Drug Compartment modelling

What do you mean by compartment modelling? Drugs are, usually, administered in our body to get relief from ill-conditions, but, can we calculate the time courses of drugs in the body? Or, what is the neccesity of calculating it? Is it related to the therapeutic efficacy of drugs? Of course, it is related to their efficacies as every drug has plasma half life so that drug will show its optimum activity within the half life, once, it crosses its half life it will show no or less effect in the body. This is the only reason, time courses of each drug, after administration, has to be measured. Earlier, it was very difficult for physicians, except, giving predictions which might work in few cases. But, now, it becomes very easy for pharmacists or doctors as there are several models available to measure the drug concentration in the body, especially, into the plasma, subsequently, full analysis of drugs, mostly, have been covered by this model. 'Compartment Modelling' is such model designed, especially, to measure simultaneous movement of the drugs in the body as it varies with time. However, One compartment model is considered as the simplest model, and, similarly it has few assumptions : 
1. The body is considered as a single, kinetically homogenous unit that has no barriers to the movement of drug.
 2. Final distribution equilibrium between the drug in plasma and other body fluids is attained instantaneously and maintainted at all times. This model thus applies only to those drugs that distribute rapidly throughout the body. 
3. Drugs move dynamically, in(absorption) and out(elimination) of this compartment.
 4. Elimination is a first order process with first order rate constant. 5. Rate input(absorption)> rate output(elimination) 
6. The anatomical reference compartment is plasma and concentration of drug in plasma is representative of drug concentration in all body tissues i.e, any changes in plasma drug concentration reflects a proportional change in drug concentration throughout the body.
However, the model does not assume that the drug concentration in the plasma is equal to that in other body tissues. Besides, the term open indicates the input(absorption) and output(elimination) are unidirected. If this model is represented diagramatically, this will look like this,

Fig.1. Explaining compartment modelling

 and this model is generally used to describe plasma levels following administration of a single dose of a drug, in case of, multi dose of several drugs can not be calculated, depending upon the rae of input, several one-compartment open models can be defined: 
1. One compartment open model, i.v, bolus administration 
2. One-compartment open model,  continuos i.v. infusion 
3. One-compartment open model, e.v. administration, zero order absorption 
4. One-compartment open model, e.v. administration, first order absorption.