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From the nose it follows the respiratory track (pharynx, trachea, bronchi) to the lungs where in the alveoli it passively diffuses into the blood enters the red blood cells and gets attached to an iron atom in haemoglobin. 

It is then transported to the body where it gets released and passively diffuses into a cell to be used in cellular respiration to accept electrons that originated in fuel molecules.
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The Pathway Air enters the nostrils passes through the nasopharynx, the oral pharynx through the glottis into the trachea into the right and left bronchi, which branches and rebranches into bronchioles, each of which terminates in a cluster of alveoli
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