Gas Exchange in Fish

Learning about the structure of the gills, the “countercurrent flow” and images from a short fish dissection…

These past two days have been very interesting, in my biology class yesterday we were lucky enough to have a fish dissection demonstration and today I joined another year 12 class and watched the whole dissection again! Not only was the second dissection great revision and a chance for a fish nerd to look at the insides of a fish again but it was also a great opportunity for me to get some photographs for my photography AS…

With these images of a dissected fish comes great responsibility… So I thought I’d share them on here along with some cool facts about the awesome way gas is exchanged within fish. This first post will cover the gills and how amazingly adapted they are to their role of gas exchange, the second post will cover the dissection of the rest of the fish.


Meet Harry. Harry is a Mackerel, he is also, unfortunately, our victim for the experiment.

Mackerels are related to the Tuna (it may be vaguely possible to see the finlets running towards the tail, typical of Tuna and their relatives). Mackerel is in the superclass of Osteichthyes (meaning it is a bony fish) it is in the group of fish with skeletons consisting of bone tissue, rather than cartilage (such as sharks or rays).

FACT: Bony fish evolved from cartilage fish and this was previously seen as an evolutionary advantage, however, cartilage is lighter and more flexible, allowing sharks to move faster and expend less energy than if they were made of bone. But! There are evolutionary advantages to Harry’s gills that sharks unfortunately do not have…

Currently we are studying gas exchange in different organisms, last week we studied bugs, this week fish and plants, next week we move on to humans. Our dissections therefore focused at looking at the gills…

Our teacher took Harry and the first thing she showed us was the gill flap, the “cheeks” of the fish head if you will, she pointed out that the passage from the mouth, through the gills, and out the gill flaps was one single path, interrupted only by the gills;



Due to the clever way the gills are engineered water flow is restricted to a one way passage; the muscles in the fish head “gulp” the water out from its mouth over the gills and back into the surrounding water.

It must be made clear that just as we breathe air, fish breathe water. BUT they respire oxygen, just as we humans do. Their gills are specially evolved to remove the maximum amount of oxygen as possible from the surrounding water (see below for the Fick’s law and countercurrent flow explanation).

Cutting off the gill flaps left the gills completely exposed for us to see. The gills are bright red, almost brown in colour, this is due to the large capillary network and the huge blood content found here. Humans are not dissimilar to fish in that we too have a large blood supply to our lungs.

(I thought I’d share a few images of it’s head because in the words of one of my friends “take a picture of his head! His head is sooo cute!)

It is possible to see on the images that the gills form a sort of fan, with a thick, paler line running through the middle called the “gill bar”, it is from this that all the gill filaments are attached. The gill filaments are stacked up inside the gills like a book. When the fish is in water, none of the gill filaments will ever touch each other, they are completely independent of each other and are each attached to the gill bar. To further increase the surface area of the gill, gill lamellae run along the gill filaments.

Gills –> Gill bar with Gill filaments –> Gill lamellae


Generally fish have four sets of gills on either sides of its heads. Four sets of what is in the image above. ^ This therefore creates a huge surface area for the oxygen in the water to diffuse across.

When the gills are in water they are as described above, none of the filaments touching each other, the filaments “fan out”, however, take the fish out of water and the filaments will clump together. This is why, if you take a fish out of water, even though there is more oxygen in air than in water, the fish will slowly (over a period of 10-15 minutes) choke and die from hypoxia or lack of oxygen. If the gill filaments are all stuck together the fish is unable to effectively diffuse oxygen into its blood stream and will suffocate to death.

If you place the petri dish filled with water and the gills under a microscope it is possible to see the gill filaments as well as the lamellae;

At various stages in this post I pointed out two amazing facts about the fish gills that are so ingeniously evolved they effectively help with the uptake of oxygen in the organism: the high blood flow and surface area of the gills. To understand the significance of this it is important to bear in mind Fick’s law of diffusion which states that the rate of diffusion is proportional to the surface area multiplied by the concentration difference all over the length of diffusion surface. Or:

If we want the rate of diffusion to be high, the surface area and concentration difference must both be high whilst the diffusion distance must be low.

The gill filaments with the lamellae hugely increase the surface area for the oxygen to diffuse across, whilst the concentration difference is also high due to the large blood supply present in the gills.

Together, the large surface area and the high concentration difference allow the rate of diffusion to be as high as possible within the fish.

Bony fish also have an adaption called the “countercurrent flow” or the “counter current exchange principle”. The main feature of this being that the water that passes over the gills and the blood that flows within the gills do so in opposite directions. This allows blood that already has oxygen in it to continue taking in oxygen. Water with high oxygen content flows over cells with blood that has high (but just under) oxygen content and water with lower oxygen content flows over cells with blood that also have low oxygen content. This allows the two mediums to never reach a state of equilibrium, they will always have a concentration difference, and the fish is therefore able to take in much more oxygen than had both the water and blood been flowing in the same direction.

Had the blood and water been in “parallel flow” (such as in the less evolved sharks and rays) the uptake of oxygen from the water would only be approximately 50% that of what it could potentially be with the countercurrent flow. This is a benefit to bony fish that unfortunately, cartilaginous fish do not have…


My thanks go to Harry the fish and Mrs Goodwin’s science class for letting me come into your dissection and take pictures! 😀

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