The property of blood capillary endothelium that allows for the selective exchange of substances between the blood and surrounding tissues and through membranous barriers such as the blood-air barrier; blood-aqueous barrier; blood-brain barrier; blood-nerve barrier; blood-retinal barrier; and blood-testis barrier. Small lipid-soluble molecules such as carbon dioxide and oxygen move freely by diffusion. water and water-soluble molecules cannot pass through the endothelial walls and are dependent on microscopic pores. These pores show narrow areas (tight junctions) which may limit large molecule movement.