Capillary permeability is the ability of capillary walls to allow the passage of water, ions, nutrients, proteins, and other molecules between the bloodstream and surrounding tissues. It is regulated by the structure of the endothelial cells and can increase during inflammation due to mediators such as histamine, bradykinin, and leukotrienes, allowing immune cells and plasma proteins to reach sites of injury or infection. Excessive capillary permeability, however, can result in leakage of fluid into the interstitial space, leading to edema, tissue swelling, and impaired organ function.