Illustration of the blood circulatory system. The heart pumps oxygenated blood into the body, via arteries (red), and to the organs where the oxygen is consumed. The now deoxygenated blood is carried by veins (blue) back to the right side of the heart, where it is pumped into the blood vessels of the lungs, where it is re-oxygenated. From the lungs it flows back into the left side of the heart and the cycle begins again.