Blood flows from the heart to the body and back to the heart. This nonstop flow is called circulation. Blood flows through blood vessels. These are hollow muscular tubes. There are 3 main types of blood vessels:
Arteries are vessels that carry oxygen-rich blood away from the heart.
Veins are vessels that return oxygen-poor blood to the heart.
Capillaries are tiny blood vessels that connect arteries and veins. Capillaries have thin walls that allow oxygen and nutrients to pass through the walls to your body parts. They also allow waste products to move from your body parts to the blood.