Final answer:
The gastrointestinal (GI) tract is the body surface that absorbs drugs most readily, primarily through passive diffusion. Factors such as metabolizing enzymes and efflux transporters can affect drug absorption in the GI tract.
Step-by-step explanation:
The body surface that absorbs drugs most readily is the gastrointestinal (GI) tract. When drugs are taken orally, they pass through the GI tract, where they can be absorbed by different mechanisms. The most important mechanism for drug absorption is passive diffusion, which accounts for about 95% of all commercial drugs. Other factors, such as metabolizing enzymes and efflux transporters, can limit the absorption of drugs in the GI tract.