instead of a unit volume or mass 

 L"^, a unit of surface L~^ is often used, and occasionally a unit of 

 length L"\ an individual, a population, a cell. In practice, metabolic 

 rate is sometimes limited to processes that involve recognized 

 chemical transformations; occasionally it means only rate of 

 oxygen consumption. Originally, of course, it had little quantita- 

 tive connotation. Often it is regarded as applying chiefly in 

 stationary states. 



Clearance started from the specific definition : ' ' the volume of 

 blood which one minute's excretion of urine suffices to clear of 

 urea" (Moller et al., '28). The volume flow (L^T^^) is a virtual 

 one and not a visible one, as is more evident in the older equivalent, 

 urea excretory ratio: urea excreted in one hour's urine/urea found 

 in one volume of blood. Usage already extends the term clearance 

 to other volumes than blood {e.g., plasma, volume of distribution^ 

 body) to other intervals of time than minutes {e.g., hour), to other 

 disposals than excretion {e.g., chemical conversion), to other paths 

 than urinary {e.g., hepatic, unknown) and to other substances than 

 urea {e.g., creatinine). Hence investigators now speak of plasma 

 clearance, galactose clearance, renal clearance, complete clearance, 

 filtration clearance, tubular clearance. The original definition is 

 no longer a sure guide to the meanings. Sometimes clearance is 

 referred to a unit of supposed body surface; accordingly the 

 dimensions are often L^L"^T"\ which overlap those for metabolic 

 rate. In supposition clearances occur in stationary states ; in prac- 

 tice they are measured as often in states of unloading. Originally 

 clearance was distinguished by the fact that no overall chemical 

 transformation but only translocation was known to be involved, 

 the same substance being measured in urine and in blood. But 

 more recently it is permissible to measure merely the rate of disap- 

 pearance from blood {e.g., Bollman et al., '35), really a rate of 

 decrement in concentration, and to call this (L^T"^) a clearance 

 without recognizing the path of exchange. It is evident that no 

 unanimity of usage prevails, even to the extent of preserving di- 

 mensions other than the factor T~\ Clearance is nearly always a 



