by 
Romiex69x
ANSWERS: 4
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Why do humans drown?
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The can't get enough oxygen from the air. Gills are designed to work in water. They don't transfer oxygen from the air as well as they do from the water. Even if you could supply moist air to the gills, their bodies require hydration as well.
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Their gills breathe oxygen under water, and there's no water on the surface.
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To answer this question sufficiently, basic gill structure must be addressed. The gill arch contains (on one side) structures called lamellae, which are evaginations that increase the surface area of the gills-thus allowing for greater exchange of oxygen between the blood capillaries and water. In water the lamellae are freely separated from each other by pockets of water, but when exposed to air they collapse upon one another (not unlike human hair when exposed to air after being in a pool). This decreases the surface area of the lamellae by hundreds or even thousands of times, and although air has around twenty times more oxygen, the drastic decrease in surface area makes it impossible for sufficient respiration.
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