How Fish Breathe

Just how do fish breathe?

Today is Good Friday, when traditionally Christians eat fish instead of meat (the abstaining from fleshy meat is due to Jesus dying on the cross on Good Friday). Anyway, I don’t eat fish as a general rule so thought I would write an interesting blog post about fish instead.

Before I go into the details of how fish breathe, I should clarify that in order for air to get into the body, it needs to travel across a barrier into the blood. In humans, the barrier is in the lungs, whereas fish do not have lungs. It makes sense that to have the maximum efficiency, any barrier which oxygen must cross must be very big and very thin. There also needs to be a constant supply of blood and air for the oxygen exchange to occur.

Fish have gills. That’s pretty common knowledge. But people don’t realise just how gills enable fish to breathe without lungs.

As fish swim along, water flows through its open mouth and out of its gills. Hence why fish always need to be moving – if they stay still for too long, not enough water can pass over the gills, so they don’t get enough oxygen. Now, that doesn’t seem like it works. Just how does enough oxygen get from the water and into the fish’s blood via the gills? Well, the gills are actually rather complex structures which have both the qualities needed to be a good surface of gas exchange – they’re thin and they have a huge surface area.

Gills are more than just slits in the side of the fish’s neck. They’re a complex structure of filaments with an excellent blood supply. They’re a bit like a very sturdy series of fans, or those irritating heavy plastic strand curtains usually found (ironically) in aquariums between exhibits. Imagine a whole tunnel filled with those. That’s a bit like what the gills are like for water filtering through them.

The structure of gills can be split into sections. Gill filaments are the rows of fine plates attached to the main bone of the fish’s skeleton. There are rows upon rows of these, and they are all covered in tiny structures called lamellae. These make the surface area of the gills even larger, and because they are so thin and have such a great blood supply, the oxygen can easily cross into them.

The main system of gas exchange is called counter-current flow. This is exactly what it sounds like. Two currents flowing in opposite directions. One being water, one being blood. Simplistically, if the water is flowing in the direction of mouth to tail, then the blood is flowing through the gills in the direction of tail to mouth. This ensures that there is always room for oxygen to enter the blood from the water, and for carbon dioxide to enter the water from the blood.

Think of a production line of cakes which need packaging, and an adjacent production line filled with the packets for the cakes. It would be much easier to have the production lines running opposite to one another, as then as the packets arrive on one production line, a worker can add them onto the cakes reaching the end of the other. If the production lines ran in the same direction, a worker would find it much harder to efficiently pick up the packaging and place it on the cake. The worker between the lines would essentially be chasing the cakes in a constant stream.

This is why the fish gills use the counter-current system. There is always high oxygen in the water when there is low oxygen in the blood, and high carbon dioxide in the blood when there is low carbon dioxide in the water. This means that the gases can move from areas where they are plentiful to the areas where they are lacking. And thus, the fish breathes.

For those wanting a more detailed view of counter-current systems, this is an excellent blog post.

Do ask me on twitter if there’s any topics you want covering, or you want to see a photo of a gill.

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