capillary, microscopic blood vessel, smallest unit of the circulatory system. Capillaries form a network of tiny tubes throughout the body, connecting arterioles (smallest arteries) and venules (smallest veins). Through the thin capillary walls, which are composed of a single layer of cells, the nutritive material and oxygen in the blood pass into the body tissues, and waste matter and carbon dioxide in turn are absorbed from the tissues into the bloodstream.

Any of the minute blood vessels that form networks where the arterial and venous circulation (see artery, vein) meet for exchange of oxygen, nutrients, and wastes with body tissues. Capillaries are just large enough for red blood cells to pass through in single file. Their thin walls are semipermeable, allowing small molecules to pass through in both directions. The smallest lymphatic vessels and minute bile channels in the liver are also called capillaries.

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