Small blood vessels called capillaries run along the walls of the air sacs. When air reaches the air sacs, oxygen passes through the air sac walls into the blood in the capillaries. At the same time, a waste product, called carbon dioxide (CO2) gas, moves from the capillaries into the air sacs. This process, called gas exchange, brings in oxygen for the body to use for vital functions and removes the CO2.
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Randy Palmer - Independent Insurance Agent in Michigan.