Capillary

Capillaries 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.

When the material to be exchanged is too big to diffuse, vesicles contained in the capillary membrane use endocytosis and exocytosis to transport material out of the blood.

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.

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.

Interesting facts

The total lenght of capillaries in humans is approximately 25,000 miles.