Capillaries are the smallest of the body's blood vessels. They connect arteries and veins. Capillaries have walls composed of a single layer of cells, the endothelium. This cellular 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.
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.
The word 'capillary' can also be used to describe any very narrow tube or channel through which a fluid can pass. The adhesive properties of water cause it to rise against gravity within a capillary until the adhesive forces equal the gravitational force on the water column. This phenomenon is known as the capillary effect.