Question

, Explain in detail the anatomy behind “first pass metabolism” and then explain in detail the physiological significance.

Answer 1

First pass metabolism-

This is the Phenominon by which the concentration of a drug which is administered orally, is reduced before it reaches the systemic circulation.

Anatomy-

From The Buccal Cavity The drug Swallowed Pass through Mouth---Oesophagus--Stomach---Small Intestine where some of the Drugs are Absorbed.

Portal System which Connects GI Tract (Small intestine) to Liver Takes the Blood into Liver.From the liver Some quantity of Drugs are metabolised into inactive compounds by the liver.From the Liver the drugs passes into Systemic Circulation.

Physiological Significance-

Due to this First Pass effect or first pass metabolism,The Bioavailabilty of a drug (percentage of administered drug that reaches the systemic circulation)which is taken Orally is less than The Concentration of Drug Taken.So For a Drug Given Orally,The Dose that is Administered Should be Greater than the Dose of Drug Required in the Blood for its action.

For Example:Nitroglycerine -If given Orally ,90 % of the Drug is Cleared from the Liver,Only 10% will be available for action.So it is not given orally,It is given Sublingually so that the Drug is absorbed directly into Circulation.