Hi everybody and welcome back to Miss Angler's biology class. I am Miss Angler. In today's video we're going to look at gaseous exchange and we are going to trace gases from the moment they enter the alveoli, then how they move through the blood and how they're transported, and then finally when they do a gaseous exchange at the tissues themselves.
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My membership has premium features like extra videos that I walk through, exam questions, extra live lessons together, as well as a free copy of my study guide. Now in order to successfully explain gaseous exchange, we need to remember the six things that must be present in order for gaseous exchange to be successful. Now this list is required because we need to make gaseous exchange as functional and as optimum as possible. So the first thing we always need is a large surface area, which is great because the alveoli does that for us because they are round and there's so many of them.
We need it to be well ventilated, which is taken care of by our intercostal muscles and our diaphragm pulling the gases in and pushing the gases out. It needs to be richly supplied with blood vessels, meaning there needs to be a capillary network wrapped around them, which it is. The gaseous exchange surface must be thin, which LV only are. They're only one cell layer thick.
It needs to be well protected, which is what we use the pleural membrane and the ribs for. And lastly, and probably most important, importantly, it must be permanently moist. Gases cannot diffuse through a dry space.
There needs to be moisture facilitating the diffusion. And our alveoli has a thin layer of moisture around it at all times, which helps us do this. Now let's have a look at the gaseous exchange that happens at the alveoli itself. Now on the left-hand side here, I have pictured an alveoli. Remember, those are those balloon or ball-like structures that we find at the end of the bronchioles.
They're clustered together. They are wrapped with a capillary. The capillary is omitted in this picture, so you can't see it. But what we've done is we've cut it open on one of these sets of alveoli so that we can see on the inside. And their round structure increases the surface area, making it way more efficient.
Now, what many people don't understand is that gases don't actually go all the way into your lungs when you breathe. And what I mean by that is when you take a breath of air, the air doesn't go all the way from your trachea all the way down to the alveoli. It's just actually not physically possible. So instead, what happens is air goes into the lungs about halfway down, and then the rest of the process is diffusion where the gas slowly diffuses down. from a high concentration to a low.
Now, this movement of air in and out of the lungs is called tidal air. Now, tidal air is when air flows in and out of your lungs, and it doesn't enter your alveoli. It makes it about halfway down your bronchi, and from there, the gases then need to diffuse down lower. Now, We diffuse from the top down. In other words, the freshest air gets to the top of your lungs and the older air sits at the bottom of your lungs.
But because of diffusion, they slowly but surely replace each other. Now it gets the name tidal air because it refers to, you know, the same way tides happen in the ocean where the water comes and goes. It's the same idea. It's just the air flowing in and out of your lungs.
Now, linked to tidal air is the fact that your air in your lungs never fully empties. You know, you breathe out and you can always like push a little more air out of your lungs. And the reason for that is you have something called residual volume.
Now, residual volume is often confused with tidal air. And so let's just key out some of the most important bits of what residual volume is. It's the air that remains in your lungs.
and it prevents your lungs from collapsing. Now, that's because it's sort of like a balloon. It's really hard to first blow up the balloon, right?
To first inflate it. But once it's inflated, it's very easy to inflate. Same idea with your lungs. You want to keep your lungs partially inflated so it's easier to inhale. Now, every time tidal air comes in, it mixes with this residual volume so that new fresh air can diffuse down into the alveoli.
So essentially, we get fresh air coming into the top portion of the lungs. And then the air slowly diffuses down the bronchi to the bronchioles, down this little bronchiole over here to the ends here where we get to the alveoli. Now, that's all diffusion.
In other words, there's no actual like mechanical. movement that's pulling the air in there. It's all what we call physical movement where the air is moving itself. Now, once we're in the alveoli, we need to exchange those gases.
And in order to do so, we need one final piece to the puzzle. And that's something called partial pressure. Now, again, something else that a lot of us get confused about what it is.
But partial pressure means that every gas that enters your lung follows its own pressure gradient. And what that means is that oxygen will have a different gradient to carbon dioxide, which sort of makes sense because when you're breathing in oxygen, it will have a different concentration to the carbon dioxide you're breathing in. In other words, there should be more oxygen in the air that you're breathing than carbon dioxide. They're not equal. So they will have their own concentration gradients.
And we calculate that out of the total pressure of gases that are going in or the total pressure of oxygen. And so if you think about it, you have air pressure going in, but it's pieces of that air pressure. And so pieces of it are carbon dioxide pressure, pieces of it are oxygen pressure. So that's what makes them partial. It sounds like it's a part of the total gas pressure that enters your lungs.
Now, it allows gases to then flow and follow a concentration gradient flowing into the lungs. Now that we've looked at how the gases get down to the alveoli, now we need to look at what is actually happening inside these alveoli and how do you actually exchange the gases. So I'm going to zoom in on one of these alveolus, and we're going to look at how the gases move across the cellular walls into the bloodstream. So we have cut one in half here. And in any exam or test, you may be asked to draw or label this.
