2. The Terminal End of the Breathing Tube

3.  After oxygen passes into the lungs through the bronchial tubes, it moves into tiny sacs called alveoli  Alveoli are spongy, air filled sacs that are surrounded by capillaries  The basic function of the alveoli is gas exchange

5.  The inhaled oxygen moves into the alveoli. The walls of the alveoli are just two cells thick.  The thin walls allow the oxygen to diffuse out of the alveoli and into the capillaries where it is taken up by red blood cells  As the oxygen moves into the red cells, carbon dioxide (a waste product), is released from the cells  The CO 2 moves into the alveoli and it is exhaled from the lungs

6.  There are about 400 to 500 million alveoli in the adult lung  Because there are so many alveoli, their surface area is very large  This provides a large area for gas exchange to occur