(i) Oxygen enters the human body through the nasal passage and reaches the lungs.
(ii) Numerous alveoli are present in the lungs. These alveoli are surrounded by many blood capillaries. From the alveoli, O2 is taken up by the red blood cells present in the blood capillaries.
(iii) Similarly, the CO2 from the various tissues is transported to the alveoli. (iv) This CO2 is then released through the nasal passage during exhalation. Alveoli are the sites of exchange of gases. Each lung contains 300−350 million alveoli. These alveoli increase the surface area for gaseous exchange, making the process of respiration more efficient.