Capillaries are the smallest of a body's blood vessels. They connect arteries and veins. Capillaries have walls composed of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillary permeability can be increased by the release of certain cytokines.
Very large molecules may be too big to diffuse across endothelial cells. In some cases, vesicles contained in the capillary membrane use endocytosis and exocytosis to transport material between blood and the tissues.
Some nutrients are also carried out by 'bulk flow', the flow of 'water' (plasma) caused by a high pressure inside the capillary. Nutrients are carried out through capillary clefts.
In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus "catching" immune cells as they pass by the site of infection and aiding extravasation of these cells into the tissue.
Sometimes people who have had diabetes for a long time find that their capillaries become weak, especially those in the kidney and the retina of the eye.
The total length of capillaries in humans is approximately 40,000 km (25,000 miles).
Capillary diameter is 5 to 10 micrometres.