The absorption of drugs into the bloodstream varies depending on a wide variety of factors. Two factors that play a major role in determining the absorption of quinine and salicylic acid from the body are the acidic and basic nature of the two drugs respectively, as well as the different pH (power of hydrogen) values of the stomach and intestine environment.
Unless assisted by membrane transporter proteins or channels, excessively polar or ionic compounds cannot be transported across the plasma membrane. Hence, compounds that have dissociated into their ionic forms are not easily absorbed, whereas compounds that have not dissociated are.
This is where the acidic nature of salicylic acid, and the basic nature of quinine, comes into play. The stomach as an acidic pH environment due to the presence of gastric juices, and the small intestine has a basic environment due to the presence of bile. Hence, salicylic acid is stable in the stomach and dissociated in the small intestine, whereas quinine is stable in the small intestine and dissociated in the stomach.
Since dissociated ions cannot usually diffuse through the plasma membrane, salicylic acid will not be effectively absorbed through the small intestine, and quinine will not be effectively absorbed through the stomach. However, the non-ionized molecules can diffuse through the plasma membrane, allowing salicylic acid to be absorbed through the stomach, and quinine to be absorbed through the intestine.