blood capillaries [AS, blod + L, capillaris, hairlike] , the minute vessels connecting arterioles and venules, the walls of which act as a semipermeable membrane for exchanges of various substances between the blood and tissue fluid. A capillary wall generally has a thickness of one simple squamous epithelial cell, permitting easy diffusion of gas molecules and other dissolved substances. Occasional small openings permit diapedesis of leukocytes, distribution of nutrients to the tissues supplied by the capillary network, and collection of waste products released by the cells.
