High school biology
The pulmonary system including the lungs, larynx, trachea, bronchi, bronchioles, alveoli and thoracic diaphragm. Created by Sal Khan.
I've done a bunch of videos already on respiration. I think even before those videos, you had a sense that we need oxygen and that we release CO2. And if you watched the videos on respiration, you know that we need the oxygen in order to metabolize our food, in order to turn our food into ATPs that can then drive other types of cellular functions-- or anything that we have to do; move, or breathe, or think, or everything that we have to do. And that through the process of respiration, we break down those sugars and we release carbon dioxide. So in this video, what I want to do is take a big step back and think about how we actually get our oxygen into our body and how we release it back out into the atmosphere. Another way to think about it is how we ventilate ourselves. How do we get the oxygen in, and how do we get the carbon dioxide out? And I think any of us could at least start off this video. It starts off in either our nose or our mouth. I always have a clogged sinus so I often have to deal with my mouth. I sleep with my mouth often. But it always starts in our nose or our mouths. Let me draw someone with a nose and a mouth. So let's say that this is my person. Maybe his mouth will be open so that he can breathe. His eyes aren't important, but just so you know it's a person. So this is my test subject or the person I'm going to use to diagram. That's his ear. Maybe he has a bit of a-- let's give him some hair. All of that is irrelevant, but this is our guy. This is the guy that's going to show us how we take air in and how we take air out of the body. So let's go inside of this guy. I can draw his outside first. Let me see how well I can do this. So this is outside the guy. That doesn't look right. Let's say the guy looks something like this and he's got-- this is his shoulders. That's our guy. All right. So in our mouth, we have our oral cavity right there, which is just the space that our mouth creates. We have our oral cavity. I could draw our tongue and all of that and maybe I will. Maybe I'll draw the tongue. But you have this space inside of the mouth-- call that the oral cavity. Oral for mouth, cavity for space, or hole, or opening. And then also you have your nostrils and they open up into a nasal cavity. So that's another big space just like this. And we know that they connect at the back of our nose or the back of our mouth. And this passage right here where they connect is called the pharynx. When your air goes through your nose-- they say breathing through your nose is better, probably because it gets filtered by your nose hairs and it gets warmed up and and what not, but you can breathe through either side. The air goes in through either your nasal cavity or your oral cavity and then comes back through your pharynx and then the pharynx splits into two pipes. One for-- well, one, air can go down either one, but the other one is for food. So your pharynx gets split. In the back you have your esophagus-- and we'll talk more about the esophagus in a future video. In the back you have your esophagus and in the front-- let me draw a little dividing line there. In the front, maybe this-- let me make it connect like that. I was using yellow. I'm going to use yellow to continue and I'm going to use green for the air. So it divides just like that. So behind your air pipe, you have your esophagus. Let me make that another color. And then right here is your larynx. And I'm going to concern ourselves with the larynx. Esophagus is where your food goes down. We know that we eat food with our mouth as well. So this is where we want our food to go-- down the esophagus. But the focus of this video is our ventilation. What do we do with our air? So I'm going to focus as the air goes through our larynx. And the larynx is also our voicebox. So as you hear me talking right now, there are these little things right about here that are vibrating at just the right frequencies and I'm able to shape the sound with my mouth to make this video. So that's also your voicebox, but I won't focus on that right now. It's called a voicebox because of this whole anatomical structure that looks something like that. But then after the air passes through the larynx-- this is on the way in-- it goes to the trachea, which is essentially just the pipe for air. The esophagus is the pipe for food. Let me write this down. And then from the trachea-- and the trachea is actually a reasonably rigid structure. It has cartilage around it and it makes sense that it has cartilage. You don't want-- you can imagine a hose-- if it bent a lot, you wouldn't be able to get a lot of water through it, or a lot of air through it. So you don't want this thing to bend a lot. So that's why it needs to have some rigidity-- so that's why it has cartilage around it. And then it splits into two tubes-- and I think you know where these two tubes are going to. And I'm not drawing this in super detail. I just want you to get the idea of them, but these two tubes are the bronchi-- or each one is a bronchus. And they also have cartilage, so they're fairly rigid, but the bronchi keep splitting. They keep splitting into smaller and smaller tubes just like that and at some point, they stop having cartilage. They stop being reasonably rigid, but they keep splitting off. So I'll just draw them as these little lines. At some point they become such thin things. They just keep splitting off. So the air just keeps splitting off and spread and goes down the different paths. And when