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They are exchanged through the process of diffusion.

The alveolar air is rich in oxygen whereas the blood surrounding the alveoli is poor in oxygen and rich in carbon dioxide. So carbon dioxide passes from the blood to the alveoli whereas oxygen passes from the alveolar air to blood. This oxygen-rich blood is then carried to the body cells. The cells are rich in carbon dioxide and deficient in oxygen. So oxygen from blood diffuses into the cells whereas carbon dioxide from tissues diffuses into the blood in capillaries. This blood is again taken to the lungs for oxygenation.