While the small intestine has all forms of drug transport, the predominant process is _______.

While the small intestine has all forms of drug transport, the predominant process is _______.

• A. Active transport
• B. Facilitated diffusion
• C. Diffusion
• D. Endocytosis

Answer: (C)

Passive diffusion dominates drug absorption in the small intestine. Most small molecules cross the intestinal epithelium by moving down their concentration gradient through the lipid membranes, driven by their lipophilicity and the unionized fraction of the drug in the local pH. This non-energy-dependent process leverages the large surface area of the small intestine and rapid blood flow to maintain the gradient. Other routes exist—active transport uses energy and carrier proteins to move substances against a gradient, facilitated diffusion uses carriers without energy for specific substrates, and endocytosis handles larger molecules—but these are not the typical route for most small-molecule drugs. Hence diffusion is the main mechanism by which drugs are absorbed in the small intestine.

Passive diffusion dominates drug absorption in the small intestine. Most small molecules cross the intestinal epithelium by moving down their concentration gradient through the lipid membranes, driven by their lipophilicity and the unionized fraction of the drug in the local pH. This non-energy-dependent process leverages the large surface area of the small intestine and rapid blood flow to maintain the gradient. Other routes exist—active transport uses energy and carrier proteins to move substances against a gradient, facilitated diffusion uses carriers without energy for specific substrates, and endocytosis handles larger molecules—but these are not the typical route for most small-molecule drugs. Hence diffusion is the main mechanism by which drugs are absorbed in the small intestine.