hi there everyone and welcome to learn a live apology for free with Miss Esther ik in this video I'm going to be going through gas exchange in fish say a festival just a bit about the gas exchange system say 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 gradients and overcome that challenge so just a recap on what three key features are in every gas exchange surface whether its gills and fish and the spiracles and insects or the lungs and alveoli 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 reach those criteria and the rate of diffusion then can be calculated using ficks 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 kills so there are four layers of gills on both sides of the head so this is where we can see here these four red V shapes that's representing a set of gills so the V shape is this bit here so we can see we've got that V shape 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 lamellae all the way along the gill filaments and they are positioned at right angles to the gill filaments so the gill filament is stretching up this way and the gill Mellie 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 lamellae 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 doing a tiny gap in the side of the head where the gills are so what we gonna focus on next is how then these different features particularly the pillories in the lamellae 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 Gill lamellae the short diffusion distance says two faxes here and the gas exchange only happens on these gill lamellae and they are very very thin so that's one factor that gives a short fusion distance but also inside of every Gill lamellae there is a capillary network and because that is so close to the outside where the option is going to be diffusing in front that provides a short diffusion distance so the final factor then is how is the concentration gradient excision maintained how do we maintain that difference stream outside and the inside concentration in the blood and that links to our last concept which is the counter current flow or sometimes called the counts current exchanged principle so I'm gonna go through it using these two opposite diagrams just to point out this one here should say con current flow not Co current say is con current and say what we mean by counter current 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 its 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 the fusion can occur across the entire length of the lamellae so if we have a look then and what we mean by that first of all we look at the opposite which is the con current flow and this is if the blood and the water were flowing in the same direction so the top bar is representing our water say the water is entering the gills at 100% saturation with oxygen and the blood is entering the gills at 0% is there to become oxygenated and that would mean which have rapid diffusion from the 100% to zero but eventually you would reach equilibrium if you had con current flow because all of that oxygen is going to diffuse rapidly in and eventually it will get to 50% 50% and you won't get diffusion occurring across the entire length of the gill Mellie in contrast the counter current 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 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 gonna 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 lamellae 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 counter current flow the water that is 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 counter current exchange principle there'd always be a mark for mentioning that as a statement that is how the diffusion gradient is maintained there be a mouth 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 lamellae now the reason I've underlined that is even if you had concurrent flow a diffusion gradient is maintained across the lamellae it's just not the entire length it might just be for three-quarters of the length so that's why in an 'mark scheme they would underline the word entire so that you know that is the advantage of counter current flow you can have diffusion of oxygen along the entire Lamoni and that is then the three different features we've gone through is surface area the large surface area short fuse and distance and how that diffusion gradient is maintained so that is it a 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]