The lungs are used for gas exchange and the air sacs are used as bellows that force air into the air capillaries in the lung that run between parabronchi. Gas exchange occurs in the air capillaries of the lung. Figure 45.9 illustrates the unique air flow seen in birds. Mollusks also have a partially open system with both arteries and veins. Vertebrates have increasingly complex hearts in their closed systems.
Fish have a two-chambered heart while mammals and birds have four chambered hearts.
Study Figure 46.4 to familiarize yourself with the changes that occur in heart structure from the more primitive to more advanced vertebrates. Note in which organisms blood is mixed and in which ones it is kept separated. Review Figure 46.5 to understand the structures of the heart and associated blood vessels. Remember that the coronary sinus also returns blood to the right atrium. Figure 46.6 illustrates the cardiac cycle .