so in the previous video we were talking about the oxygen dissociation curve and what we've learned about the oxygen dissociation curve at least the most obvious thing is the shape of it okay the shape is kind of sigmoid or it is also referred to as an s-shaped curve um as you can see there I'm just showing you how the curve looks like right that's the S shape right there so the first thing we have to understand about the s-shaped curve is to give ourselves a situation now I don't need you to memorize this part at least I'll tell you which part you need to memorize but the problem with this part of the chapter uh is the fact that I do need to give a bit of explanation in my situation over here imagine for a second there are a few lung alveoli that I'm drawing at the top and blood capillary at the bottom of the alveolus because that's where gas exchange happens now for example with my oxygen dissociation graph over here if let's say there are four situations where the lung alveolas have has two kilopascals of oxygen four kilopascals of oxygen six and also eight kilopascals of oxygen now for example let's say when the red blood cell passes the lung alveolus with two kilopascals of oxygen it will have an oxygen saturation of 20 so what I love to do is I would love to ask my students this question if two kilopascicles of oxygen gives you 20 saturation um what will four kilopascals give you then most students will say well if two gives me 20 4 gives me 40 6 kilopascals give me sixty percent and eight kilopascals give me eighty percent so if I were to plot a graph of oxygen saturation against partial pressure of oxygen I will get a linear line of a straight line so this is a problem because the oxygen dissociation curve is a cuff it's not a sweet light what that means is so this is wrong so what happens in reality in reality something slightly different takes place what happens is the partial pressure of oxygen Still Remains the Same two kilopascals four six and eight kilopascals of oxygen okay so two kilopascals of oxygen will still give me 20 which is fine okay so let's plot that in the cloth but four kilopascals of oxygen instead of giving me 40 oxygen saturation it actually gives me 60 percent there is a sudden sharp increase in the percentage oxygen saturation of the red blood cell so the question is is this good or is this bad this is extremely good because you want your red blood cells to be carrying as much oxygen as possible to your body cells because your body cells constantly need oxygen so the more you fill up the red blood cells with oxygen the better and lucky for us the hemoglobin behaves in such a way if the graph was linear 4 kilopascals will only give us a 40 saturation but because it's a curve four kilopascals of oxygen gives us 60 oxygen saturation I would like to remind you again that these values are theoretical different species and even amongst different people the oxygen dissociation curve is slightly different so these values of two two giving us 20 four kilopascals giving us 60 percent these values you don't need to memorize the part that I just want you to memorize at least the first part at least is a small increase in the partial pressure of oxygen will give us a large increase in the percentage oxygen saturation uh that is why you'll get the s-shaped curve but of course then the question in the exam is why is it a curve why does hemoglobin behave in such a way the reason is because okay so let's start again when did lung alveoli only has two kilopascals of oxygen what happens is the there is a low amount of oxygen and some of the oxygen wants to bind with the heme group here's the weird thing about hemoglobin inside the red blood cell I've drawn out that hemoglobin there for you and those away those four red dots represent the human group and there is an oxygen molecule near it O2 do you see that so the oxygen wants to bind to the heme group but the heme group is kind of um what's the word I'm looking for it's hidden it's hidden or it's shrouded by the polypeptide chains which are those green and purple color uh circular squiggly things that I've drawn around the heating group okay so the oxygen molecule has difficulty binding to the heme group right so a lot of students assume just because I have oxygen it will automatically bind to the heme group that is a wrong assumption okay oxygen has difficulty binding to the first hem group because all the heme groups are hidden within the polypeptide chains so we would see here that the Affinity of oxygen and the heme group is low which I'm drawing out to be like a small heart shape over there so they have like a small attraction to each other which means to say it takes at least two kilopascals of oxygen to make sure at least that the hemoglobin is 25 saturated what that means I don't need you to memorize that part I just need you to understand that to make sure that at least one hem or one oxygen molecule binds to the human group we need at least two kilopascals of oxygen that is what the graph is saying okay and I'm putting a reminder that don't memorize that part now the next part is the important one for the exam and that is the one that you need to understand and memorize and explain it again if the questions ask you as you can see when the first oxygen molecule binds to the human group notice what happens to the shape of the hemoglobin you're right the shape of the hemoglobin becomes distorted so you might be thinking oh if the oxygen distorts the shape of the hemoglobin which is a protein every time proteins get distorted this is bad the 3D structure changes in this case it's not necessarily a bad thing because look at what happens to the second hymn group the second hymn group is no longer hidden by the polypeptide chains it is now exposed when it is exposed it makes it easier for the second oxygen molecule to bind to the second aim group so two kilopascals of oxygen gives you 25 okay uh oxygen saturation but you don't need four kilopascals of oxygen to reach a 50 saturation you might just need 2.5 kilopascals of oxygen accumulatively so because the second oxygen molecule has a higher affinity for the second hymn group that is why a small increase in the partial pressure of oxygen from 2 to 2.5 causes a large increase in the oxygen saturation from 25 to 50 percent and that's a good thing okay so you don't need a lot of oxygen molecules to saturate your hemoglobin and then once the second oxygen molecule binds to the second hem group same thing happens for the third hemp High Affinity it makes it easier and it keeps on going with the fourth hem group so a small increase in the oxygen partial pressure will cause a large increase in the percentage oxygen saturation because when the first oxygen molecule binds to the um heme group uh it distorts the shape of hemoglobin it exposes the second one and then it exposes the third one and it exposes the fourth hem group making it easier for subsequent oxygen molecules to bind to it that's basically what you need to explain for the exam so to summarize a small increase in partial pressure of oxygen makes it causes a large increase in percentage oxygen saturation the reason is because when oxygen binds to the first hem group it distorts the hemoglobin exposing the second hymn group and the second hymn group will have a higher affinity for oxygen making it easier to bind to oxygen that's all we need to know for the exam foreign