What occurs when aspirin (ASA) is introduced into the small bowel?

When aspirin (acetylsalicylic acid) is introduced into the small bowel, the pH environment of the small intestine, which is more alkaline than the stomach, leads to the ionization of aspirin. In this more alkaline environment, aspirin, which is a weak acid, ionizes more compared to its state in the acidic pH of the stomach.

As aspirin ionizes, it becomes less able to cross cell membranes, which are lipid bilayers that facilitate the passage of non-ionized, or unionized, drugs. This decreased ability to diffuse across the enterocyte membranes can result in reduced absorption of aspirin in the small bowel, ultimately leading to a loss of its effectiveness as it reaches systemic circulation. Thus, the correct conclusion is that upon reaching the small bowel, aspirin ionizes and loses its effectiveness as an unbound agent capable of passive diffusion across membranes.