The Lung

The lung is an exchange surface of many flat cells. The surface, about the size of a tennis court or about one hundred and thirty square meters, has the form of three hundred million grape-like sacs or alveoli spread over one side of a thin membrane on the other side of which courses a bed of capillaries. Gases must traverse this alveolar-capillary membrane as they cross in both directions between alveolus and capillary. Our lung surface is crumpled within our chests, each folded half court taking up about five liters of space. Networks of capillaries surround each alveolus.

The capillaries around all our alveoli together must receive blood as evenly as possible, so that the alveoli high up in our chests and others lower down near our diaphragms all receive the blood they need to match the air they get: no more and no less.

So one-half of our distribution problem is how to divide up the stream of blood coming into the lung efficiently so that blood reaches all points of the combined alveolar surface evenly in approximately the same time. The other half of the problem entails getting air uniformly to the alveoli. Alveoli, no matter where they are in the chest, high up or low down, must receive adequate fresh oxygen from each inspired breath. Airflow and blood flow must match so that blood in some capillaries does not meet poorly ventilated alveoli and return to the heart empty-handed and also so that well ventilated alveoli receive more blood than poorly ventilated ones. So the goal of a matched supply system must be that ventilation and perfusion of the exchange system must match up almost perfectly over the area on both sides of the alveolar capillary membrane.

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