in a previous video we talked about how gas exchange occurs between the lungs and the atmosphere and how Alvi are the sites of actual gas exchange where oxygen from the air that is inhaled is picked up by the blood and transported to all parts of the body in this video we're going to specifically talk about how oxygen is transported within the body so at the alveoli at the actual sites of gas exchange oxygen diffuses out of the alveoli and enters the bloodstream where it is picked up by red blood cells or erthrocytes so if you think of oxygen as the cargo that needs to be transported to all cells and red blood cells is the truck that transports the cargo but what is the actual carton box or the container in which oxygen is transported inside the red blood cells that is hemoglobin hemoglobin is a Metallo protein it is made of four subunits and each subunits contain a iron or a heem group so the heem group contains a Fe Fe atom ion atom to which oxygen goes and binds and it is this hemoglobin that transports oxygen to all parts of the body this is probably why your parents and Elders always ask you to eat a lot of green leafy vegetables apples and other sources of iron because iron is very important for the formation of hemoglobin transport of oxygen and the proper functioning of the body so if you think of oxygen as the cargo the hemoglobin is the carton box in which the cargo is transported and the red blood cells are the trucks pickup trucks in which the carton boxes are transported your blood has millions of red blood cells and within each red blood cell there are millions of hemoglobin as well so I just told you that hemoglobin has four of these ion centers right which means that each hemoglobin protein can transport four oxygen molecules so if these are the sites that contain ion these are the sites where oxygen can come and bind so the mechanism by which oxygen binds to hemoglobin is known as Cooperative binding now what does this mean so when oxygen is binding to hemoglobin all four molecules do not bind at the same time initially when the oxygen enters the red blood cells only one oxygen molecule initially binds to the hemoglobin it comes and bumps into hemoglobin and binds to one of the ion containing centers now after one molecule of oxygen has bound to hemoglobin then this protein under goes a confirmational change which causes an increased Affinity to oxygen now with this increased Affinity three more molecules can bind to the three more open now exposed sites so this is known as Cooperative binding now this protein now which is bound to oxygen is known as oxy see hemoglobin apart from oxygen binding sites hemoglobin has other binding sites known as allosteric sites and at these sites two different substances can bind to it which are hydrogen ions or protons and carbon dioxide now where do we get this hydrogen ions from so what happens is within the cells carbon dioxide is produced as a result of cellular respiration right so this carbon dioxide diffuses out of the cells and enters the red blood cell we'll talk more about how carbon dioxide is transported in a later video but for now let's just understand that carbon dioxide diffuses out of the cells and enters the red blood cells where this enzyme Carbonic anhydrases catalyzes the reaction between carbon dioxide and water this results in the formation of a very shortlived compound known as carbonic acid now if you remember property of acids is that when acids dissociate they dissociate into conjugate base and protons right this carbonic acid is highly unstable and it immediately dissociates into hydrogen ions and bicarbonate ions we'll talk more about the bicarbonate ions in the video for carbon dioxide transport but this is where we get the hydrogen ions from so hydrogen ions and carbon dioxide have allosteric sites on this hemoglobin apart from the sites where oxygen canb so we can say that hemoglobin is allosterically inhibited by protons and carbon dioxide so now let's move on to the actual transport of oxygen it is the partial pressure that is involved in the movement of oxygen from the lungs into the red blood cells so the pathway that the oxygen takes inside the body is that at the alveoli oxygen is diffused into the red blood cells and inside the red blood cells it binds to hemoglobin and then is transported to all cells and tissues now this difference in partial pressure if you remember what is partial pressure it's the pressure exerted by each gas in a mixture of gases so when we talk about gases in our body we speak mainly about oxygen and carbon dioxide they are the two main gases in our body so we're going to talk about partial pressure of oxygen and carbon dioxide so the partial pressure gradient is what is needed to move oxygen from the Alvi into the red blood cells and from the red blood cells into the tissues so if you remember gases always move from a region of higher pressure to a region of lower pressure so from that itself you can guess that the pressure gradient is like this in this direction so the partial pressure of oxygen is the greatest in the lungs it decreases in red blood cells and then it is the least in the cells and tissues why do you think partial pressure of oxygen is least in the cells and tissues so after the cells and tissues have obtained oxygen the blood is going to go back to the lungs and get oxygenated again conversely for carbon dioxide it's going to be in the reverse Direction carbon dioxide is produced in the cells and tissues as a result of cellular respiration right from that it enters the blood and some of it is also picked up by the red blood cells we just saw here about this equation right so carbon dioxide is going to be picked up by red blood cells and then that's going to be transported back to the alveoli from where it is exhaled out so you can guess the partial pressure of carbon dioxide is in the reverse Direction it is the highest in the cells and tissues and the least in the lungs and Alvi so this partial pressure is important for oxygen to leave the lungs and enter the red blood cells but what about binding of oxygen to hemoglobin so we'll talk about the loading of oxygen in the Alvi in the lungs the partial pressure of oxygen is high we just saw that also to allow for oxygen to bind with hemoglobin the concentration of hydrogen ions is less how because when this hydrogen ions and these bicarbonate ions reach the Alvi the reverse of this reaction is going to take place it binds with bicarbonate gives carbonic acid which then the reverse of this reaction results in the production of carbon dioxide and water that is why the concentration of hydrogen ions is going to be less in the Alvi so now because there is no hydrogen or carbon dioxide to compete with hemoglobin the unloaded hemoglobin this can be called as deoxyhemoglobin the unloaded hemoglobin that's come to the alveoli for oxygenation can now pick up the oxygen from the alveoli so you have four oxygen here that's going to diffuse into the red blood cells and then it's going to bind to hemoglobin with the help of Cooperative binding now this oxyhemoglobin is going to be transported to all peripheral tissues where the oxygen has to be unloaded it needs to unload from the RBC and enter the cells and tissues basically oxygen needs to dissociate from hemoglobin leave the RBC and enter the cells how does that happen so in the cells and tissues we just saw that the partial pressure of oxygen is low and conversely there is a high concentration of protons and the partial pressure of carbon dioxide is also going to be high so now the hemoglobin that has come to the cells is loaded with oxygen it contains four oxygen molecules somehow the oxygen needs to dissociate or unbind from hemoglobin and then exit the red blood cells and enter the cells so what happens now is that the H+ ions in the RBC is going to come and tell oxygen hey oxygen you've reached your address you have to get down now and it's my turn to be carried back to the lungs so you have to get down now so hydrogen is going to come and nudge the hemoglobin protein slowly slowly that's going to cause the unbinding of oxygen from hemoglobin and as one of the oxygen molecules unbinds then again the reverse of Cooperative binding occurs as one is unloaded all the other three are also unloaded and because of the difference in the partial pressure gradient this is going to have more partial pressure than the cells right partial pressure of oxygen because of the partial pressure gradient oxygen is going to move out of red blood cells and into the peripheral cells and tissues so this is how unloading of oxygen works and which is why this equation and the presence of hydrogen ions is going to be very important so now once this oxygen has been unloaded into the cells this uh free hemoglobin you can think of it as a free hemoglobin that is known as deoxyhemoglobin so this is the loading and unloading of oxygen now based on this based on the partial pressure of oxygen and based on how much of hemoglobin is bound to oxygen we can plot an oxygen dissociation curve and what this oxygen dissociation curve shows is on the xaxis it's going to have the partial pressure of oxygen in mmhg and on the y axis it's going to have the saturation of hemoglobin which means how much of hemoglobin is bound to oxygen and if you plot it it's going to give a sigmoid curve this shape is known as a sigmoid curve so initially as the partial pressure is less there is no oxygen that is bound to it so as the partial pressure slowly increases oxygen starts binding to hemoglobin and oxygen is binding to hemoglobin Cooperative binding is going to increase the saturation of oxygen in hemoglobin at high partial pressure of oxygen or hemoglobin is going to be completely saturated at 100% of oxygen so at this stage no more oxygen can come and bind with hemoglobin is fully saturated so hemoglobin's job now is to unload oxygen at the peripheral cells and tissues so with this we are ending the transport of oxygen we'll talk more about the transport of carbon dioxide in another video