The circulatory system transports blood throughout the body, delivering oxygen and nutrients to tissues and removing waste. It has three main parts: the heart, arteries, and veins. The heart has three layers of tissue in its walls and pumps blood through the body, producing a lub-dub sound. The three types of major vessels are arteries, veins, and capillaries. Veins have valves and muscle contractions that help blood return to the heart. The average adult contains 5 liters of blood composed of plasma, red blood cells, white blood cells, and platelets. Red blood cells contain hemoglobin which carries oxygen and white blood cells help fight infection.