RESPIRATION 



FIG. 38. 



AIR 



FIG. 39. 



FIG. 38. Diagram to show the double movement of air and blood 

 through the lungs. The blood leaves the heart by the pulmonary artery 

 and returns by the pulmonary veins. The air enters and leaves the lungs by 

 the same system of tubes. 



FIG. 39. Diagram to show air and blood movements in a terminal 

 air sac. While the air moves into and from the space within the sac, the 

 blood circulates through the sac walls. 



