Video: Pulmonary respiration

In humans, as in numerous  other air breathing animals, gas exchanges occur thanks to the lungs.

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During inspiration, Oxygen rich air penetrates the lungs  via the respiratory passages. It passes through the oral cavity or the nasal cavities, then descends down the trachea and the bronchi.

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The air then fills the bronchioles which terminate in several million pulmonary alveoli. Each of these is surrounded by a network of very small blood vessels: the capillaries.

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The assemblage of alveoli makes for a very large surface for the exchange of air between the blood and the exterior.  Indeed, if one unfolded all the alveoli in the lungs of an adult, they would cover an area of more than 100 square meters (….), that is the same area as half a tennis court.

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Oxygen from the air passes through the very thin walls of the alveoli into the blood. It is then transported by the circulation of the blood to the organs, where it will be  used.

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In return, these organs produce  Carbon Dioxide, which is a waste product that has to be removed.

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It is via the circulation of the blood that the Carbon Dioxide is transported to the pulmonary alveoli. It passes from there into the air in the lungs,  which expel it to the exterior, during expiration.