REDISTRIBUTION

 

REDISTRIBUTION

The phenomenon of redistribution is seen with drugs which are highly lipid soluble.

After drug administration, when the drug reaches systemic circulation, the drug is preferentially distributed in tissues with the maximum blood supply.

The brain is 2% of the body weight but is highly vascular and receives 15-20% of  the total blood supply. So highly lipid soluble drugs quickly enter the brain and the onset of action is very fast.

The faster the drug molecules enter the brain, the quicker they exit because it is a two way process and remember the blood supply is high both for entry and exit (Voila! Yes it is so!). 

Simultaneously, the drug molecules are also moving into and out of the lesser vascular tissues like the fat but albeit at a slower rate. As the amount of drug in peripheral tissues (read fat) increases, the amount of drug in brain decreases. So the action is terminated. (This is up to which you have already understood O)

However, on repeated administration, the peripheral sites gets gradually filled up and redistribution from brain to adipose tissue slows down and prolonged action of the drug will be observed.

A schematic diagram herewith will help you understand.