Hi there everyone and welcome to learn A level biology for free with Miss Estrich. In this video I'm going to be going through gas exchange in fish. So first of all just a bit about the gas exchange system. So fish are waterproof because of their scales and they do have relatively a small surface area to volume ratio. So they can't simply diffuse oxygen across their surface. So they do require a gas exchange surface which is the gills in fish. Another consideration is the fact that fish will be obtaining their oxygen from the water in their environment. But water contains 30 times less oxygen compared to air. So therefore they also have a special adaptation to help maintain the concentration gradient um to overcome that challenge. So just to recap on what three key features are in every gas exchange surface whether it's gills and fish um spiracles and insects or the lungs and alvioli in mammals. They always have to have a large surface area to volume ratio. They always have to have a short diffusion distance and they need some mechanism to maintain the concentration gradients. So that's what we're going to focus on in this video. What are the three adaptations in fish gills to um reach those criteria? And the rate of diffusion then can be calculated using fixed law where diffusion is proportional to the surface area times the difference in concentration divided by the length of the diffusion path. So if we focus then on the fish gills. So there are four layers of gills on both sides of the head. So this is where we can see here these four red Vshapes that's representing a set of gills. So the Vshape is this bit here. So we can see we've got that Vshape and you have four of those on both sides of the fish head. Now we call these long parts sticking out the gill filaments. And the gill filaments align in stacks. So you have lots and lots of gill filaments stacked up. And zooming in on each gill filament, you have lots of these very very thin gill lamelli all the way along the gill filament and they are positioned at right angles to the gill filaments. So the gill filament is stretching out this way and the gil lamelli is going horizontally across each of them. And that structure on top of the fact we have lots and lots of stacks of gill filaments and we have four on either side of the head. That's how we create a really large surface area. So the diffusion of the gases only happens on the lamelli and the water is going to be rushing in through the mouth which we can see here. This is representing the mouth. The water then rushes over the gills and it comes out through a tiny gap in the side of the head where the gills are. So what we're going to focus on next is how then these different features particularly the um capillaries and the lamelli provide the three gas exchange features that we mentioned. So first of all we've already talked about the large surface area to volume ratio. the fact that we've got many gill filaments covered in many gil lamelli the short diffusion distance there's two factors here the gas exchange only happens on these gill lamelli and they are very very thin so that's one factor that gives a short fusion distance but also inside of every gill lamelli there is a capillary network and because that is so close to the outside where um the oxygen's going to be diffusing in from that provides a short diffusion distance. So the final factor then is how is the concentration gradient of oxygen maintained? How do we maintain that difference between the outside and the inside concentration in the blood? And that links to our last concept which is the countercurren flow or sometimes called the countercurren exchange principle. So I'm going to go through it using these two um opposite diagrams. Just to point out this one here should say concurrent flow not co-current that is concurrent. Um so what we mean by countercurren is when the water that is flowing over the gills is flowing in the opposite direction to that of the blood flowing in the capillaries. So it's counter it's opposite and the purpose of this is it makes sure that you never reach equilibrium. So you'll never get the same concentration of oxygen in the blood as you have in the water and because you don't reach equilibrium that means diffusion can occur across the entire length of the lamelli. So if we have a look then at what we mean by that. First of all, we'll look at the opposite which is the concurrent flow. And this is if the blood and the water were flowing in the same direction. So the top bar is representing our um water. So the water is entering the gills at 100% saturation with oxygen and the blood is entering the gills at 0% because it's there to become oxygenated. And that would mean we'd have rapid diffusion from the 100% to zero. But eventually you would reach equilibrium if you had concurrent flow because all of that oxygen is going to diffuse rapidly in and eventually will get to 50% 50% and you won't get diffusion occurring across the entire length of the gil lamelli. In contrast, the countercurren flow, which is what happens in fish, again, we're starting with this top bar is representing the water. The water flows into the gills or into the mouth and then over the gills at 100% saturated with oxygen. But this time, because the blood is flowing in the opposite direction, the blood that is reaching that 100% saturated water is already very oxygenated. Now, it's never going to reach 100% oxygenated. It's just below. And because it's just below 100%, you still get diffusion of oxygen from the water into the blood. So that will then mean that as the water continues to flow through and over the lamelli, it again is losing oxygen. So it goes from 100% to maybe 90 80 70 60 50 until we get very close to zero. But because of that countercurren flow, the water that is um flowing over this bit of the blood, this bit of blood is at 0% saturation. So as the water is flowing over, it's near zero, but it might still be at about 5% saturation. So you'll still get diffusion of oxygen from the water into the blood. So in an exam question the key things of the countercurren exchange principle there'd always be a mark for mentioning that as a statement that is how the diffusion gradient is maintained. There'd be a mark for pointing out that that is when the water flows in the opposite direction to the blood in the capillaries. But the two key points of these here number one it ensures that equilibrium is not reached which would happen in concurrent flow and this is your key reason why that's an advantage. So that means that a diffusion gradient is maintained across the entire length of the lamelli. Now the reason I've underlined that is even if you had concurrent flow a diffusion gradient is maintained across the lamelli. It's just not the entire length. It might just be for three quarters of the length. So that's why in a mark scheme they would underline the word entire so that you know that is the advantage of countercurren flow. You can have diffusion of oxygen along the entire lamelli. And that is then the three different features we've gone through surface area the large surface area short diffusion distance and how that diffusion gradient is maintained. So that is it for gas exchange in fish. I hope you found it helpful. If you did, please give it a thumbs up and click subscribe to keep up to date with the latest videos. [Music]