The most permeable capillaries are those exhibiting what?

The most permeable capillaries are those exhibiting fenestrations.

Fenestrations are small pores or openings in the capillary endothelial cells that allow for the rapid exchange of substances between the blood and the surrounding tissues. This type of capillary is found in tissues where there is a high demand for nutrients and oxygen, such as the kidneys, liver, and intestines.