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→Absorption
==Absorption==
==Absorption==
Drugs are most usually small molecules (<1000 molecular weight) and thus can pass through cell membranes through '''passive diffusion''' and '''facilitated transport'''. For further information on these processes please look [[Transport Across Membranes - Physiology|here]]. To cross vascular endothelium drugs usually are able to squeeze through the gaps between the cells. The size of these gaps varies between different locations in the bosy; in the liver the gaps ate large but in the central nervous system these gaps are tight junctions.
The ability of a drug to cross the phospholipid bilayer not only infuences the rate and extent of its absorption but also the rate and extent of it's distribution, metabolism and elimination.
Below are the main factors affecting absorption:
===Properties of the Drug===
* Lipid solubility - the more lipid soluble a drug the easier it is absorbed across the bilayer
* Chemical nature - ie. Is it basic or acidic? Is it ionised or not?
* Molecular weight
* Stability in the gastro-intestinal tract - only applies to orally administered drugs
===Physiological Variables===