Video on aspirin pharmacokinetics: the role of the ion trapping phenomenon

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.

image Non-ionized form a at the gastric lumen. It has 8 white spheres.





image 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!


Recommended reading

  • Pocket Guide: Pharmacokinetics Made Easy (2009)
  • Basic Clinical Pharmacokinetics (2009)
  • Concepts in Clinical Pharmacokinetics (2010)
  • Clinical Pharmacokinetics, 4th Edition (2008)
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