PHARMACOLOGY

 ABSORPTION VIA PARENTERAL SITES ::

Drugs when injected intravenously (IV) are rapidly distributed as they reach the blood stream directly without crossing any membrances 

The factors which influence the rate of absorption from intramuscular (IM) and subcutaneous (SC) injection sites are :: (1) Drug concentration (2) Solubiliy of the drug and (3) Local blood flow : Absorption following IM and SC injections usually occurs by simple diffusion in the direction of the concentration gradient from the injection site of the plasma or lymph 

    Absorption of drugs from IM sites is usually more repid than from SC sites because of the higher vascularity of the muscle compared to substaneous tissue some of the important factors controlling absorption from SC or IM sites are the area of the absorbing capillary membranes and the solubility of the drug in the interstitial fluid Filtration through channels in the endothelial capillary membrane is also very efficient as these channels are relatively large and can allow passage of most lipid -insoluble drugs 

            The dosage rulation can also affecrabsorption from parenteral sites e.g. drugs in aqueous solutions are usually absorbed more rapidly than drugs in suspension e.g. benzathine penicillin with procaine penicillin G Often drugs are suspended in certain vehicles like oil to reduce their rate of absorption and provide a prolonged action e.g. long -acting hormone preparations 

        Decreased peripheral blood flow in conditions of shock significantly reduces the rate of absorption of injected drugs Blood flow to an area can be increased and absorption enhanced by application of heat local vasodilators and massage In contrast the absorption can be delayed by application of cold a tourniquet or vasoconstrictors all the above mentioned factors have a profound effect on the onset and duration of action of a drug