So be very aware of that when you are preparing for your exams. But what we're looking at here is we have some oxygen coming in to the alveoli. And what it does is it needs to get into the bloodstream, right? It needs to get into the red blood cells.
But in order to do that, it has to diffuse into the moisture layer. that is lining the alveoli. So there's actually moisture here.
The reason for that is gases cannot enter a tissue without moisture. It makes sense because you can't breathe through your skin, for example. You can't pull in gases through your skin because there's no moisture on the outside of your skin.
Whereas someone like amphibians, they can do that. They can breathe through their skin because their skin has a lot of moisture on it that allows gases to diffuse into it. So The gases diffuse into our moisture lining.
From there, they are then going to diffuse across our lining of our alveoli, then into the endothelium, which is the tissue that lines the capillary. And then finally, we end up into the capillary itself, where we can make our way into the red blood corpuscles or the red erythrocytes or red blood cells, whatever you want to call them. to call them, and off we go to the rest of the body. Now, on the other side, we have to exchange a gas, hence the name gaseous exchange. We have to exchange one gas for another.
So in this instance, we are exchanging oxygen for carbon dioxide. So if we have a look on the other side of the diagram, we will notice that carbon dioxide is making its way from the red blood corpuscle through the endothelium into the lining of the alveoli, and again into the moisture lining. of the alveoli into that moisture and then evaporating into the empty area inside of the alveolus. Once it's evaporated there, it can make its way up and out of the alveoli and we've now successfully exchanged gases. You should be able to explain this fairly well and clearly in an exam focusing on how the gases dissolve into the liquids and then how they diffuse across membranes.
Now that we have successfully exchanged gases, we need to now move those gases around the body. And so we need to have a look at this diagram here, which is demonstrating the way in which we move gases around. Now, this particular one focuses on carbon dioxide, which I'm going to get to very soon. But the one that we have just done now with the alveoli is in reference to our red blood cells or our erythrocytes. And those erythrocytes are responsible for picking up the oxygen and then moving it from the lungs and taking it to the tissues of the body.
And to do that, it has a special pigment called hemoglobin inside of it. And when oxygen joins it, it becomes oxyhemoglobin. That red blood corpuscle then goes off, delivers this oxygen and starts the whole process again of picking up the carbon dioxide and taking it to the alveoli so we can exchange.
Now, when it comes to moving carbon dioxide around, It's a little bit more complicated, a little more tricky. And there are three ways that carbon dioxide can be moved around the body. Now, remember, where is this carbon dioxide coming from? If we have a look at the diagram alongside, the carbon dioxide is coming from your cells. And so as your cells respire, they are producing carbon dioxide.
And that carbon dioxide is moving out of the cells through diffusion into the bloodstream. And now we need to move it around and get it. out of the body. So these are the three ways that you can do that. The first way that we can move carbon dioxide through the body is it combines with the water in your bloodstream and it forms bicarbonate ions.
Now it needs to do that pretty quickly because if carbon dioxide joins with water, it can become an acid, carbonic acid, which I'm going to mention soon. So what it does is it joins with the water, but it becomes a more stable, friendlier substance, which is a bicarbonate ion. And bicarb ions are basic in nature.
So they're more alkaline, which is a good thing. The other way we can transport carbon dioxide in your bloodstream is a small amount, and it's key that it's small, can combine with hemoglobin. Now, the reason why we want it to be small is if carbon dioxide gets stuck to hemoglobin, it's incredibly difficult for them to break apart and to sever their bond. And if you fill up all your red blood cells with carbon dioxide, it's really difficult to put oxygen back into your blood and you could end up dying.
So this is the least amount of carbon dioxide is traveled in this route because it is not very efficient. Now the final amount of carbon dioxide is dissolved in the cytoplasm of your red blood cells and it forms carbonic acid. Now, acids in general are not great for your tissues, but the carbonic acid plays a really important role in gaseous exchange, which I'm going to elaborate in the next slide after this, so you can see how important it actually is. So now we have successfully transported the gases through the body.
The gases are now arriving at the tissue. So the blood's arriving full, fresh of oxygen. Now we need to get the oxygen out of the bloodstream and into the tissue. And we need to then exchange that oxygen for carbon dioxide so we can take the waste gas away.
Now... Whenever you are explaining this or drawing this, you're going to need to have some kind of diagram like this one above. And it's one of the more common ones that you would see in exams and tests and things.
And what we have here is an inflow of red blood cells. They're bringing oxygen with them. So as we can see here alongside, the oxygen is flowing and diffusing out of the bloodstream and into the tissues.
On the other side of that. however, we have the carbon dioxide, which is a waste product of cellular respiration. That's leaving the cells and it's entering the bloodstream.
And that's normal because that's gaseous exchange. You must exchange those two gases all the time. Now, when explaining this in an exam or a test, you always need to speak about the fact that these gases are diffusing down a concentration gradient.
