Changes

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 body; in the liver the gaps are large but in the central nervous system these gaps are tight junctions.

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 body; in the liver the gaps are 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.

The ability of a drug to cross the phospholipid bilayer not only influences the rate and extent of its absorption but also the rate and extent of its distribution, metabolism and elimination.

Below are the main factors affecting absorption:

Below are the main factors affecting absorption: