[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the structure of the gas exchange system in bony fish you should then be able to describe how the gas exchange system in bony fish has evolved for the maximum rate of gas exchange and finally if you're following the ocr or edxl spec you should be able to describe how bony fish maintain constant water flow through the gas exchange system in this video we're looking at gas exchange and bony fish bony fish are a large group of fish which have evolved a skeleton made of bone this group includes tuna salmon cod and trout now fish face a significant issue when it comes to gas exchange bony fish are large and active organisms with a very high oxygen requirement and because of their large size they've got a very low surface area to volume ratio and the scaly surface of bony fish does not allow gases to pass through fish get their oxygen from the water but the concentration of oxygen in water is much lower than an air so for these reasons bony fish have evolved a specialized gas exchange system which can extract the maximum amount of oxygen from water if we look closely at bony fish we can see a flap of tissue on either side slightly behind the head this flap is called the operculum and behind the operculum we have the opercular cavity inside the opercular cavity we find the gills we're going to be looking in detail at the gills later oxygen-rich water enters the fish through the mouth the water then passes over the gills in the gills oxygen diffuses from the water into the blood and carbon dioxide diffuses from the blood into the water and finally the water passes out through the opercular opening now the fish i'm showing you here was born with a missing operculum and we can clearly see the gills here in the impercular cavity okay let's take a closer look at the structure of gills gills consist of several bony gill arches extending from each gill arch are a large number of gill filaments i'm showing you a diagram of the guild filaments here as you can see many pairs of gilt filaments extend from each gill arch gill filaments are covered with numerous gill lamellae which are also sometimes called gill plates and the gill lamellae are where gas exchange takes place water flows between the gill lamellae oxygen diffuses from the water into the bloodstream and carbon dioxide diffuses from the bloodstream into the water now gill lamellae are adapted for efficient diffusion of gases firstly gill lamelli have a massive surface area for gases to diffuse over secondly there's a very short diffusion distance through the walls of the lamellae and into the bloodstream and lastly gill lamellae have an extensive network of blood capillaries once oxygen diffuses into the blood it's carried away and this maintains a steep concentration gradient for oxygen now there is one other way that bony fish are adapted for efficient diffusion of gases and that's called a counter current exchange system this can look tricky at first but it's not as difficult as it looks once you get the idea in order to understand this we need to take a closer look at the gill lamellae i'm showing you two gill lamellae and their blood supply here blood with a low concentration of oxygen passes into the capillaries of the gill lamellae as the blood passes through the gill lamellae oxygen diffuses from the water into the blood oxygen-rich blood now passes out of the gill-lamellae and leaves the gills now a key idea you need to understand is that the flow of blood is in the opposite direction to the flow of water and i'm showing you the direction of water flow here because the blood and water move in opposite directions this is called a counter current system and the counter current system has one major advantage which is that it always maintains a steep concentration gradient for oxygen i'm showing you here what would happen if the blood and water flow in the same direction scientists call this parallel flow the numbers give us an approximate idea of the relative concentration of oxygen initially the water has a much greater oxygen concentration than the blood so initially there's a very high rate of diffusion of oxygen from the water into the bloodstream however after a short distance the concentration of oxygen is the same in both the blood and the water scientists call this equilibrium and at this point diffusion stops so this means that no more than 50 percent of the available oxygen in the water can diffuse into the blood here's what happens if the blood flows in the opposite direction to the water which is the case in bony fish in this counter current system there is always a concentration gradient for oxygen this means that equilibrium is never reached and diffusion of oxygen takes place right across the length of the lamellae with a counter current flow up to around eighty percent of the oxygen in the water diffuses into the bloodstream okay now if you're following the ocr or edxl spec then you need to be able to describe how bony fish maintain constant water flow through the gas exchange system now in non-bony fish such as sharks the flow of water through the mouth and over the gills is caused by the fish swimming forward however bony fish have evolved a different mechanism which allows water flow to occur even when the fish is not swimming when a bony fish opens its mouth water flows into the mouth space which is also called the buccal cavity the flow of the buccal cavity drops down increasing the volume available for water the fish shuts the operculum and increases the volume of the opercular cavity which contains the gills due to the increased volume the pressure in the opercular cavity falls at the same time the floor of the buccal cavity lifts upwards this increases the pressure of the water causing the water to flow over the gills in the opercular cavity now the fish closes its mouth and opens its operculum at the same time the sides of the opercular cavity squeeze inwards on the water this increases the pressure of the water forcing it out of the operculum okay so hopefully now you can describe the gas exchange system in bony fish