Volume of distribution (V_d or V_D) defines the distribution of a drug between the blood plasma and other bodily tissues after oral or parenteral administration. V_D can be considered as the size of the compartment in to which the drug distributes following absorption. It is the theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma. The higher the V_D, the greater the degree of drug distribution in to the tissues.