Content About General Pharmacology and Absorption of drugs

Introduction: 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 are not absorbed from the gut.

Passage of drugs across cell membrane: Drugs can pass through cell membrane by two processes. They are:

1. Passive transfer

(a) Simple diffusion

(b) Filtration

(c) Pinocytosis

2. specialized transport

(a) Active transport

(b) Facilitated diffusion

1. Simple diffusion: It is a bidirectional process. Polar water-soluble and non-polar lipid-soluble substance can be transported by this process.
2. Filtration: Only water soluble substances can be transported by this process. It involves passage through pores present in the cell membrane.
3. Active transport: It is a selective process which requires energy. Also it requires a carrier and so is called as carrier transport.
4. Facilitated diffusion: It is very similar to carrier transport, but it does not require energy.
5. Pinocytosis: Proteins and macromolecules are transported 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 and peritoneal cavity are large surface areas from where drugs can quickly absorbed.
6. Circulation to site of absorption: Increased blood flow to the area of absorption can increase the absorption of a drug. This can be achieved by message 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.

Bio-availability: It is defined as the quantity of the drug that is absorbed and reaches systemic circulation after non-vascular injection. But it is less after oral administration. Bio-availability is affected by factors like

1. Physical properties
2. Dosage form
3. Physiological factors.

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