hi everybody this video we are looking at the um concept of the B shift or the bore effect uh so we're basically looking at how the Affinity of hemoglobin for oxygen changes in different parts of the body so this is an oxygen dissociation curve and what we can see here along the x-axis we've got the partial pressure of oxygen in kilop pascals it's basically just the same idea the same concept as the concentration of oxygen so as we move from left to right here we have an increasing um concentration of oxygen and then the y- axis we can see we've got the percentage saturation of hemoglobin with oxygen so up in this area here where we would have a very high partial pressure of oxygen this is what it's like in the lungs so obviously the lungs are being wly ventilated with a so there's a lot of oxygen present and when we have these very high uh partial pressures of oxygen we see that there's a very high saturation of hemoglobin so 98% of the hemoglobin is bound to oxygen so there is a very high Affinity of hemoglobin for oxygen at these high partial pressures lower down this way though and this is what we would see in the respire ing tissues um and the partial pressure of oxygen respiring tissues varies can be anywhere sort of in this sort of range it depends on what tissue it is it depends on the activity level um at the time if we just pick an example of 3 kilopascals so that is quite low but we can see that we've got a percentage saturation of hemoglobin of only 36% so in those respiring tissue areas then hemoglobin has has a much lower affinity for oxygen than it does in the lungs so what that means is as the blood moves from the lungs to the respiring tissue as it gets to the respiring tissue that means the oxygen gets unloaded it unbinds from the hemoglobin which then releases it for use in aerobic respiration in the tissues so we need to explain uh a couple of things we need to explain why that happens so why is it that oxygen unloads um and we also need to think about why this is uh this is a curve why is it not a straight line because what we can see with this curve is that uh a very small so this steep section here uh just quite a small decrease in the partial pressure leads to a large decrease in the percentage saturation it's not a straight line so there's a few things that we need to explain so in terms of why the oxygen um binds and unbinds this is all to do with the fact that we've got a reversible reaction now I will say at the moment that later in this uh video we'll also look at the effect of carbon dioxide because the presence of carbon dioxide um also has an effect on the Affinity of the hemoglobin with oxygen but even without that we need to understand a little bit about why the oxygen um binds and unbinds or it just doesn't really make much sense you will not need to explain this bit about the reversible reactions but I think it helps to sort of understand what's going on so this is the reaction we're talking about so um HB um that's just what we write to represent hemoglobin obviously hemoglobin is a protein so this is not the formula for hemoglobin but we write it as HB so hemoglobin plus oxygen uh will form oxyhemoglobin and this is a reversible reaction so it can go both ways so in the lungs where we know there's a high partial pressure of oxygen um we could represent the reaction like this now again this is not um supposed to show any sort of a balanced equation it's just a representation so we can understand what's happening so here we've got hemoglobin molecules here we've got oxygen molecules and here we've got oxyhemoglobin so what you can see here this is what we might see in terms of uh the sort of equilibrium state if we increase though the concentration of oxygen the partial pressure of oxygen so this is a very high partial pressure as we see in the lungs when you have a reversible reaction the um the balance of the reaction can shift depending on the situation so in this situation when you have a high um concentration of your reactants then the forward reaction is favored so that means that the forward reaction happens more than the reverse reaction so what that means is that hemoglobin binds to Ox oxygen to form oxyhemoglobin so because there is more oxygen available more oxyhemoglobin is formed because the forward reaction is favored in the respiring tissues then we we basically see the reverse we've got a low partial pressure of oxygen now if we say that here we've got the oxyhemoglobin which has traveled in the blood from the lungs but we've now got a very low partial pressure of oxygen in the lungs so so we are going to favor the reverse reaction and this is this is uh chemistry principles so if you want to know more about this and understand this a bit more have a look at the chemistry principles the chatelles principle um if you want to you don't need to but if you want to that's what you can have a look at so the reverse reaction is favored which means that the oxygen unbinds from the hemoglobin excuse me and therefore more oxygen is available here so that's why oxygen will bind or unbind to the hemoglobin in those different parts of the body it's all to do with the reversible reaction and the equilibrium balance in terms of explaining why we have this very steep section here and why we've got this s-shaped curve overall is to do with something called Cooperative binding uh so here we have the lungs so we know that in the lungs High partial pressure of oxygen and if we imagine the hemoglobin we know it's made of four polypeptide chains two alpha globin two betag globins so in the lungs all four of those hemoglobin um polypeptide chains will have an oxygen molecule bounds to them so one oxygen molecule is able to bind to each polypeptide chain in a hemoglobin molecule obviously bear in mind there are thousands and thousands of hemoglobin molecules in each red blood cell so 98% of oxygen saturation so not every single red uh hemoglobin molecule will have four bound that would be 100% but the majority do if we move down uh to here so we reduce the partial pressure of oxygen then what happens is rather than having an oxygen molecule bound to each of the polypeptide chains we get oxygen uh dissociation so the oxygen will unbind so one of those oxygen molecules unbinds um and is released if we then move further and decrease the concentration of oxygen the partial pressure of oxygen even more then we get an additional oxygen being released but the important thing is that as one oxygen gets released here that changes the configuration of the whole hemoglobin molecule and it makes it less able to stay bound to the other hemoglobin molecules so at this point so as soon as an oxygen molecule is released it Alters the configuration of the uh the polypeptide chains which means it is much easier to lose the second molecule and that's why you need only a small decrease in the partial pressure to lose that second molecule whereas here to lose the first molecule you need needed quite a large decrease as soon as one molecule of oxygen is lost it makes easier to lose the next and then again as soon as we lose the second it makes easier to lose the less the the the the third so this is the idea of Cooperative binding it goes the other way as well so as soon as one oxygen molecule binds to one of the polypeptide chains it makes it easier for a second molecule to bind which then makes it easier for a third molecule to bind so because of that we see this very steep section in the middle here and overall we get this s-shaped dissociation curve so now we want to look at the effect of carbon dioxide so we've already explained that the oxygen will bind and unbind anyway as the partial pressure changes um but as I said before carbon dioxide Al has a very big effect and this is really important uh when we're thinking about what's happening in the body so here's our regular dissociation curve so uh this curve represents sort of like a normal carbon dioxide um concentrations or pressures if we add this line here though this represents the um oxygen dissociation curve when there's a much higher partial pressure of carbon dioxide so this would be the say of the case in the tissues for example so um if you take any partial pressure of oxygen and if we take a partial pressure of five because that's what we get in respiring tissues it's normally about five um kilopascal partial pressure of oxygen in the respiring tissues what you can see here this is um the percentage saturation that you would see if you had normal carbon dioxide levels so normal carbon dioxide levels You' see around sort of a 70% saturation of uh of the hemoglobin with the oxygen so obious as we've seen already what that's telling us is that if you have oxygen bound to the hemoglobin in the lungs as that uh hemoglobin as as the blood travels to respiring tissues uh we see this decrease in pressure and so we're going to see um oxygen dissociate until only about 70% of um the hemoglobin is bound to the oxygen so that's what would happen normally however as you can see from the graph if we are in the respiring tissues there is a much higher partial pressure of carbon dioxide compared to if we're not in the respiring tissues so because we've got a high partial pressure of carbon dioxide that shifts the oxygen dissociation curve so instead of being in this position here in the respiring tissues we're actually at this position here for the same partial pressure of oxygen so that means that when the blood moves to the respiring tissues the oxygen is going to unload anyway but because there's a much higher concentration of carbon dioxide in the respiring tissues even more oxygen unloads than it would otherwise which means that we get only 40% saturation of hemoglobin with oxygen in the respiring tissues it unloads because the partial pressure of oxygen has reduced but it UNS even more is high it means that in the respiring tissues where lots of oxygen is needed that's the whole point we need the oxygen to be released for AIC respiration so because of this effect of the carbon dioxide which is increased in the tissues because of respiration that then means that more oxygen gets released for the respiration what we can see is that when there is more carbon dioxide the hemoglobin has a lower oxygen Affinity or it has a lower affinity for oxygen than it does when there's a higher um partial pressure of carbon dioxide the blue curve here shows what we would have if there was a low carbon dioxide concentration so um this is what we would see in the lungs because in the lungs obviously the carbon dioxide um in the blood is removed as we breathe out so there's much lower partial pressure of carbon dioxide so again if we were to pick um any partial pressure that we might see in the lungs of oxygen so let's say about 7 if it was just like a regular carbon dioxide concentration then we would see um maybe about a little bit under 90% of the hemoglobin would be saturated but as soon as we have uh as soon as we decrease the carbon dioxide concentration which is what we see in the lungs we see this increase um in the oxygen Affinity so that means that in the lungs because there is less carbon dioxide even more oxygen is able to bind to the hemoglobin so even more oxygen is able to bind and load and therefore the hemoglobin carries more oxygen from the lungs down to the tissues this whole process here is called the bore effect so it's the idea that as we move from lower to higher partial pressure of carbon dioxide the curve shifts to the right so the more carbon dioxide there is present the further to the right the oxygen dissociation curve will shift what that means is that as you increase the pressure of carbon dioxide hemoglobin has a lower oxygen Affinity which means that as you increase the um concentration of carbon dioxide the pressure of carbon dioxide more oxygen is going to be released and that is the bore effect so it's the effect that increasing carbon dioxide has on reducing the oxygen Affinity of hemoglobin so that process helps to um increase the ability of the body to release oxygen in the respiring tissues so that it is available for respiration okay there's some quite complicated Concepts there um so you might need to go over it again um but that's all for now thank you