the bronchi no longer have cartilage around them, they're no longer rigid. The first of those are called-- or actually all the tubes after that point are called bronchioles. These are bronchioles. So for example, that we could call a bronchiole. And there's nothing fancy here. It's just a pipe that just gets thinner and thinner and thinner. We've labeled the different parts of the pipes different things, but the idea is, let's take it through our mouth or our nose and we just keep dividing and keep dividing this main division into two different paths that takes us into each of our lungs. Let me draw this guy's lungs here. And these bronchi-- or the bronchi split into the lungs-- the bronchioles are in the lungs and eventually the bronchioles terminate. And this is where it gets interesting. They keep dividing smaller and smaller, thinner and thinner and thinner, into these little air sacs, just like that. At the end of every super small bronchiole are these little air sacs-- super small air sacs-- and I'm going to talk about these air sacs in a second. And these are called alveoli. So I've used a lot of fancy words, but the general idea is simple. Air comes in through a pipe. The pipe gets thinner and thinner and thinner and they end up at these little air sacs. And you're saying, well, how does that get the oxygen into my system? Well, the key here is that these air sacs are super small and have very, very, very thin walls-- or I guess thin membranes. So let me zoom in. So if I were to zoom in on one of these alveoli-- and just to give you an idea, these are super duper duper small. I've drawn them fairly large here, but each alveoli-- let me draw a little bit bigger. Let me draw these air sacs. So you have these air sacs like this. And then you have a bronchiole that's terminating in that air sac. Maybe a bronchiole is terminating in another air sac just like that-- another set of air sacs just like that. Each of these are only 200 to 300 microns in diameter. So that distance right there-- let me switch colors-- that distance right there is 200 to 300 microns. And in case you don't know what a micron is, a micron is a millionth of a meter-- or you can view it as a thousandth of a millimeter. So this is 200 thousandths of a millimeter. Or you can think of them as-- and this is actually a very easy way to visualize it-- this is about one fifth of a millimeter. So if I actually try to draw it on the screen-- if you made this full screen, a millimeter is about that far. Maybe a little farther than that. Maybe about that far. So imagine a fifth of that and that's what we're talking about the diameter of one of these things. And just to put it in the whole scheme relative to cells, the average cell in the human body is about 10 microns. So this is only about 20 or 30 cells in diameter or relative to the average cell in the human body. So these have a super thin membrane. If you view them as balloons, the balloon is very thin-- pretty much one cell thick and they're connected to the bloodflow-- or actually, a better way to think about is that our circulatory system passes right next to each of these things. So you have blood vessels that come from the heart and they want to be oxygenated. In general, the blood vessels that don't have oxygen-- and I'm going to do this in a lot more detail when I make the videos about the heart and our circulation system-- the blood vessels that don't have oxygen-- de-oxygenated blood is a little bit darker. It looks a little bit purplish. So I'll draw it as blue. So these are vessels that are coming from the heart. So this blood right here has no oxygen in it or it's been de-oxygenated or it has very little oxygen in it. And the word for the blood vessels that comes from the heart are arteries. Let me write that down. I'll review that again when we cover it in the heart. So arteries are blood vessels from the heart. And you've probably heard of arteries. Vessels that go to the heart are called veins. This is really important to keep in mind because later on, you're going to see that arteries don't always carry oxygen or they're always not de-oxygenated and veins always aren't one way or the other. We're going to go into a lot more detail when we actually cover the heart and the circulatory system, but just remember, arteries go away. Veins go to the heart. So here, these are arteries going away from the heart to the lungs, to the alveoli because they want the blood that's traveling in them to get oxygen. So what's going to happen is that the air is flowing through the bronchioles and circulating around the alveoli, filling the alveoli-- and as they fill the alveoli, the little molecules of oxygen are allowed to cross the membrane of the alveoli and essentially be absorbed into the blood. I'll do a lot more on that when we talk about hemoglobin and red blood cells, but you just have to realize that there's just a lot of capillaries. Capillaries are just super small blood vessels that allow air to pass-- essentially oxygen and carbon dioxide molecules to pass between them. These have a lot of capillaries on them that allow the exchange of gases. So the oxygen can go into this blood and so once the oxygen-- so this is the vessel that's coming from the heart and then it's just a tube. So then once it gets the oxygen, it's going to go back to the heart. And so essentially this is the point where this vessel, this pipe, part of our circulatory system, goes from being an artery-- because it's coming from the heart-- to a vein because it's going back to the heart. And there's a special word for these arteries and veins. They're called pulmonary arteries