If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

How does lung volume change?

Learn about how muscle contraction and lung recoil actually help the lungs change their volume with every breath! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

Want to join the conversation?

  • blobby green style avatar for user Ras.Strand
    Ok, so basically we're not consciously "sucking"/"blowing" in/out air? We're just increasing/decreasing lung volume by "stretching" our lungs out by muscle contraction which in turn causes lower/higher pressure compared to the outside atmosphere which automatically causes air to flow in/flow out?
    (55 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Minish Patel
    How do scientist find out how many alveoli are in our lungs?
    (12 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user rgadde06
      There are many ways. One way is to measure how much air a single alveoli can take in and then see how much a large breath takes in. You could then divide the large breath volume by a single alveoli's volume.
      Or you could fill a corpse's lungs with plastic and the plastic would create nodes. You could then scan this with an MRI.
      (18 votes)
  • piceratops tree style avatar for user Andres Politi
    When exhaling, is only the elastic force of the elastin proteins what drives the air out, or there are also muscles involved? When we are forcing air out, are we just relaxing the diaphragm and intercostal muscles more rapidly, or are we actively using muscles to squeeze the lungs?
    (12 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user kmd227
    In the video it talks about elastin in the elastic recoil process, what role does surface tension and surfactant play in the process?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Yushy Zhou
      Inside the alveoli, there is water moisture. One of the properties of water is that they adhere to each other very strongly in a phenomenon known as surface tension. This means that with every expiration of breath, the alveoli will collapse (due to their elastic recoil) and the alveoli will "stick together" due to this surface tension. This makes taking another breath very difficult because the pressure difference must be great enough to overcome this surface tension. Surfactants, which are secreted by type II alveolar cells, are a complex mixture of phospholipids and other compounds. They disrupt the adherence of water molecules to each other and thus disrupt surface tension. This means that after each expiration of breath, the alveoli will not stick together and consequently the next breath does not need to be as forced.
      (21 votes)
  • piceratops ultimate style avatar for user courtneyliston
    When the diaphragm flattens out when you inhale, is that contraction or relaxation? and for exhaling?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user JBF
      Inhalation is considered active and is when the volume of the thoracic cavity increases. This causes pressure in the lungs to decrease, and air flows in because of the differences in pressure. The diaphragm is CONTRACTED at this point (aka it is flattened). When the diaphragm relaxes, the thoracic cavity goes back to its original size, and air is forced out of the lungs. Exhalation is considered to be passive.
      Here is a little flow chart:
      Inhale-->Diaphragm and intercostal muscles expand the thoracic cavity -->Cavity gets bigger-->Diaphragm flattens -->Chest wall moves out -->Intrapleural space volume increases -->Intrapleural pressure decreases -->Air moves into the lungs
      Exhale-->Diaphragm and external intercostals RELAX-->Chest cavity moves in-->Intrapleural space volume decreases-->Pressure of intrapleural space goes up because volume goes down (think Boyle's Law here)-->Air is pushed out of the lungs
      (13 votes)
  • leafers seedling style avatar for user R-Mg-X aka Mr.Grignard
    If breathing is involuntary then how am I able to stop while I halfway breath air ?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user Tybalt
      You are able to control your breathing, despite it being involuntary, because of the motor cortex. The motor cortex is capable of overriding the signals sent by the brainstem (the brainstem controls involuntary breathing), which can cause one to hold their breath underwater or blow out a candle instead of the typical respirations that happen without conscious thought.

      Does this help?
      (6 votes)
  • blobby green style avatar for user Minish Patel
    What does ATP stand for?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • leaf yellow style avatar for user ReviewerOfficial
    When were muscles discovered?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user briancsherman
      I imagine the first person who ate meat discovered muscles. We've learned a lot more about them since then. Luigi Galvani discovered that electrical charges could make muscles move in 1780. Guillame Duchenne discovered in the 1840's that groups of muscles worked together to create movement. Wilhelm Kühne in 1864 discovered myosin, giving us more information about how muscles work. Brunó Ferenc Straub discovered actin in 1942. H. Lee Sweeney discovered in 1998 that the IGF gene increases muscle density. Muscle research is still going on today and more things are being learned.
      (4 votes)
  • starky tree style avatar for user Elijah
    Will i have to know this for going into nursing
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Henry Spooner
    What is the point in yawning? it seems to serve no purpose to the respatory system what do ever
    (2 votes)
    Default Khan Academy avatar avatar for user

