The actual sites of gas exchange within the lungs are within tiny air sacs called alveoli. They are found at the end of the bronchial tubes.
When a person inhales, the partial pressure of oxygen is lower in the alveoli, causing gas to diffuse into the surrounding pulmonary capillaries, which move it into the bloodstream. Simultaneously, the partial pressure of carbon dioxide is higher in the capillaries, allowing it to diffuse from the capillaries to the alveoli. As a person exhales, the carbon dioxide is removed from the lungs. The ventilation/perfusion ratio describes the amount of blood and gas the alveoli need for adequate gas exchange.