Pharmacology: Bioavailability

Introduction

Drug Properties

Physiology

Medicine Properties

Bioavailability

Activity

Assessment

Resources

Factor 2: Important physiological factors

Physiological factors that reduce the absorption of a drug include rapid intestinal transit time, reduced splanchic circulation (such as occurs in shock) and the presence of food in the stomach.

However, one of the most important factors is first pass metabolism - that is the extent to which the drug is destroyed on its first passage through the liver. If a drug is rapidly metabolised by the liver, the amount of active drug the reaches the systemic circulation will be reduced. Propranolol is one drug that is significantly transformed during a single pass through the liver whilst, if swallowed, glyceryl trinitrate (GTN) is rendered ineffective by first pass metabolism.