CBSE Class 10 Answered

When you inhale, the alveoli fill with air. Because the oxygen concentration is high in the alveoli and low in the blood entering the pulmonary capillaries, oxygen diffuses from the air into the blood. Likewise, because the concentration of carbon dioxide is higher in the blood that's entering the capillaries than it is in the alveolar air, carbon dioxide passes from the blood to the alveoli. The nitrogen concentration in the blood and the alveolar air is about the same. So no diffusion of nitrogen occurs.