Science Questions

How do fish gills work?

Sat, 2nd Oct 2010

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Question

Hugh Sinclair asked:

How do fish metabolize oxygen? Specifically, how does the fish's gill extract oxygen from the H2O molecule?

Answer

Diana -   Well I think they work a little bit like really efficient lungs.  So, when a fish opens its mouth, in goes the water, it goes into the gill, and they've got a really thin membrane over which the water flows, and on the other side of membrane, blood is flowing and itís flowing in the opposite direction which means that any oxygen which is dissolved in the water can then go through this membrane into the blood.  And they're also really, really, finely made so they've got these things call lamellae and what happens when a fish is out of water, the reason it can't breathe is that these little structures collapse in on each other.  And so, it becomes less efficient, thereís less surface area exposed to the available oxygen, and they essentially asphyxiate in the air, but yeah, thatís how it does it.

Chris -   Because human lungs don't collapse like that because they have a surfactant which reduces the surface tension in the water on the surface of these little tiny air sacs, so they don't collapse, but fish don't have that surfactant because obviously, it all would be washing away and they don't need it.

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Hugh Sinclair asked the Naked Scientists: How do fish metabolise oxygen? Specifically, how does the fish's gill extract oxygen from the H2O molecule? What do you think? Hugh Sinclair, Fri, 1st Oct 2010

Fish gills are the aquatic version of a mammal's lungs. They bring an oxygen-rich fluid (water) close to the bloodstream of the fish, enabling oxygen to move from the region of relatively high concentration (in the water) to the area of lower concentration, in the blood.

The oxygen that is being removed from the water, however, is not the oxygen that forms part of the water molecule (H2O). Instead it is oxygen that has dissolved in the water either through contact with atmospheric oxygen or through the release of oxygen into the water by aquatic plants (which liberate oxygen when they photosynthesise).

Chris chris, Sun, 3rd Oct 2010

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