Some key facts before you watch the video on aspirin absorption:
- Aspirin is a weak acid (pKa 3.4).
- Acidic drugs in an acidic environment (like gastric lumen) are most likely to be in their neutral form ( non-ionized )
- Non-ionized drugs are more lipid soluble. Hence, they can be readily absorbed
- Since aspirin is a weak acid and gastric pH is an acidic environment, it can be readily absorbed.
- Bloodstream has a pH of around 7.4, therefore aspirin tends to be ionized. This prevents the drug from diffusing back to the stomach.
Source: University of Rhode Island Pharmacy Animations
Now, watch again the video paying attention to the following details.
Non-ionized form a at the gastric lumen. It has 8 white spheres.
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Ionized form of aspirin, in the bloodstream. It has 7 white spheres, this means that it has lost a proton.
Deprotonated form = ionized = non-lipid soluble.
Now,since the drug is ionized it can’t diffuse back to the stomach and remains in the bloodstream.
This is ion trapping!