152 



PHYSIOLOGICAL KEGULATIONS 



as ''the volume of body fluid dissolving the substance at the same 

 concentration as the plasma. ' ' More generally, a component added 

 may supplement some already present, or if subtracted may decre- 

 ment that already present; this (positive or negative) increment 

 spreads through a virtual volume of distribution. Going beyond 

 the limits of body fluids as distributees, of substances (heat or pres- 



TABLE 5 



Mean volumes of distribution, as measured in plasma or whole blood of three species of 



mammals, after introduction of excess substance into the plasma or into the 



stomach. The concentrations of the substances marked * were 



measured in whole blood; the others were 



measured in plasma 



sure might be distributed) as distribuends, and of plasmas as dis- 

 tributors, I define a volume of distribution ( Vd) as a virtual volume 

 computed to contain an added or subtracted quantity (AGo) in that 

 increment of concentration (AGs/Vs) found by analysis of a chosen 

 sample. Or, Vd equals AGo introduced into or removed from the 

 living unit (of volume Vo or weight Bq), divided by AGs measured 

 in the specified tissue per unit of its volume Vs. Usually Vd is 



