Our body is composed by millions of cells, but only the outside cells are directly exposed to air, and the other millions are too far from the outside air to make use of diffusion to supply oxygen. Therefore, the ventilation system, together with the circulatory system, works in order to gather oxygen molecules in the inner portion of the lungs and transport that oxygen to all the body cells.
Also, ventilation system is needed to maintain the concentration gradients of gases in the alveoli. to ensure the diffusion of each gas in the direction that it is supposed to follow - the body needs to get rid of carbon dioxide and needs to take oxygen in.
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