3.15 : Factors Influencing Drug Absorption: Anatomical Parameters

Drug absorption involves the movement of drugs from the point of administration into the systemic circulation. Initially, Gastrointestinal (GI) motility propels the drug through the digestive tract and into the stomach. However, the stomach's high acidity and limited surface area restrict its role in drug absorption for most drugs. The drug then moves from the stomach to the small intestine via gastric emptying, which can be slowed by various factors, including interactions with other medications, other disease states, high-fat meals, or cold beverage consumption. This results in prolonging the absorption time.

The small intestine (including the duodenum, jejunum, and ileum) is typically the primary site for drug absorption due to its large surface area and favorable environment. The duodenum neutralizes the acidic content received from the stomach. It also has an intricate design featuring folds, villi, and microvilli, which offer an extensive surface area for absorption, and its proximity to a dense capillary network facilitates rapid drug movement along the concentration gradient. However, the specific site of drug absorption can vary depending on its properties, such as its solubility, stability, and the mechanisms involved in its absorption.

Intestinal peristaltic motion helps to mix the drug with digestive fluids, thereby enhancing contact with intestinal mucosal cells. Rapid intestinal motility, as seen in diarrhea, can lead to inadequate absorption, whereas extended intestinal transit times facilitate complete drug absorption. Drug absorption is a finely tuned orchestration of various physiological processes.