Gas exchange: diffusion across thin surfaces — Biology, 14–17
Oxygen and carbon dioxide cross specialised surfaces by diffusion. Efficient lungs, gills and leaves all provide a large, moist, thin exchange surface and maintain a difference in concentration.
Gases spread across a boundary
Gas exchange is the movement of oxygen and carbon dioxide between an organism and its surroundings, or between blood and tissues. The molecules diffuse from where they are more concentrated to where they are less concentrated, across a very thin barrier.
Why need a special surface?
A large animal cannot rely on gases crossing only its outer skin: the distance to deep cells is too great. Evolution repeatedly favoured folded or branched surfaces, because more area and shorter distances make diffusion faster.
Following oxygen through a lung
You inhale air containing about 21% oxygen. In an alveolus, oxygen concentration is higher than in the arriving blood, so oxygen diffuses across the moist, one-cell-thick wall into a capillary. Carbon dioxide moves the opposite way and is breathed out.
The breathing-equals-absorption trap
Breathing moves air in and out, but it does not by itself put oxygen into the blood. Gas exchange happens at the alveoli, where a concentration difference, moisture and a thin barrier allow diffusion; air movement and diffusion are connected but not identical.
Sport, altitude and fish tanks
During exercise, muscles use oxygen faster and produce more carbon dioxide, so breathing and blood flow increase. At high altitude, the air pressure makes oxygen uptake harder. In an aquarium, a pump helps keep water moving over the gills of fish.
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