The smallest blood vessels in the human body are capillaries, which connect arteries and veins. Capillaries can be as small as 5 micrometers wide. The average adult body has about 10 billion capillaries.
Capillaries are part of the circulation system in the human body. They help move oxygen, water, carbon dioxide and other nutrients throughout the body. Arteries transfer blood out of the heart. Arteries narrow into capillaries, then widen into venules and veins. The veins transport blood back into the heart. Capillaries work as a group and not individually. During exercise, capillaries assist the lungs with meeting the need for increased blood flow.