Gastrointestinal absorption of drugs .
●The oral route of drug administration is the most common for systemically acting drugs.
●before proceeding to discuss absorption aspects, a brief description of cell membrane structure and physiology is necessary.
●cell membrane:structure and physiology
-for a drug to be absorbed and distributed into organs and tissues and eliminated from the body,it must pass through one or more biological membranes/barriers at various locations. Such a movement of drug across the membrane is called as "drug transport".
-the cellular membrane consists of a double layer of amphillic phospholipid molecules.
-the membrane is a mayonnaise sandwich where a biomolecular layer of lipids is contained between 2 parallel monomolecular layers of proteins.
-Hydrophobic core of the membrane is responsible for the relative impermeability of polar molecules.
-aq.filled pores or perforations of 4 A° in diameter are also present in the membrane structure through which inorganic ions&small organic water-soluble molecules like urea can pass.
-biomembrane acts like a semipermeable barrier permitting rapid and limited passage of some compounds while restricting that of others.
-the GI lining contains absorption barrier allows most nutrients like glucose,fatty acids,vitamins,etc,.to pass rapidly through into the systemic circulation but prevents the entry of certain toxins and medicaments.
-thus for long to get absorbed after administration, it must first pass through this biological barrier.
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