Red blood cells transport oxygen throughout the body and remove carbon dioxide, while white blood cells defend against infection and foreign materials. Platelets prevent bleeding. The circulatory system includes arteries that carry blood away from the heart, veins that carry blood toward the heart, and capillaries that connect arteries and veins and allow for gas and nutrient exchange. The four main blood vessels connected to the heart are the inferior and superior vena cavae, aorta, and pulmonary artery.