Gas exchange occurs in the alveoli of the lungs. In these alveoli, carbon dioxide moves from the red blood cells into the alveoli, and oxygen moves from the alveoli into the red blood cells.
Oxygen enters the body through the act of inhaling. Oxygen molecules travel through the nose down the windpipe or trachea. The gas then enters the bronchial tree, which consists of the two bronchi and their thousands of branches, which are called bronchioles.
At the end of the bronchiole lies a microscopic sac called an alveolus that holds the gases for exchange. The lungs contain millions of alveoli. Epithelial cells line the alveoli. Surrounding the entire alveolus is a network of blood vessels called capillaries. The oxygen in the interior of the alveoli passes through the alveolar epithelial cell. The gas then traverses the basement membrane material sitting in the space between the alveoli and the smallest of blood vessels, the capillaries. This oxygen then diffuses through the capillary walls through the walls of the red blood cells. The red blood cells then carry this oxygen back to the heart to be pumped to the rest of the body.
Carbon dioxide travels through the red blood cell walls and through the walls of the capillaries. This waste product then moves through the basement membrane of the extracellular space. From this space, carbon dioxide goes through the membrane of the alveolar epithelial cells into the alveolar space. Here, the carbon dioxide awaits expulsion from the lungs to the air outside the body.