 How does oxygen get carried in the blood? Well, I have a little hint here for you. We've talked about a lot of parts of blood. We've talked a lot about different functions of blood. It's involvement in capillary exchange. It's involvement in blood pressure. But we haven't really talked about the composition of blood. And when we deal with oxygen transport, here's our main player. It's the red blood cell. So let's review what blood even is in the first place. And I'm going to just give you a little test tube of blood and tell you that about 44% of blood, if you were to take a test tube of blood and then centrifuge it, about 44% of it is going to be made up of red blood cells for erythrocytes. About 1% of it is this stuff called the Buffy Coat. The Buffy Coat includes platelets. We're going to talk about them when we talk about how blood clotting takes place. And white blood cells, which are our immune warriors. So we're going to talk about those guys when we deal with the immune system. And the remaining 55% is plasma. Plasma contains proteins, as we know, because that's where the osmotic pressure in the blood comes from. It also contains various immune substances and some ions and some other stuff. But that's basically the fluid that the oxygen is transported in. Now, here's the scoop. Are you ready for this cool fact? We know that we can carry, we can get our partial pressure of our blood up to 100 millimeters of mercury of oxygen. However, that's only 2% of the oxygen that is carried in your blood is actually dissolved in your plasma. The rest of the 98% is carried on the red blood cells. So let's take a second to draw ourselves a red blood cell and look at, dude, what, how? How does a red blood cell carry oxygen? Well, that, my friends, has to do with the fact that in one red blood cell, in one erythrocyte, there are, let me get this right, 250 million hemoglobin molecules. Seriously, 250 million in one red blood cell. So take a deep breath and look at, dude, what is this hemoglobin you speak of? And I'll tell you what this hemoglobin is. It's a protein. It has four subunits that look just like this. Each subunit is just a string of amino acids. So, you know, the alpha subunits have like 141 amino acids. The beta subunits have 146 amino acids and they just fold in on themselves to give you this nice little structure. Then four of them stick together and now you have a molecule, a single molecule of hemoglobin. There's something really important that I did not draw here. Each string of amino acids, each subunit on the hemoglobin contains an iron atom. And so there's actually four iron atoms associated with this hemoglobin molecule and this whole thing is one hemoglobin. Now, the iron is the significant part. The iron loves, and I mean loves, oxygen. So oxygen literally will stick to the iron each found in each subunit of the hemoglobin molecule. So how many molecules of oxygen can be carried in one hemoglobin molecule? Four. There are, I like to think of it as like a bus and each iron atom is like a seat for an oxygen atom. And so there are four seats for oxygen here. However, there are four seats on each one of our 250 million hemoglobin molecules. So one red blood cell, do you agree with this, could carry one billion oxygen molecules? There's like a billion seats for hemoglobin in this one red blood cell. You got to be joking me. Now, here's the thing that I want you to think about. When the alveoli gets filled with oxygen, oxygen is going to diffuse down its partial pressure gradient from 100, I can't help it, I've got to draw you a picture. Okay, I'm going upside down, ready? Except I want it to be black, like I've drawn all my tracheas. Here's my trachea, okay? And you're cool with this having an upside down lung, right? Because that's just my alveolus. And you remember that my partial pressure of freshly inhaled breath is 100 millimeters of mercury. And this whole thing is going to be, this whole thing is my blood vessel. Whoa, did you follow what I just did? Okay, I just wanted to put my red blood cell inside a blood vessel, surrounded in plasma. Okay, do you see my, you got my concept here? Of course, I have the blood vessel going through my alveolus, that's not quite accurate, but you're cool, you got it covered. So watch what happens. Oxygen is going to diffuse into the plasma where the partial pressure gradient was 40. And of course it's going to diffuse in. As it diffuses in, it's almost instantaneous that the oxygen that's now in the plasma is going to go, dude, why would I go down this water slide of plasma when there's an awesome seat inside this red blood cell bus that I can sit on? I'm just going to go in and sit on this hemoglobin seat. And I'm taking four of my buddies with me. And so the oxygen almost immediately says, I'm not hanging out here, and goes into the red blood cell and sticks there. And the oxygen moved into the plasma. Briefly, the partial pressure of the plasma increased. But as soon as it gets sucked into the red blood cell, it goes back down, so it's at 40 again. Which means you have the exact same pull to pull oxygen out of the alveoli again. And you will continue to pull because when oxygen comes in and combines with hemoglobin, it's now called oxyhemoglobin. It's a new molecule, you guys. It isn't oxygen anymore, so it doesn't count as a partial pressure of oxygen. So it keeps the partial pressure of oxygen in the plasma at 40 until all the red blood cells are full. Once all the red blood cells are full, then the plasma will fill to approximately 100. And then we'll head off. Does that work for you? Okay, there we can look at blood and we can quantify how many of our red blood cell seats are filled. And that's the saturation of hemoglobin. So let's talk about that next.