Recently I wrote about the evolution of plants and how, through photosynthesis, they filled the atmosphere with oxygen. Now, this allowed animals to evolve because, as we all know, animals need oxygen to survive.
There are several ways animals get oxygen into their bloodstream. Insects have tiny holes in their shells called spiracles through which air enters, some amphibians can breathe through their skin, whilst mammals, birds and reptiles all use lungs. But fish live in water, so how can they breathe? You probably know that they use their gills, but how exactly do gills work?
Water contains much less oxygen than air (about 50,000 times less!) but gills can extract enough of it for a fish to breathe. They are remarkable organs and very efficient. When a fish takes a gulp of water it passes over the gills which are folded over themselves, making a vast surface area. Each oxygen molecule which passes over the gills is absorbed into the fish’s bloodstream, in much the same way that oxygen in our lungs is absorbed.
So, if gills are so wonderful at taking oxygen, why can’t fish breathe in air, where there is so much more of it? The reason is that gills are just too heavy. You see, gills need to have massive surface areas. This means that they must be supported by water – in air they just collapse. They can extract some oxygen, but only a fraction of what they need. That’s why fish drown (or, more accurately, suffocate) on land.
The reason that humans (and any other air-breathing animal) can’t breathe water is not because it’s a liquid. It’s because there just isn’t enough oxygen in water for lungs to work. There are some liquids, believe it or not, which we can breathe. Scientists have been working on fluids which can be used by deep sea divers, or to help premature babies with underdeveloped lungs.
As long as a liquid contains enough oxygen and can remove carbon dioxide, then there’s no reason we can’t breathe it. But DO NOT TRY THIS AT HOME! No liquid you might come across will be breathable. Let’s just stick to breathing air for now shall we? It’s much more pleasant in my book – and it keeps us alive!