During gas exchange, oxygen from the air breathed in diffuses into the bloodstream, and carbon dioxide from the bloodstream diffuses into the air in the alveoli. This gas exchange occurs through the thin wall of the alveoli.
The alveoli are small, round protrusions in the lungs. The interior of the alveoli is maximized to provide a large surface area for gas exchange. Capillaries cover the outside walls of the alveoli. These capillaries contain deoxygenized blood that is high in carbon dioxide waste. The carbon dioxide moves through the thin capillary wall into the alveoli where it mixes with air and is exhaled. At the same time, oxygen in the inhaled air passes into the capillaries, which carry it to nourish other parts of the body.