Question

Aspirin is a weak acid with a pKa of 3.5. It is absorbed into the blood through the cells lining the stomach and the small intestine. Absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. The pH of the stomach contents is about 1.5, and the pH of the contents of the small intestine is about 6. Is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? Clearly justify your choice.

Answer 1

More aspirin is absorbed into the bloodstream from the small intestine.

Step-by-step explanation:

The pH of the stomach contents is highly acidic (about 1.5), which is below the pKa of aspirin (3.5). In an acidic environment, aspirin remains predominantly in the non-ionized form, which is more lipophilic (hydrophobic) and can pass through the plasma membrane more easily. However, the small intestine has a higher pH (about 6), closer to the pKa of aspirin. In this less acidic environment, a larger proportion of aspirin molecules will be ionized, which slows down their passage through the plasma membrane. Therefore, more aspirin is absorbed from the small intestine where the pH is closer to the pKa, and a greater proportion of the drug exists in the ionized form.