Capillary permeability is the ability of capillary walls to allow fluid, nutrients, gases, and blood cells to pass between the vascular bed and tissues. It is regulated by the structure of the endothelium, the action of inflammatory mediators, and other physiological factors.

Increased permeability can lead to the release of plasma and proteins into the interstitial space, causing edema and inflammatory reactions. Impaired capillary permeability is observed in the case of allergies, infections, injuries, and some systemic diseases.