Answer:
Carbon dioxide (CO2) is a waste product of cellular metabolism. You get rid of it when you breathe out (exhale). This gas is transported in the opposite direction to oxygen: It passes from the bloodstream – across the lining of the air sacs – into the lungs and out into the open.
The smallest bronchioles end in tiny air sacs, called alveoli, which inflate during inhalation, and deflate during exhalation. Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs.