 We've only looked at half of the story. We've taken care of oxygen. We know that 98% of the oxygen is going to combine with hemoglobin, and the other 2% is just going to dissolve in the plasma, and that's how we carry it to the cells. Once we get it to the cells, the cells just suck the oxygen right off of the hemoglobin, depending on how needy they are, and then the red blood cell picks up the carbon dot, or the blood picks up the carbon dioxide. I'm going to draw you a picture of this new setup. And in my picture, I want to include a cell. This is my Joe cell that's metabolizing, and through the process of metabolism, producing carbon dioxide. I want to draw the blood with, okay, a gigantic, thick, very strangely shaped red blood cell. I apologize, it kind of looks like a tooth, but I need more room to show you what's going to happen inside this red blood cell. But it's also got plasma in there, so this is just the blood. And rather than draw another zone of blood, I'm going to show you the alveoli as well. And so this right here is just going to be also representing what happens, and we're going to get rid of the carbon dioxide into the alveoli. Let's make a note of what our partial pressures for carbon dioxide were in these various scenarios. Remember that PCO2 in the alveoli, from a fresh breath of air, was 40 millimeters of mercury. You might want to review that original video that we did. Let's review the fact that our cells are constantly metabolizing and using oxygen and going through cellular respiration in the mitochondria, and we can just make the assumption that the partial pressure of carbon dioxide in most metabolizing cells, let's just call it 46 millimeters of mercury, just to call it good. That will vary depending on how active this cell actually is. My blood, since I have it right next to the cell and right next to the alveoli, the blood actually depends on what we're talking about. But at the fresh end, we're going to have a PCO2 of 40, and at the used up end, we're going to have a PCO2 of 46. So we're going to be thinking of our blood in the middle here in both ways. Now, there are three ways that we can transport carbon dioxide in the blood, and I'm going to tell you about the first way, knowing that there are three of them, 70%. This is the first thing to fill. This is the, oh, you might think of this as like the stickiest, most attractive way to ride, for carbon dioxide, to ride to the alveoli in a scenario where PCO2 is 46 millimeters of mercury. 70% of the carbon dioxide is made into bicarbonate ions, HCO3 negative. Now, I already did this once, but remind yourselves, carbon dioxide plus water, is there any water in your cells for your blood? Yeah, you're surrounded. When you combine carbon dioxide and water, you're going to get carbonic acid, H2CO3, which dissociates into hydrogen ions and bicarbonate ions. Bicarbonate ions are in baking soda. Baking soda is sodium bicarbonate, so you add sodium onto a bicarbonate ion and you have sodium bicarbonate or baking soda. Bicarbonate ions or baking soda are very important in your system for helping maintain pH homeostasis. If you add hydrogen ions, you're going to get acidic. If you add bicarbonate ions, it's going to make things more basic. So, 70% of your carbon dioxide is actually made into bicarbonate ions, but how does that happen? Well, watch and be amazed. I'm going to make my carbon dioxide purple. So, imagine a carbon dioxide molecule. It's going to actually diffuse into the blood, the partial pressure of the blood. Let's just go ahead and write this down here. Let's just say PCO2 at the beginning is going to be about 40, just like in the alveoli. Okay? So, you can imagine that we've gotten rid of it. So, now there's a gradient with this fresh new blood that has approached this cell. We have a gradient. The carbon dioxide is going to travel down its partial pressure gradient. And you might think, dude, just stay in the plasma. That's great. That's not the first... If it stayed in the plasma, you would not be able to carry very much carbon dioxide. This carbon dioxide actually diffuses into the red blood cell because there's not very much carbon dioxide inside that red blood cell. And it comes in contact with my friend, an enzyme known as carbonic anhydrase. I'm drawing carbonic anhydrase as a circle. I'm going to write its name down here. Carbonic anhydrase. You'll see it again. So, it's worth committing to memory. Carbonic anhydrase takes the carbon dioxide, combines it with a little bit of, you know, water, and produces what? H2CO3, which is going to dissociate into hydrogen ions, which just actually combine with hemoglobin. And so, they don't change the pH of the red blood cell. Brilliant. And bicarbonate ions. And what happens to the bicarb? It's transported. This is so awesome. There's a transporter in the red blood cell that switches trades bicarbonate ions for chloride ions. Seriously? Yeah. So, chloride comes in, bicarb goes out, and now you have bicarb in your blood. Bicarbonate ions, 70% of our carbon dioxide got turned