Patient Factors
The rate of drug penetration across a membrane is related to the surface area of the membrane. For this reason, drug absorption from the intestinal lumen is more efficient than across the stomach, as the surface area of the intestinal microvilli is about 200 times larger than that of the stomach. Absorption may be impaired in patients who have had sections of small bowel removed (e.g. Crohn's disease).
The health of the absorbing surface is also a factor. For example, drug absorption may be impaired in conditions that lead to intestinal malabsorption (e.g. coeliac disease), whereas the absorption of drugs applied to areas of inflamed skin may be enhanced (e.g. dermatitis).
Another important patient factor is the perfusion of the tissue where absorption is taking place, which determines how fast drug molecules are removed from the area once the absorbing surface has been penetrated. This in turn depends on the vascularity of absorbing surface, which relates to the nature of the tissue (blood flow through the intestinal capillaries is 6-7 times that through the gastric capillaries) and blood flow in the general circulation. Thus, in hypovolaemic shock, absorption after subcutaneous administration may be so unreliable that intravenous injection becomes necessary.
Whereas the rate-limiting step for absorption of drugs with low lipid solubility is the rate of membrane penetration, highly lipid-soluble drugs cross membranes so rapidly that the rate of absorption is limited primarily by the perfusion of the tissue.
Some drugs require specific mechanisms for their absorption (Fig 4). For example, intrinsic factor produced in the stomach is essential for vitamin B12 absorption from lower ileum. Iron and calcium salts are transported across the intestinal mucosa by specific carriers.
Fig 4 Mechanisms of drug absorption across membranes.
