The gills are ventilated continuously and unidirectionally with water. The blood flows in the opposite direction of the water creating a countercurrent exchange mechanism that further enhances the efficiency of gas exchange. Figure 45.6 illustrates the countercurrent exchange mechanism found in fishes. Bird Lungs and Air Sacs Birds have a system of a lung and air sacs that are interconnected. 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.