Absorption is the process by which the drug enters into the systemic circulation from the site of administration (except intravenous or intra-arterial routes) through biological barrier. In case of intravenous or intra-arterial administration the drug is not absorbed and it enters into the circulation directly.
The drug is absorbed from different sites of administration such as from the alimentary tract, patenteral sites except intravenous route, skin, and respiratory tract:
* From the alimentary tract:
Buccal cavity:
The blood flow through the buccal mucosa is high and drugs are absorbed in to the systemic circulation by simple diffusion. The area of absorptive surface is relatively small and the time during which absorption occurs is also short.
Stomach:
Only a limited amount of drug is absorbed from the stomach. The extensive vascular supply to the stomach provides a good area for the absorption of drug. But the surface area of absorption is less and the drug remains within the stomach for a short time. However, a significant amount of aspirin and alcohol are absorbed from the stomach.
Intestine:
Blood flow to the intestine is much greater than that of stomach. Most drugs in non ionized form are absorbed from the upper part of the small intestine by simple diffusion. Ionized drugs are incompletely and slowly absorbed by filtration. Some drugs such as methyldopa, fluoro-uracil, levodopa are absorbed from the intestine by an active transport mechanism.
Rectum:
The absorption of drug from the lower two-thirds of the rectum by-passes the first-pass effect in the liver. Because middle and inferior rectal veins collect blood from these parts of rectum and then drain into the inferior venacava via hypogastric vein. On the other hand the drug absorbs from the upper part of the rectum passes through the liver as because the superior rectal vein drains into the mesenteric vein and then into the portal circulation.
The absorption of drug from the rectum is slow and incomplete; and occurs by simple diffusion. However, the extent of absorption is not large. The absorption of drug from the rectal suppository is relatively inadequate. For example, less than 40% of an aspirin dose is absorbed from a rectal suppository.
* From the parenteral site:
The absorption of drug into systemic circulation from the site of subcutaneous and intramuscular injection occurs by simple diffusion process. The absorption of drug from the intramuscular site is usually more rapid and uniform than from subcutaneous site because of the more extensive vascular supply of the muscle compared with the subcutaneous fatty tissue.
* From the skin:
The absorption through the skin is a passive process. The absorption occurs more easily through the cell lining, sweat and sebaceous glands, and hair follicles than through kera-tinized epidermis because of its multiple cell layers.
* From the respiratory tract:
The degree of penetration of drug into the respiratory tract depends upon the size of drug particles in solid and liquid aerosols. The rate of absorption is rapid due to high surface area of the alveoli and high vascular supply of the lung.
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