CapillaryCapillaries are the smallest of the body's blood vessels. They connect arteries and veins. Capillaries have walls so thin that oxygen, water and lipids can pass through them by diffusion and enter tissues. Waste products such as carbon dioxide can pass back into the blood to be carried away by hemoglobinand taken out of the body.
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.