Capillary - Relating to or resembling a hair; fine and slender. Having a very small internal diameter: a capillary tube. Anatomy. Of or relating to the capillaries. Physics. Of or relating to capillarity. n., pl. -ies. Anatomy. One of the minute blood vessels that connect arterioles and venules. These blood vessels form an intricate network throughout the body for the interchange of various substances, such as oxygen and carbon dioxide, between blood and tissue cells. A tube with a very small internal diameter.
Membrane - A flexible layer surrounding a cell, organelle (such as the nucleus), or other bodily structure. The movement of molecules across a membrane is strictly regulated in both directions.