Below Figure shows drug passing backwards and forwards between blood and a series of different tissues. It is assumed that drug will distribute into and out of tissues at varying rates; these are indicated by the width of the arrows. In the figure, drug moves most rapidly into Tissues 1 and 2 with Tissue I being fastest of all. Movement into Tissues 3 & 4 is slow, with Tissue 4 being slowest.

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An exact description of drug handling within the whole body would be immensely complex as it would have to model each tissue separately. To achieve a practically useful model, we make two simplifications. Neither of these is strictly realistic, but they give us access to models that are good enough for practical purposes without undue mathematical complexity. The two simplifications are:

We allow for just two rates of distribution – ‘Rapid’ and ‘Slow’. All tissues are then assumed to belong to one class or the other. All the slow ones are assumed to permit drug distribution at one fixed (slow) rate and for all other tissues, distribution occurs at a common (rapid) rate. For the ‘Rapid’ tissues, we consider movement to be instantaneous; as soon as drug arrives in the blood, it is assumed to spread immediately into all the space provided by the ‘Rapid’ tissues.

For the situation shown in above Figure, blood and Tissues 1 and 2 would be considered as a single space into which drug moves instantly following injection or absorption. Tissues 3 and 4 would form another space into which drug moves at some fixed and relatively slow rate.

 

The term ‘Compartment’ is used to describe these aggregations of tissues that are treated as allowing drug to enter at a common rate.

A ‘Compartment’ is a collective term for all those areas of the body into which a drug distributes at approximately the same rate.