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Chapter: Medicine Study Notes : Pharmacology

Absorption - Pharmacokinetics

Food: may slow gastric emptying, alter ionisation, decrease first pass metabolism. Affects different drugs differently

Absorption

 

·        3 routes


o  Passive diffusion:

§  Most important

§  Net transfer depends on concentration gradient and lipid:water partition co-efficient

§  Lipid soluble drugs absorbed more quickly

§  Non-ionised drugs absorbed quicker (more lipid soluble)

§  Absorbed along whole GI tract: basis for slow release preparations


 

o   Active transport: highly specific – usually for transporting naturally occurring substances (e.g. amino acids)

 

o   Filtration through pores: absorbed via paracellular route in small intestine. E.g. frusemide, atenolol, digoxin


·        Factors affecting oral absorption:

o   Formulation: affects solubility and bioavailability.  Presence of other drugs (e.g. Fe, Ca)

 

o   Gastric emptying: ¯emptying ® ¯ absorption rate

 

o   Food: may slow gastric emptying, alter ionisation, decrease first pass metabolism. Affects different drugs differently


·        Alternative sites of absorption: 


o   Plasma concentration rises quickest for iv > im > sc (absorption similar to im but ¯ blood supply)


o   Intramuscular injection: affected by lipid solubility and blood flow


o   Rectal absorption: avoids GI irritation, good in vomiting, doesn‟t avoid 1st pass metabolism


o   Pulmonary absorption: small particle size


o   Absorption from mouth: nitrates/buprenorphine


o   Absorption from nose: desmopressin & other peptides


o   Percutaneous: zero order absorption – for nicotine, nitrates, scopolamine

 

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Medicine Study Notes : Pharmacology : Absorption - Pharmacokinetics |


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