Drug distribution total equivalent volume

Total equivalent volume of distribution Vtot (units volume). This is the total volume of the system seen from the accessible pool it is the volume in which the total amount of drug would be distributed, assuming the concentration of material throughout the system is uniform and equal to the concentration in the accessible pool. 

Pharmacokinetics After ingestion, ethanol is rapidly and completely absorbed the drug is then distributed to most body tissues, and its volume of distribution is equivalent to that of total body water (0.5-0.7 L/kg). Two enzyme systems metabolize ethanol to acetaldehyde (Figure 23-1). 

Theoretical limits for apparent volume of distribution will be as low as 7 to 10 L (equivalent to the volume of the body fluid if the drug totally fails to penetrate the tissues or the drug is extremely hydrophilic) to as high as 500 L or even greater. The most commonly reported number, though, is as low as 7 to 10 L and as high as 200 L. 

Now let us translate the natural-limit concept back to the real-world quantity of dose. If a midrange volume of distribution of 50 L is assumed for a typical drug, then a meaningful detection limit of 1 M for a metabolite implies a concentration of 10 M for the parent drug (i.e., metabolite is 10% of parent), which is equivalent to a total dose of 500 attomol. If the drug has a molecular weight (MW) of 400, then, the dose would be 0.2 pg. For comparison, what is... 

---