There are three main types of blood vessels.One of the blood vessels are arteries. Arteries carry blood away from the heart. The arteries also have a thick wall with small passageways for blood. The second blood vessels are veins. Unlike arteries veins carry blood towards the heart. The veins have thin walls and have valves to prevent blood from flowing backwards. The last type of blood vessels are capillaries. Capillaries are found in the muscles and in lungs. Capillaries are usually where gas exchange takes place. Oxygen passes through the capillary wall and into the tissues, carbon dioxide passes from the tissues into the blood.

Blood vessels