In other words, they're moving from a high concentration to a low. Be specific about which side is which. So where is the high in the blood or in the tissue? And then go on to say that the one gas moves from point A to point B and vice versa for the other gas.
Now, again, I'm mentioning this because we said at the beginning of the video, but each gas follows its own concentration gradient, which makes sense because in the lungs there's going to be more oxygen but in the tissues there's going to be more common dioxide and because of these differences in concentrations it influences and encourages gaseous exchange in a slightly different way because of how much gas there is you know we want to put in as much oxygen into the blood in the lungs and when we're at the tissues like now We want to put as much carbon dioxide into the blood so we can move it out because it's a waste. And this whole concentration gradient is affected by that partial pressure that we mentioned earlier, where each gas has a piece or a part or a partial pressure of the full gaseous pressure that's occurring in the in the alveoli. Now, when explaining. this process of gaseous exchange at a cellular level, so at the tissue level, you're going to need to speak about something called a state of acidosis.
Now this is interesting and this is probably the hardest part of this topic because you need to understand what's going on inside of the tissues. So when I use the word acidosis, what I am referring to is the acidic pH of your blood. And the reason why the blood has gone a little bit more acidic, as we mentioned in the previous slide, when carbon dioxide leaves the cell. So like we can see here, we've got carbon dioxide leaving the cell and entering into the bloodstream. If it fuses with the cytoplasm of the cell, it can form carbonic acid, which lowers the pH. Now, too much of this is a bad thing.
But we need just a little bit of carbonic acid to hang around so that acidosis can occur. Now, why does acidosis need to occur? Because acidosis makes it easier for oxygen to break its bonds with hemoglobin.
In other words, as the carbon dioxide leaves the tissues, it makes the bloodstream lower in pH, acidic. And that's important. because as the oxygen is trying to go into the cells, it struggles to break its bond with the hemoglobin.
So basically what I want you to imagine is something like this, if it's difficult to imagine. Imagine this is the red blood corpuscle and attached to it is our oxygen. Now they hold a really strong bond with one another, but if you make the blood slightly acidic, the acid in the blood...
eats away at this bond, separating the two. So now what you have is the oxygen, which is this little guy over here, breaking away and going into the cells. And the red blood cell continues on its journey so that it can start all over again, pick up some carbon dioxide in return.
So now we have CO2 joining this little molecule here. And so now that red blood cell can go off and move the CO2 all the way to the lungs so you can breathe it out. And this is only possible when you have acidosis, which is a slight acidic pH of the blood.
Now, as always, I like to finish off my lessons with a terminology recap. You can use any of these terms to use as flashcards. And remember that.
Terminology is really important for exams when you're trying to explain or describe things for full marks. So we started off looking at the site of gaseous exchange which is the alveoli, those sac-like structures and we spoke about how the air enters the lungs and exits the lungs and what is that flow called? It's called tidal air and the tidal air represents the air that flows into the top of your lungs and then slowly diffuses down to the bottom of your lungs. Now you Your lungs are never fully empty. There's always some kind of oxygen and gas left inside of your lungs.
And so we call this residual volume. It's the leftover air in your lungs that prevents your lungs from collapsing. Now, when these gases move into your bronchi and then into your bronchioles and alveoli, the gas needs to move from a high concentration to a low, which we call diffusion. Now, that diffusion is only possible.
because of the structures that we found wrapped around the alveoli, which are the capillaries. And they are that thin, one-cell thick layer of tissue that creates these little blood vessels that transport the gases around for us. And floating in our capillaries are our erythrocytes, which are our red blood cells.
They love picking up oxygen, but they also pick up some carbon dioxide as well. And basically, they're like the wheelbarrows of the... blood vessel system, they pick up gases and move them around from place to place. Now lining the capillary are endothelial cells as well as there being epithelial cells lining the alveoli. The epithelial cells have a moisture content, so they're actually wet, and that's because gaseous exchange cannot happen without moisture because you need gases to diffuse through moisture.
Now, once we're in the erythrocyte, so we've got the oxygen in there, we need to hold on to it. And we do that with some hemoglobin, which is an iron compound or an iron pigment, picks up the oxygen and moves it around for us. Now, on the flip side of that, we've been speaking about oxygen, but you also need to move around carbon dioxide. And the majority of carbon dioxide that comes out of your cells and then moves into the bloodstream before you exhale it is in the form of bicarbonate ions, which is a pretty harmless substance.
However, on the other hand, if carbon dioxide joins with water, it can create carbonic acid. Now, carbonic acid in very large amounts is not a good thing. However, in small amounts, it's really important to be in the blood to create a state of acidosis. Because acidosis allows for the bonds between the oxygen and the hemoglobin to break so that you can actually get the oxygen out of the red blood cell and get it into the tissue where it wants to go. Now, if you've liked this video, please give it a thumbs up and make sure your notifications are turned on.
And I will see you all again soon. Bye.