and veins. So going away from the heart to the lungs, to the alveoli, these are the pulmonary arteries. And going back to the heart are the pulmonary veins. Now you're saying, Sal, what does pulmonary mean? Well, pulmo comes from the Latin for the lungs. It literally means the arteries that are of the lungs or that go to the lungs and the veins that come from the lungs. So anytime people talk about pulmonary anything, they're talking about our lungs-- or maybe something related to how we breathe. So it's a good word to know. So anyway, you have your oxygen coming in through your mouth or your nose, through the pharynx-- it could fill your stomach. We can blow up our stomach like a balloon, but that doesn't help us actually get oxygen to our blood stream. But the oxygen will come through our larynx, into our trachea, through the bronchi, eventually in bronchioles, ending up in alveoli and being able to be absorbed into what where the arteries, but then we're going to go back and then essentially oxygenate the blood. The red blood cells become red once the hemoglobin becomes very red or scarlet once it actually has the oxygen and then we go back. But at the same time, this isn't just about getting oxygen into our arteries or onto the hemoglobin. It's also about releasing carbon dioxide. So these blue arteries coming from the lungs are also going to release carbon dioxide into the alveoli. And these will be exhaled. So we have oxygen coming in. Other things will be coming in, but the O2 is what gets absorbed in the alveoli. And then when we breathe out, we're going to have carbon dioxide that was in our blood, but then it gets absorbed into the alveoli and they get squeezed out. I'm going to tell you in a second how it gets squeezed. It's actually that squeezing out that actually-- when the air goes back out, it can vibrate my vocal cords and it'll allow me to talk, but I'm not going to go into too much detail about that. So the last thing to consider about this whole pulmonary system or about our lungs is, how does it force the air in and how does it force the air out? And the way it's done is really kind of like a-- you could imagine it's kind of a pump or a balloon-- is that we have this huge layer of flat muscles. Let me pick a good color here. Right below the lungs-- and this is called a thoracic diaphragm. And so when it's relaxed, it kind of has this arch shape, and so the lungs are kind of squeezed in. They don't have a lot of volume. But when I essentially breathe in, what's happening is this thoracic diaphragm is contracted and when it's contracted, it gets shorter, but more importantly, it opens up the space where my lungs are. So my lungs can fill up that space. So what it does is, it essentially-- it's like pulling a balloon larger, making the volume of my lungs larger. And when you make the volume of something larger-- so the lungs will become larger as my thoracic diaphragm is contracted and it kind of arches down, creates more space-- and as the volume of something becomes larger, the pressure inside of them goes down. If you remember from physics, the pressure times volume is a constant. So when we breathe in, our brain is essentially telling our diaphragm to contract. We have more space around our lungs. Our lungs expand to fill the space. We have less pressure here than we have outside-- or you can view it as negative pressure. So air always wants to go from high pressure to low pressure and so air is going to flow into our lungs. And hopefully there'll be some oxygen there that can then essentially go into our alveoli and end up in our arteries and then go back in the veins as oxygen attached to hemoglobin. I'll talk more about that in detail. And then when we stop contracting the diaphragm, it goes back to this arched position. It contracts. It's kind of like a rubber band. It contracts back the lungs and it essentially expels the air back and now that air's going to have a lot more carbon dioxide. And just to get a sense of-- I can look at my lungs-- I can't look at them, but they don't seem too large. How do I get enough oxygen in them-- and the key is, is that because of this branching process and the alveoli, the inside surface area of the lungs are actually much larger than you can imagine-- or actually than I could have imagined had someone not told me. So it actually turns out-- I looked it up-- the internal surface area of your alveoli-- so the total surface area where the oxygen can be absorbed in or the carbon dioxide can be absorbed out from the blood-- it's actually 75 square meters. That's meters, not feet. If you think about it, that's like a-- imagine some type of a tarp or a field. That's almost nine by nine meters. That's almost 27 by 27 square feet. That's the size of some people's backyards. That's how much surface air you have inside of your lungs. It's all folded up. That's how it gets jammed into what look like relatively small lungs. But that's what gives us enough surface area for enough of the air, enough of the oxygen, to cross the alveoli membrane into our blood system and enough of the carbon dioxide to go back in. And just to have a sense of how many alveoli we had-- I told you that they're very small-- we actually have 300 million in each lung. In each lung, we have 300 million alveoli. So anyway, hopefully that gives you a decent sense of how we at least get oxygen into our blood system and carbon dioxide out of it. In the next video, I'll talk more about our actual circulatory system and how we get the oxygen from the lungs to the rest of the body and how do we get the carbon dioxide from the rest of the body into the lungs?