Absorption of Drugs

How drugs passes through our cell membrance.

A drug can enter into circulation and reach the site of action only after absorption. The absorption of a drug involves its passage across cell membrane. As a rule, lipid insoluble and water insoluble drugs not absorbed from the gut.

Passage of drugs across cell membrane:

Drugs can pass through cell membrane by two processes. They are:

Under Passive Transfer the drugs transfers under two processes, they are as follows:

a. Simple Diffusion
b. Filtration

a. Simple Diffusion: It is a bidirectional process. Polar water-soluble and non-polar soluble substances can be transported by this process.
b. Filtration: Only water soluble substances can be transported by this process. It involves passage through pores present in the cell membrane.

Under Specialised Transport the drugs transfers under three processes, they are as follows:

a. Active transport
b. Faciliated diffusion
c. Pinocytosis

a. Active transport: It is a selective process which requires energy. Also it requires a carrier and so is called as carrier transport.
b. Facilitated diffusion: It is very similar to carrier transport, but it does not requires energy.
c. Pinocytosis: Proteins and macromolecules are transportetd by this process. It is similar to phagocytosis where cells engulf fluids or macromolecules from the surroundings.

Factors modifying drug absorption: The absorption of a drug can be influenced by the following factors:

1. Physical state: Drugs in the form of liquids are well absorbed than solids. Gases are quickly absorbed through lungs.
2. Particle size: Smaller the particle size, better is the absorption of a drug. If the particle size is large, the drug is slowly absorbed and hence the action is delayed.
3. Solubility: An easily soluble drug is quickly absorbed. Also drugs in the form of solutions are quickly absorbed than solids.
4. Concentration: Concentrated forms of drugs are quickly absorbed than dilute solutions.
5. Area of absorbing surface: Greater the area of absorbing surface, quicker is the absorption of a drug. For example, lungs are peritoneal cavity are large surface areas from where drugs can be quickly absorbed.
6. Circulation of site of absorption: Increased blood flow to the area of absorption can increase the absorption of a drug. This can be achieved by massage or local application of heat. Vasoconstrictors decrease blood flow and so decrease the absorption of a drug.
7. Route of administration: This is a very important factor which determines drug absorption. Some drugs are absorbed only on parenteral administration and they fail to get absorbed on oral administration e.g.insulin. so it is necessary to carefully choose the route of administration of a drug.

BIOACTIVITY: It is defined as the quantity of the drug that is absorbed and reaches systematic circulation after non-vascular administration. The bioavailability is 100% after intramuscular injection. But it is less after oral administration. Bioavailability is affecte4d by factors like
1. Physical properties
2. Dosage form
3. Physiological factors


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