Blood in the pulmonary artery is low in O2 concentration because it has returned from dropping off oxygen to all cells in the body. It is high in CO2 concentration because it picked up CO2, a metabolic byproduct, from those cells.
The layer of cells between an alveolus and blood is only two cells thick, which allows for diffusion of gas in both directions following separate concentration gradients.
Carbon dioxide diffuses into the alveoli from the CO2-rich blood and is exhaled.
O2 is inhaled into the alveoli and diffuses into the oxygen-poor blood traveling towards the heart.
The concentration of both CO2 and O2 equilibrate through diffusion until there are roughly equal concentrations of both in the alveoli and in the blood.
The pulmonary vein runs from the alveoli to the left atrium of the heart, and is eventually pumped out to the body. This blood is O2-rich and CO2-poor, because it has just picked up oxygen and dropped off carbon dioxide at the alveoli.
The other exchange that occurs at the alveoli is heat exchange. Breathing is a way of contributing to body temperature regulation by breathing out warm air if the body temperature is high, or retaining heat from the air if the body temperature is low.
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