ABSORPTION OF DRUGS

ABSORPTION is the process that makes the movement of the drug from its site of administration into the circulation.

Absorption - pharmacyconcepts

The absorption of a drug involves its passage across cell membrane. Not only the fraction of administered dose that gets absorbed, but also the rate of absorption is important.

As a rule, lipid insoluble and water insoluble drugs are not absorbed from the gut.

Passage of drugs across cell or biological membrane : Drugs are transported across the membranes by......

(A) PASSIVE TRANSFER :

       1-Simple Diffusion

       2-Filtration

(B) SPECIALISED TRANSPORT :

       1-Active Transport

       2-Facilitated Diffusion

       3-Pinocytosis

(A-1) SIMPLE DIFFUSION : It is a bidirectional process. The membrane plays no active role when the drug diffuses across the membrane. Polar water soluble and non polar lipid soluble substances can be transported by this process. This is the most important mechanism for majority of the drugs.

(A-2) FILTRATION : It involves passage through pores present in the cell membrane (aqueous pores or paracellular spaces). Only water soluble substances can be transported by this process.

(B-1) ACTIVE TRANSPORT : It is selective process which requires energy. It also requires a carrier so it is called Carrier mediated transport.

(B-2) FACILITATED DIFFUSION : It is very similar to Carrier mediated transport, but it does not require energy.

(B-3) PINOCYTOSIS : It is the process of transport across the cell in particulate form by formation of vesicles. Proteins and macromolecules are transported by this process. It is similar to Phagocytosis where cells engulf fluids or macromolecules from the surroundings.