The gills are made up of tiny structures called lamellae. Lamellae are very similar to the villi in the intestine and have a lot of similar features. The role of the lamellae is to increase the surface area of a gill, this will increase the rate of diffusion. Two other properties of the lamellae are:
A thin layer of epithelial cells (lining) which provides a short diffusion pathway
A good blood supply
These both further increase the rate of diffusion.
For the fish to respire they need oxygen which they get from the water. Unlike land animals such as us ,Humans, fish can not get the oxygen from the air they rely on the oxygen dissolved in the water.
Fish get oxygen from the water. The oxygen enters the fish through the fishes mouth. For the oxygen to reach the bloodstream of the fish it must pass through the gills. The gills, much like the lungs in humans, allow gas exchange to occur.
In order to achieve oxygen-rich water in mouth or buccal cavity of the fish, there needs to be some sort of gradient dragging water into the water. The gradient, in this case, is pressure. I think the best way to explain this is in phases…
In this phase, the fish has its mouth open, meaning the gill cover/ operculum is closed. The gill cover is a bony plate used to let water out after it has passed through the gills. When the fishes mouth is open water is drawn in. This occurs because of the pressure gradient. There is a much higher pressure outside the fish than in. Another way of saying this is that: There is a very low pressure in the mouth cavity (caused by the lack of water) and there is a much higher pressure of water outside the fish this forces the fishes mouth to open thus causing water to enter the fish.
Once the pressure in the buccal cavity exceeds that of the pressure outside the fish, the mouth will close thus increasing the pressure even more so. After, the gill cover will open allowing the rush of water to exit the fish.
Then Phase 1 will start again forming a cycle. The numbers of the phases are interchangeable and don’t change the concept behind it. 🙂
Countercurrent flow is when the blood and the water flow in opposite directions. It does this to maintain a constant diffusion gradient of oxygen from the water into the bloodstream. This means the most oxygen-rich blood is in contact with the most oxygen-rich water. And the most oxygen-poor blood is in contact with the most oxygen-poor blood. This ensures a constant concentration gradient.
The best way to understand why this is the most effecient way is to look at the opposite.
Parallel current is not a real process in fish
As shown in the graph, the diffusion of oxygen reaches equlibrium much faster than counter-current meaning the fish will not acquire as much oxygen.
If the fish has less oxygen this may lead the fish to suffocate. Less oxygen means less respiration. Respiration allows the fish to grow and repair itself. Without the necessary amount of oxygen, the fish will have a low rate of respiration, leading the fish to not be able to function correctly.
Luckily this is not how fish do gas exchange.
Very active fish like mackerel and sharks ventilate a bit differently. There are two types of ventilation in bony fish:
As you can probably guess, active fish are RAM. The main differences between the two are that active fish always have there mouths open and always need to swim forward (to actively push water into there mouths). They need to do this as they have a very high rate of respiration which means they also need a very high rate of water flow.
If you have any questions leave a comment and I will respond 🙂