Okay, so we looked at how oxygen goes from the air in the alveolus and the lungs into the blood in the capillary. And that blood is then pumped by the heart all over the body, and the oxygen then goes from the blood. into the cells. Okay?
Now in the cells you're doing cellular respiration. And in those cells you are making carbon dioxide. And we need to get rid of the carbon dioxide. We need to move the carbon dioxide out of the cells and send it out. So how do we do that?
So here I have a setup. This is a tissue cell somewhere in your body. Could be your brain, could be your toe. Okay so this is a cell. This is a capillary right next to the cell.
Here I'm doing a lot of cellular respiration and I am accumulating a lot of carbon dioxide. Okay so let's pretend that these red buttons I have represent carbon dioxide molecules. And I'm going to put 20 of them. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. OK. So I have had cellular respiration and I have 20 molecules of carbon dioxide.
I don't want them there. I need to get rid of them. So I don't have any in the plasma.
So now there is a concentration gradient. And so things will move out of the cell and into the plasma until there is equilibrium. Okay.
So I have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. So I have sent 10. out, but now I'm at equilibrium. I've lost my gradient. But I still have 10 left.
I want to get rid of as much of this as possible. So I want to establish concentration gradient again. So there are three ways carbon dioxide is removed from the cells. In this video, we're just going to show you one of those ways. And then later, when we talk about this in lecture, we will talk about the other two ways and tie it all together.
Now, one of the things there is no shortage of in blood is water. So let's just pretend... Yeah, like this is, these little green buttons are water. And there is as much water as you need.
Okay, 92% of the blood is water, so there is no shortage. Okay, one of the things that carbon dioxide will do when there is water is there is carbon dioxide and a water molecule. combine and make H2CO3, carbonic acid. So this is actually a really simple equation. You just add these two together, two hydrogen, one carbon, and the two and the one oxygen, so the three oxygens.
So H2CO3, this is carbonic acid. Carbonic acid is not carbon dioxide. So as carbon dioxide becomes carbonic acid, the carbon dioxide concentration here will go down. Now I'm going to represent carbonic acid with purple buttons. So one carbon dioxide and one H2O will make H2CO3.
So one carbon dioxide, one water will be replaced by one H2CO3. Okay, another carbon dioxide, another water, H2CO3. And carbon dioxide will come to equilibrium with carbonic acid.
Okay, so we now have carbon dioxide and carbonic acid in equilibrium in the water, but we have high concentration of carbon dioxide. So you have to imagine carbon dioxide will keep moving from the tissues into the blood. And that would help with the concentration gradient, but our body has one more trick here, which is the carbonic acid does not stay as carbonic acid. Each carbonic acid dissociates into H plus and HCO3 minus, so a hydrogen ion and a bicarbonate ion.
Okay, so Carbonic acid will try to come to equilibrium with this set. So what I can do, I'm going to represent H plus with a black button and a CO3 minus with a blue button. This is a sort of a button.
Okay. So every time I remove a purple button, I can replace it with... H plus and HCO3 minus. Another one, replace it with another H plus and HCO3 minus.
So now what happened is, carbon dioxide is not in equilibrium with carbonic acid anymore. So carbon dioxide and carbonic acid has a concentration gradient, so more carbonic acid will become carbonic acid. and carbonic acid will become hydrogen ion and bicarbonate ion. And so I have a concentration gradient here, I have a concentration gradient here, so carbon dioxide will keep moving out, so more carbon dioxide will move out, more of those carbon dioxide will become carbonic acid, And then carbonic acid will keep converting to H plus and HCO3 minus. And because of this, a lot of the carbon dioxide will get removed from the blood.
So 66, 67% of our carbon dioxide is transported by ultimately getting converted into bicarbonate ions. Okay, so this concentration gradient keeps flowing. Equilibrium is bad. Remember, we want concentration gradient.
And so the concentration gradient keeps... is maintained and the carbon dioxide keeps moving from cell to blood and then within blood it converts into carbonic acid and then from carbonic acid it converts into hydrogen ion and bicarbonate ion. And this is one of the ways carbon dioxide is removed.