What are the thinnest and smallest blood vessels?

The thinnest and smallest blood vessels are called capillaries. Capillaries are the smallest type of blood vessel in the body and are responsible for the exchange of substances between the blood and the tissues. They are found in all organs and tissues of the body, except for the cornea of the eye and cartilage. Capillaries are typically only about 5-10 micrometers in diameter, or about the size of a single red blood cell.