Figure 32.6: The Human Heart and Circulation In the human heart, blood flows from the right heart to the lungs, then to the left heart, and from there to the systemic tissues. Color indicates whether the blood is O2-rich (red) or O2-depleted (blue). Thus the pulmonary arteries (carrying blood to the lungs) are blue, and the pulmonary veins (carrying blood to the heart from the lungs) are red, although arteries in the systemic circuit are red and veins in that circuit are blue. The atrioventricular valves prevent blood from flowing back into the atria when the ventricles contract. The pulmonary and aortic valves prevent blood from flowing back into the ventricles from the pulmonary artery and aorta, respectively, when the ventricles relax.