The irreversible shift of a drug from one part of the body to other part of system is known as elimination. It is from the more perfused parts to the lesser perfused parts. Elimination may be by:
Rate of elimination (ER) = Q (Ca-Cv)
Where Q is the quantity of drug going to the organ (blood flow to organ), Ca is the arterial concentration of the drug and Cv is the venous concentration of drug.
Factors affecting Elimination
1. Perfusion of Organ
If the drug is rapidly or completely eliminated, then the rate of blood flow does not effect elimination. E.g. mannitol is almost completely eliminated from kidneys; therefore increasing the blood flow to the kidneys has no effect on elimination of mannitol. In cases of drugs not completely eliminated, increased blood flow increases the rate of elimination.
2. Binding to proteins/ Bound and unbound forms
If a drug is in bound form, its elimination is less and vice versa.
Increased metabolism means decreased plasma half life and increased elimination.
Dopamine and prostaglandins cause vasodilatation of renal vessels, increasing their elimination. Adrenaline causes vasoconstriction, leading to decreased blood flow and decreased elimination. Renin is another vasoconstrictor.