PHARMACOLOGY

 MOLECULAR WEIGHT ::

Substances with a high molecular weight are not usually absorbed intact except in minute quantities Thay may also be altered by enzymatic ation e.g. on oral administration proteins will be broken down to their constituent amino acids Thus insulin undergoes enzymatic breakdown in the gut and for practical purposes is not effectively absorbed 

Chemical stability ::

Unstable drugs are inactivated in the gastrointestinal tract Benzylpenicillin is unstable in and acid medium and cannot be relied upon to produce satisfactory results on oral administration because a high proportion of the drug is inactivated by the acid in the stomach in contrast phenoxymethylpenicillin is more stable in an acid medium than benzylpenicillin and adequate oral doses are therapeutically effective 

            Thus higher pKa is required for acidic drugs for getting non-ionized form of drug required for the absorption Similarly lower pKb is required for acidic drugs for their greater absorption 

Concentration of drug :: 

Higher concentration of drug favours absorption 

Area of the absorbing surface ::

Larger the absorption area greater is the absorption e.g.in small intestine the surface area is very large so absorption of drugs is greater in this part 

Blood supply to the area of absorption ::

Greater the blood supply greater is the rate of absorption