Figure 49.2 The Human Heart and Circulation (A) The major blood vessels of the body either carry blood away from the heart (arteries) or return blood to the heart (veins). (B) In the human heart, blood flows from right side of the heart to the lungs to the left side of the heart and then out to the body. 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 arteries when the ventricles relax.