Why do capillaries have to be leaky?

Capillaries need to be leaky to facilitate the exchange of nutrients, gases, and waste products between blood and surrounding tissues. This permeability allows essential substances like oxygen and glucose to diffuse into cells, while carbon dioxide and metabolic waste can exit the bloodstream. The leaky nature of capillaries is particularly important in organs where rapid exchange is necessary, such as in the lungs and kidneys. Additionally, the leaky characteristics can help in immune responses by allowing white blood cells to reach sites of infection or injury.