There are three main types of blood vessels, arteries, veins and capillaries. ARRTERIES USUALLY CARRY OXYGENATED BLOOD AWAY FROM THE HEART, except for the pulmonary artery which carries deoxygenated blood to the lungs. VEINS USUALLY CARRY DEOXYGENATED BLOOD BACK TO THE HEARTFROM THE TISSUES, except for the pulmonary vein in the heart which carries oxygenated blood from the lung to the left atrium. Veins have valves. Substances diffuse in and out of the blood in the capillaries.