These drugs are usually absorbed from sites in the small intestine. Active transport processes have been identified for various ions, vitamins, sugars, and amino acids. Fluid or particles are engulfed by a cell. The cell membrane invaginates, encloses the fluid or particles, then fuses again, forming a vesicle that later detaches and moves to the cell interior. This mechanism also requires energy expenditure.
Pinocytosis probably plays a minor role in drug transport, except for protein drugs.
Oral Administration For oral administration, the most common route, absorption refers to the transport of drugs across membranes of the epithelial cells in the GI tract. Absorption after oral administration is confounded by differences in luminal pH along the GI tract, surface area per luminal volume, blood perfusion, the presence of bile and mucus, and the nature of epithelial membranes. Acids are absorbed faster in the intestine than in the stomach, apparently contradicting the hypothesis that un-ionized drug more readily crosses membranes. However, the apparent contradiction is explained by the larger surface area and greater permeability of the membranes in the small intestine. The oral mucosa has a thin epithelium and a rich vascularity that favors absorption, but contact is usually too brief, even for drugs in solution, for appreciable absorption to occur.