The percentage of carbon dioxide is slightly higher in air samples collected from the alveoli than in exhaled air because the alveoli are where gas exchange occurs in the lungs. When we inhale, oxygen is taken up by the blood in the lungs and carbon dioxide is released into the alveoli. When we exhale, the air that is released comes from the dead space in our lungs, which is air that did not participate in gas exchange. Therefore, the air we exhale has a lower concentration of carbon dioxide than the air in the alveoli.