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The actual exchange of oxygen and carbon dioxide take place in the alveoli. One mammalian lung has millions of alveoli. The alveoli are surrounded by a network of blood capillaries.

During the inhalation the air accumulates in the alveoli. There is a higher concentration of oxygen in the alveoli than in the bloodstream. Therefore, oxygen diffuse out of the alveoli into the blood in the capillaries. It combines with haemoglobin to form oxyhaemoglobin.

Then the oxygen is transported to the tissue, once in the tissue, the oxyhaemoglobin breaks down to release oxygen and haemoglobin, then the tissue use oxygen and release carbon dioxide.

This cause the level of carbon dioxide to become higher in the tissue than in the blood. Carbon dioxide therefore diffuse into the blood in the capillaries and combine with haemoglobin  to form carbaminohaemoglobin.

The capillaries transport carbon dioxide in this form to the alveoli, the concentration of carbon dioxide is higher in the blood in the capillaries than in the air in the alveoli. Carbon dioxide therefore diffuse from the capillaries into the alveoli, it then transported through the bronchioles, trachea, glottis, pharynx, and finally nostril into the atmosphere.


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