into bicarbonate ions. And now they're floating in the blood. Now, I can't help it, but the whole chair analogy just works for me. There's 70 seats in the blood for bicarbonate ions. As soon as we hit 70 bicarb ions, done. You're not going to fit anymore in there. It's the nicest way to travel. Carbon dioxide is like, please turn me into a bicarb, because those are the best seats on the bus. But only 70 seats are available that way. So there's got to be something else. Once you hit your 70 seats, we've got to have another option and guess what it is. I can't believe this. Hemoglobin. I got to draw you a picture. What? Remember my little hemoglobin molecules? I can't remember. Did I make the little things green? Remember my little chairs for oxygen in a hemoglobin molecule? Guess what? You're not going to believe this, because it's this cool. There are 23 chairs available. 23% of carbon dioxides can actually come in and bind to hemoglobin. There's 23 seats available here, and they're going to ride on the hemoglobin molecule with oxygen, which in this environment is probably going to be 40 here, don't you think? In this environment, we're going to have three oxygens hanging out on our hemoglobin, but now we're going to throw a carbon dioxide on the hemoglobin as well. Are you kidding me? That's awesome. We can't fit any more bicarbs in the plasma. We can't fit any more carbon dioxides on our hemoglobin, but we can actually squeeze a little bit more in. In fact, if you did the math, you will see that we have 7% of our carbon dioxides left, and they're just going to dissolve in the plasma. So, watch. Do you agree with this? When we turned carbon dioxide into bicarb, it is no longer carbon dioxide, so it did not change the partial pressure of carbon dioxide in the plasma. When we stuck it on hemoglobin, that's not the partial pressure. That's not going to change the partial pressure. So, now our last thing is going to be just to dissolve carbon dioxide directly in the blood plasma. And as we do that, PCO2 is going to increase up to 46. But first, we filled all these other seats. Now we have... Now, you can think of dissolving the carbon dioxide in the plasma as being the least strong. Like, that's going to be the first... They're going to run off that bus. They're going to be like, dude, I'm just a carbon dioxide molecule hanging out in the plasma. I really don't feel entirely comfortable. When they get turned into bicarb, they're like, dude, I got the chillest boat on the planet. This is the best boat to travel in the plasma in. It's like traveling in a yacht. But those who had to go down without any boat at all because they're just pure carbon dioxide, they're really not stoked about this. So, imagine, we're going to take this whole thing today I'll be a lie. And now we have the PCO2 of the blood is at 46 because of these carbon dioxide that are dissolved directly in the blood and now all of a sudden they're exposed to a very different partial pressure of carbon dioxide and that is 40 in the alveoli. And guess what's going to happen? We're going to pull those guys out of the blood plasma. They're going to go down their partial pressure gradient. As they move out of those seats, the whole thing is going to reverse. The alveoli, this pressure keeps pulling carbon dioxide molecules in. Now you've got room. We could have more carbon dioxide directly in the blood plasma. And so we're going to reverse the whole thing. First, the carbon dioxide from the plasma just dissolves in. Second, the carbon dioxide on the hemoglobin is just going to dissolve in. It's going to fall off of its little connector to the hemoglobin and once it falls off it's going to be in the plasma and once it's in the plasma it's going to defuse down its partial pressure gradient. And then the whole thing reverses. Now there's these, the whole environment is changing because of this right here. This whole thing reverses. By carbonate ions are actually going to go backwards. They're going to travel back in in exchange for who? Chloride ions, which are going to go back out. We're just reversing the whole thing. The bicarb ions are going to combine with the hydrogen ions that we're hanging out in there anyway. Carbonic anhydrase is going to reverse the reaction. We're going to end up with carbon dioxide that, whoa, is now diffusing into the alveoli. True story. If you can accept that we fill the easiest place first and why did we fill it? There was a push. There was a push from partial pressure gradients. But once it's filled, if we remove that push by changing the partial pressure gradient, then we're going to have a whole new scenario and we reverse the whole thing. It's like I shoved you all into the corner and now I release the pressure that kept you in the corner and you're going to go back the way you came. And now we have carbon dioxide in the alveoli. Are you happy? Do you think you should draw that like 82 times so that you're comfortable with it? Yes, you should. Good work, team. That's dense. Kidneys are coming next, which means holy density.