Tissue distribution (2) - Apparent volume
Now assume that the 12 units of drug do not distribute evenly into the plasma and interstitial layer of fluid. In this example, 8 units of the drug are bound to the wall of the interstitial layer. Now only 3 units are free in the interstitial fluid layer. For the concentration to be equal in both the plasma and interstitial tissue, there is now 1 and 3 units respectively with a concentration of 1 unit/3 litre = 0,33 units/litre.
Now if we determine the volume of distribution, we find that 12 units (the dose) at 0,33 units/litre (= unit plasma concentration) will give us a volume of 36 litres. But we know that there is no more than 5 litres of blood. This demonstrates that the volume of distribution is an apparent value and does not reflect a real physiological volume. Thus, a volume of distribution that is larger than the blood volume just signifies that the drug is distributed predominantly to other body fluids than blood.