Physiological factors
1. Gastric motility
It has large effect. Some disorders like diabetic neuropathy causes gastric stasis and decrease the drug absorption.
Due to drug treatment some time motility is increased or decreased. The excessively rapid movement of GIT impairs absorption. This is because movement of GIT wall the drug particle can easily comes in contact with mucosa and get absorbed, so due to faster movement of GIT the drug particles are not able to come in contact with mucosa
2. Ionization and gastric emptying
Acidic drugs are absorbed faster in acidic pH because acidic drugs remain unionized in acidic medium (stomach). So they can be absorbed through lipidic cell membrane. Basic drugs are not absorbed well in acidic pH because the basic drugs ionize in acidic medium. Basic drugs remain unionize in basic medium (small intestine) and can be easily absorbed. Acidic drugs ionize in basic medium so can’t be absorbed.
Thus, if acidic drugs remain for long time into stomach, they get absorbed at a faster rate. And if basic drug remains for a short time in stomach and being more time in small intestine, they get easily absorbed.
It means, for acidic drug gastric emptying time should be more and for basic drugs gastric emptying time should be faster.
3. Splanchnic blood flow
Some drugs are achieving higher plasma conc. after food, this is because food increase splanchnic blood flow.
Example: propranolol, chloramphenicol, lithium carbonate.
The absorption of some drugs is reduced due to presence of food like ampicillin, aspirin, L-dopa.
In hypovalemic state, the splanchnic blood flow is reduced. So absorption of the drug is also decreased.
4. Enterohepatic circulation
This increases bioavailability of the drug. For example, morphine.
The morphine is less potent when given orally. It undergoes enterohepatic cycling. If more is the cycling of drugs, then more absorption of drugs.
5. Metabolism of drug
If any drug is undergoing first pass metabolism and it may be degraded by liver or by the enzyme present in the gut, the bioavailability of the drug is decreased.
Thus drug absorb well after oral administration but may not be effective due to first pass metabolism.
6. Pharmacogenetic factors
Enzymes influencing metabolic reactions are under genetic control. A study of variability of action of drug in different human is called as pharmacogenetic.
Differences observed in the metabolism of the drug among different races are called ethnic variation.
For example, alcohol dehydrogenase enzyme, present in gastric mucosa, converts the alcohol into acetaldehyde, which is not intoxicating.
- females have less quantity of this enzyme. So they often develops more adverse effect of alcohol than man.
- Asian males also have less level of this enzyme.
Some time, the drug is pseudotolerated by the patient, which can’t be pseudotolerated by another one.
7. Disease state
Some diseases like mal absorption, thyrotoxicosis, achlohydria, cirrhosis of liver and billiary obstruction can affect he absorption & bioavailability of drugs.
In case of mal absorption the pseudotolerance is developed and drug can’t be absorbed.
In thyrotoxicosis(hyperthyroidism), the weight loss occurs due to less appetite.
In achlorhydria(decrease in Hcl level in stomach), acidic drug can’t be absorbed.
In cirrhosis of liver the drug is more metabolized and produces toxic effect.
8. Route of administration
Parenterals are absorbed at a faster rate than oral administration. Bioavailability of intra venous is 100%.
Some drugs like insulin can’t be given by oral route due to destruction by GIT enzymes. Thus, the drug is to be administered by the selective route.
9. Presence of other substance
Vitamin c favours the absorption of iron from GIT while phytates retard it.
Absorption of fat soluble vitamin is increased in presence of liquid paraffin.
Cholesterol absorption is reduced by sitosterol and calcium complexes with tetracyclines & retards its release from drug and decrease absorption.
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