THE RESPIRATORY SYSTEM
The respiratory system consists of two lungs and the respiratory tract that carries
air to and from the lungs (figure 3-2). When a person inhales, air enters the nose or
mouth, travels down the trachea, and into one of the two bronchi. These air tubes
divide into smaller and smaller tubes. Finally, the air reaches the tiny alveoli. These
alveoli are like tiny sacs. When air enters an alveolus, the capillaries surrounding the
alveolus absorb oxygen and give off carbon dioxide. When a person exhales, the air
carrying the carbon dioxide travels from the alveoli through the air tubes, up the trachea,
and out of the nose or mouth. Of course, not all of the air inhaled reaches the alveoli
nor is all of the oxygen removed from the air in the alveoli. The average adult takes in
about 500 milliliters of air each time he inhales, and he exhales the same amount. Even
after the person exhales, the lungs still contain about 2300 milliliters of air.
Figure 3-2. The respiratory system.