Alright, everybody. It is now time to talk about internal respiration. Remember, internal respiration, we said, is the exchange of gases, including oxygen and carbon dioxide, between blood and the internal tissues of the body. Here in internal respiration, we are going to have a net movement of carbon dioxide or CO2 into the blood and oxygen or O2 into the tissue. Also, we want to remember that in any tissue, just like in the alveoli, the total amount of carbon dioxide exchanged is going to equal the amount of oxygen exchange. If we were to count the total number of molecules crossing here, these two would be equal. And again, that's because of cellular respiration. Cellular respiration occurs in the tissues, and it uses the same amount of oxygen as carbon dioxide it produces.
Alright. So to look at this more closely, we're going to bring back this analogy of the tissue tower. The tissue tower, from the blood chapter, represents the tissues where all these molecules are either going to or coming from. Going by the tissue tower here, we have the street, and the street represents the capillary. Inside the capillary, so the space in the capillary, the space on the street or on the sidewalk there, represents the liquid portion of the blood or the blood plasma. And then we also have these buses coming through, and those buses represent the hemoglobin molecules.
Alright. So we'll start over on the left there. You see that hemoglobin molecule coming in. It's loaded up with oxygen. There's also oxygen dissolved in the blood plasma there, and we can look at the gradients here. Well, the partial pressure of oxygen on the street there is 100 millimeters of mercury. Well, in the tissue tower, it's less than or equal to 40 millimeters of mercury. Remember that less than or equal to depends on just how metabolically active that tissue is. Alright. So we have this big gradient. It's just way more crowded on the street, so there's a lot more pressure on the street that's going to be pushing these oxygen molecules off the street and into the tissue tower where there's less pressure. So these molecules move into the tissue tower, and as there becomes more room on the street, well, some of those molecules are going to hop off the bus. Those oxygen molecules hop off the bus, onto the street. It's more crowded now as they're getting off. That pushes more molecules into the tissue tower, and that process just continues until the partial pressure of the blood plasma well, on the street, equalizes to the partial pressure in the tissue tower. So here now it equals 40 millimeters mercury in both places, so no more oxygen molecules are going to cross.
Alright. But then that bus turns around. Well, actually, we just want to note we showed here this bus emptying, but really remember in every red blood cell, there's 250,000,000 of these buses, 250,000,000 hemoglobin molecules. We just want to note most of those actually don't drop off all their oxygens. There's actually a huge reservoir of oxygen still bound to hemoglobin even after this exchange happens. So that's just to say if you really need a lot of oxygen, don't worry. There's a lot of it in your blood. That hemoglobin carries a lot of oxygen around all the time.
Alright. But some of those hemoglobins do drop off their oxygen. We can follow that one along. So now it's this deoxyhemoglobin over here, and we can look at these gradients for the carbon dioxide. Well, you can see it's greater in the tissue tower, 46 millimeters of mercury in the tissue tower compared to 40 millimeters of mercury out here on the street. So it's crowded in the tissue tower. There's a lot of pressure. Those molecules get pushed out of the tissue tower. They come out. They come out onto the street. Now the street's getting more crowded. Some of those molecules hop on the bus. As they hop on the bus, you know, they're sort of riding on the outside of the bus here. Remember, that's because carbon dioxide binds to a different place on the hemoglobin than the oxygen does. But these molecules hop on the bus. This continues. That creates a little bit more room on the street. More molecules are pushed out until, right, that partial pressure on the street equalizes to the partial pressure in that tissue tower, and now they are both equal to 46 millimeters of mercury.
Alright. From here, this blood rolls on, and it's going to go through the veins back up to the alveoli, and it's going to continue this entire process again. Alright. But let's look at these gradients. So that oxygen gradient here, well, that gradient again was 60 millimeters of mercury. It's 60 millimeters of mercury greater in the street, in the plasma, than it is in the tissue tower than in the tissues, and that's why that oxygen moves into the tissues. Well, the CO2 gradient was 6 millimeters of mercury. Again, greater in the tissue tower than on the street. That's why those molecules move out of the tissue and into the blood plasma.
Alright. Those should look familiar. That's the same gradients that we had for external respiration. So, hopefully, those are easy to remember. It's always 60 and 6. Alright. Again, quick note, those gradients do depend on metabolic activity. If this is a really metabolically active tissue, like skeletal muscle when you're exercising, those gradients will be larger. Alright. So now we've followed along how these molecules move in and out of blood, both external respiration and internal respiration. We're going to review this all some more and some examples and practice problems to follow. I will see you there.
