Passive diffusion allows small lipophilic drugs to cross the cell membranes along a concentration gradient. During this process, the drug absorption rate increases linearly with the concentration increase.

The rate of diffusion depends on four factors: the membrane surface area, the drug's lipid-water partition coefficient, the concentration gradient, and membrane thickness.

Orally taken drugs create a significant concentration difference between the GI lumen and the blood, facilitating quick diffusion into the bloodstream.

Highly lipid-soluble drugs having a higher partition coefficient diffuse more readily across the membrane.

In the small intestine, duodenal microvilli enhance the surface area, leading to rapid drug absorption into blood vessels.

Thin, permeable capillary membranes efficiently diffuse drugs into blood vessels, whereas a thick blood-brain barrier lined by glial cells limits drug diffusion into the brain.

For drugs that are weak acids or bases,  changes in the pH levels can influence their diffusion across membranes.