If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:14:34
In this video, Sal says "nucleuses" but means "nuclei."

Video transcript

I've talked a lot about the importance of hemoglobin in our red blood cell so I thought I would dedicate an entire video to hemoglobin 1 because it's important but also it explains a lot about how the hemoglobin or the red blood cells depending on what level you want to operate know and you know I have to use no.1 quotes these aren't sentient beings but how do they know when to pick up the oxygen and when to drop off the oxygen so this right here this is actually a picture of a hemoglobin of a hemoglobin protein right there this is hemoglobin hemoglobin it's made up of four amino acid chains that's one of them d and that those are the other two we're not going to go into the detail of that but these look like little curly ribbons if you imagine them they're a bunch of molecules and amino acids and then they're curled around like that so this on some level describes its shape and then within each of those groups or in each of those chains you have a heme group you have a heme group here in green heme group that's where you get the heme and hemoglobin from you have four heme groups and the globins are essentially describing the rest of it the protein structures the four peptide chains now this heme group this is pretty interesting it actually is a porphyrin a structure and if you watch the video on chlorophyll you remember a porphyrin structure but at the very center of it in chlorophyll we had a magnesium ion but at the very centre of hemoglobin we have an iron we have an iron ion iron ion and this is where the oxygen binds so on this hemoglobin you have four major binding sites for oxygen you have right there may be right there a little bit behind right there and right there now why is hemoglobin well you know oxygen will bind very well here but hemoglobin has a several properties that one make it really good at binding oxygen and then also really good at dumping oxygen when it needs to dump oxygen so in it exhibits something called cooperative binding cooperative cooperative binding and this is just the principle that once it binds to one oxygen molecule so let's say I have one oxygen molecule binds right there it is what it changes the shape in such a way that the other sites are more likely to bind oxygen so it just makes it you know one binding one binding makes the other bindings more likely makes other bindings more likely now you say okay that's fine that makes it a very good oxygen acceptor when it's traveling through the the capillary the pulmonary capillaries and oxygen is diffusing from the alveoli that makes it really good at picking up the oxygen but how does it know when to dump the oxygen this is an interesting question when to dump oxygen it doesn't have eyes or some type of GPS system that says oh you know what this guy is running right now and so he's generating a lot of carbon dioxide right now and in these capillaries and he needs a lot of oxygen in these capillaries surrounding his quadriceps I need to deliver action it doesn't know it's in the quadriceps how does how does the hemoglobin know to let go of the oxygen there and that's a byproduct of what we call allosteric inhibition which is a very fancy word but the concepts actually pretty straightforward allosteric inhibition and when you talk about allosteric anything it's often used in the context of enzymes you're talking about the idea that things bind to other parts allo means other so you're binding to other parts of the protein or the enzyme and what enzymes are just proteins and it affects the ability of the protein or the enzyme to do what it normally does so hemoglobin is allosteric ly inhibited by carbon dioxide by carbon dioxide and by protein by protons so carbon dioxide can bond to other parts of the hemoglobin I don't know the exact spots and so can proton so remember acidity just means a high concentration of protons so if you're in an acidic environment protons can bond reveal to the protons and this pink color protons which are just hydrogen without electrons right protons can bond to certain parts of our protein and it makes it harder for them to hold on to the oxygen so when you when you're in the presence of a lot of carbon dioxide or an acidic environment this thing is going to let go of its oxygen and it just happens to be that that's a really good time to let go of your oxygen let's go back to this guy running he's running there's a lot of activity in these cells right here in his quadriceps they're releasing a lot of carbon dioxide into into the capillaries into well at that point they're turning they're going from arteries into veins and they need a lot of oxygen which is a great time for the hemoglobin to dump their oxygen so it's really good that hemoglobin is allosteric ly inhibited by carbon dioxide carbon dioxide the joins on certain parts of it starts letting go of its oxygen that's exactly where in the body the oxygen is needed now you're saying wait what about this acidic environment how does this come into play well it turns out that most of the carbon dioxide is actually disassociated it actually disassociates it does go into the plasma but it actually gets turned into carbonic acid so I'll just write a little formula right here so if you have a little bit of if you have some co2 and you mix it with the water I mean most of our blood the plasma that's its water so you take some carbon dioxide you mix it with water and you have it in the presence of a of an enzyme and this enzyme exists in red blood cells it's called carbonic anhydrase carbonic anhydrase a reaction will occur that'll that'll essentially you'll end up with carbonic acid you'll end up with we have h2 co3 it's all balanced we have three oxygens two hydrogen's one carbon and this is called carbonic acid because it gives away hydrogen protons very easily acids disassociate into their conjugate base and their hot end there and hydrogen protons very easily so carbonic acid carbonic acid this associates very easily it's an it's an asset although right in some type of an equilibrium right there if any of this notation really confuses you if you want more detail on it watch some of the the chemistry videos on acid dissociation and equilibrium reactions and all of that but it's essentially can give away one of these hydrogen's so you're left with but just the proton and it keeps the electron of that hydrogen so you're left with a hydrogen proton plus well you gave away one of the hydrogen so you just have one hydrogen this is actually by a bicarbonate ion so this is bicarbonate right there but it only gave away the proton kept the electron so you have a minus sign so it's the the all of the charge adds up to neutral and that's neutral over there so if I'm in my if I'm in a capillary of the leg let me see if I can draw this so let's say I'm in the capillary of my leg let me do a neutral color so this is a capillary of my leg I've zoomed in just one part of the capital you know it's always branching off and over here I have a bunch of you know muscle cells right here that are generating a lot of carbon dioxide they're generating a lot of carbon dioxide and they need oxygen well what's going to happen well I have my red blood cells flowing along and it's actually interesting red blood cells that the Kappa are 25% their diameter is 25% larger than the smallest capillaries so essentially they get squeezed as they go through the small capillaries which a lot of people believe helps them release their contents so releases it maybe some of the oxygen that they have in them so you have a red blood cell that's coming in here it's being squeezed through this capillary right here it has a bunch of hemoglobin and when I say a bunch you might as well know right now so each red blood cell has 270 million hemoglobin proteins 270 million and if you total up the hemoglobin in the entire body it's huge because we have let me write this number down this is we have 20 to 30 trillion red blood cells we have 20 to 30 trillion red blood cells and each of those 20 to 30 trillion red blood cells have 270 million globen hemoglobin proteins in them so we have a lot of hemoglobin so anyway that was a little bit of so actually red blood cells make up roughly 25% of all of the cells in our body we have about 100 trillion or a little bit more give or take I've never sat down and counted them but anyway we have a bunch we have 270 million hemoglobin particles or proteins in each red blood cell explains why the red blood cells had to shed their nucleuses to make space for all of those hemoglobins they're carrying oxygen so right here so right here we're dealing with this is an artery right it's coming from the heart the red blood cell is going in that direction and then it's going to shed its oxygen and then it's going to become a vein now what's going to happen is what's going to happen is you have this carbon dioxide you have a high concentration of carbon dioxide in the mussel salad eventually by just by a diffusion gradient ends up let me let me do that same color ends up in the blood plasma ends up in the blood plasma just like that and some of it can make its way across the membrane into the actual into the actual red blood cell in the red blood cell you have this carbonic anhydrase which makes the carbon dioxide disassociate into or essentially become carbonic acid which then can release protons so let me do the protons out here well those protons we just learned can allosteric ly inhibit the uptake of oxygen by hemoglobin so those protons start bonding to different parts and even the carbon dioxide that hasn't been reacted with that can also allosteric ly inhibit the hemoglobin so it also bonds to other parts and that changes the shape of the hemoglobin protein just enough that it can't hold on to its oxygen set well it starts letting go and just as we said we had cooperative binding the more oxygens you have on the the better it is it accepting more the opposite happens when you start letting go of oxygen it becomes harder to retain the other one so then all of the oxygens let go so this is at least in my mind it's a brilliant brilliant mechanism because the oxygen gets let go just where it needs to let go doesn't it doesn't just say oh I'm in I've now I'm now I've left an artery and I'm now in a vein you know maybe I've gone through some capillaries right here I'm going to go back to a vein let me release my oxygen because then it would just release the oxygen willy-nilly throughout the body this system by being allosteric ly inhibited by carbon dioxide and an acidic environment it allows it to release it where it is most needed where there's the most carbon dioxide where respiration is occurring most rigorously so it's or vigorously is so it's a fascinating fascinating scheme and just to get a better understanding of it right here I have a this little chart right here that shows the oxygen uptake by hemoglobin or how saturated can be and you might see this in maybe your biology class so it's a good thing to understand so right here we have on the on the x axis or the horizontal axis we have the partial pressure of oxygen and if you watch the chemistry lectures on partial pressure you know that partial pressure just means how frequently are you being bumped into by oxygen pressure is just generated by gasses or molecules bumping into you or it doesn't have to be gas but just molecules bumping into you and then the partial pressure of oxygen is the amount of that that's generated by oxygen molecules bumping into you so you can imagine as you go to the right there's just more and more oxygen around so you're going to get more and more bumped into by oxygen so this is just essentially saying how much oxygen is around as you go to the right axis and then the vertical axis tells you how saturated are your hemoglobin molecules you know this hundred percent would mean all of the heme groups on all of the hemoglobin molecules or proteins have bound to oxygen zero means that none have so when you have an environment with very little oxygen and this actually shows the co-operative binding so let's say we're we're just dealing with you know an environment with very little oxygen so once a little bit of oxygen binds then it makes it even more likely that more and more oxygen will bind as soon as a little that's why the slope is increasing I don't want to go into algebra and calculus here but as you see we're kind of flat ish and then the slope increases so as we bind to some oxygen it makes it more likely that will bind to more and at some point it just you know it's hard for oxygens to bump just right into the right hemoglobin molecules but you can see that it kind of accelerates right around you now if we have an environment with a lot an acidic environment that has a lot of carbon dioxide so that the hemoglobin is allosteric ly inhibited it's not going to be as good at this so in an acidic environment this curve for any for any level of oxygen partial pressure or any amount of oxygen we're going to have less bound tema globin so let me do that in a different color so then the curve would look like this the saturation I wanted to do darker the saturation curve will look like this so this is an environment right here so this is an acidic environment maybe there's some carbon dioxide right here so the hemoglobin the hemoglobin is being allosteric ly inhibited inhibited so it's more likely to dump the oxygen at this point so I don't know I don't know how exciting you found that but I find it brilliant because it really is the simplest way for these things to dump their oxygen where needed no GPS needed no no robots needed to say oh I'm now in the in the quadriceps and the guy is running let me dump my oxen and it just doesn't naturally because it's a more acidic environment with more carbon dioxide it gets inhibited and then the oxygen gets dumped and ready to use for respiration