Video transcript

So we talked about inhaling and exhaling. And I'd mentioned that the key first step for both of them is this change in volume, going up in volume or going down in volume. But I didn't really talk about how that happens exactly, so I thought I would jump into that now. And let me begin by telling you that in the middle of your chest, you have this enormous kind of bone that goes down. And I'm drawing it out of proportion just to make it very clear where this bone is. But you can go ahead and feel on your own body this bone, which we call either the breastbone, or the more technical name is the sternum. So I'll write that down here. The sternum is this middle bone, and all the ribs on both sides attach there. So you've got a total of 12 ribs and seven pairs of them. Actually, I should say 12 pairs of ribs. I don't want you to think there are 12 total. We actually have 24 total and seven pairs of the ribs. So 14 ribs actually attach directly to this sternum bone. So in white, these are the ribs. And between the ribs, you actually have muscle. So I'm going to draw in some of these muscles between the ribs. And these muscles are all going to have their own nerve that allows them to contract. So these muscles are controlled by your brain, and their name-- let me just jot down here on the side-- is intercostal muscle. And inter just means between, so this is the name of the muscle. And costal refers to the ribs. So when you see that word costal, you'll know we're talking about the ribs. So what's between the ribs is these muscles, intercostal muscles. And they are going to start moving outwards when your brain says, hey, I want to take a deep breath. So these muscles are going to contract. The ribs, I should say, are going to move outwards. So these go out. And you also have-- let me just make a little bit of space on this canvas-- another muscle that kind of rides down here and has kind of an upside down U shape to it. So I'm drawing it kind of like a dome. You can think of it as a dome. And this dome is the floor-- if you remember, we talked about the floor of the thorax. So this is, of course, our diaphragm muscle. So we've got our diaphragm muscle. And this one when it contracts, instead of going out, it's going to go down. So it's going to kind of flatten out. And I can actually draw this. If you can now just stick with me for a moment, I'm going to erase this dome-like shape. And I'm going to draw what it looks like as it contracts. So when it contracts, it's actually going to be more flat. And this flat diaphragm, as you can see, is now further down than it used to be. And as it goes down, all of the structures that are inside this space-- so the two lungs. And of course, I didn't draw the heart here. But the heart would be kind of in this cardiac notch. If you want, maybe I could even draw that heart here. They're all going to kind of physically move down. So this is our heart and our lungs. They're physically going to be kind of drawn downwards and out. They are going to also move out as the intercostal muscles move out. So you have expansion of these lungs. That's basically the idea. And if you were to kind of zoom in on this to kind of see exactly what this expansion looks like, when I say you have more volume in the lungs, really what I should be saying, if I wanted to be more exact, is that all the alveoli-- if these are the alveoli, let's say this is another branch. And this is another alveoli right here. All these alveoli, they are actually expanding. And you have about 500 million alveoli. If you can just kind of fathom how big a number that is. It's an enormous number of alveoli. And if I was actually drawing them, I would be here drawing forever. It would take forever to write out this many different alveoli. But basically what happens is that when the ribs go out and the diaphragm moves down, these alveoli are actually being pulled out. They're actually being pulled outwards, so they are actually going to be getting larger in size. They literally look like they've grown in size, and this is what they look like. And actually, if you were to take an even closer look, you'd see that these alveoli have around them a bunch of protein. The cells around them have a bunch of protein, and this protein is called elastin. And you can guess what elastin might do. It has kind of a similar sound to the word elastic. And elastin is basically kind of like a rubber band, so you can kind of think of elastin as a rubber band. And when the muscles move down and out and the alveoli are pulled open-- let me actually now scroll up, because you can kind of go back to the idea of inhaling-- what is happening, then? Well, you have a couple things happening. One, you have muscles-- I'll just write muscles contracting. And when I say muscles, you know I'm talking about all those intercostal muscles in your diaphragm. And as a result of the muscles contracting, you have now the alveoli are stretched open. So those rubber bands, those elastin proteins, are literally physically being stretched opened. And keep that in mind, because what's going to happen then is when the muscles relax, which is what happens when you exhale, what do you think is going to happen to that elastin? Well, if it's like a rubber band, if that's what I'm saying it's going to be like, then the alveoli are going to recoil. And that's actually the driving force for why the volume goes back down. So if you have a bunch of rubber bands that you're stretching out-- let me actually bring up the picture. You'll see it really clearly. If you're physically kind of using your muscles to help pull this stuff open, then the moment that you stop pulling open, the moment that you stop contracting those muscles now that you have a nice big volume, what's going to happen? Well, all these elastin molecules are going to snap back. Let me do it with a different color. Let's do this color. They're going to snap back like this. All that protein is going to want to snap back into the original size. And when they do, this thing gets smaller. So my alveoli goes back to its original size, which was much smaller than this. Let me actually just quickly show that and show you that even though contraction is what opened up things, it's the recoil that brings things back down to their normal size. And let me erase this to make it kind of a neater drawing. So you can see it now. Inhaling, the way that we actually increase the volume, is by pulling things open through contraction. And this actually requires energy. Remember, you can't contract a muscle without spending chemical energy. So this takes chemical energy, and we usually think of this molecule ATP as the specific type of chemical energy we're going to use. And to exhale, when you reduce the volume, it's going to be driven by this elastic recoil. So that's a type of elastic potential energy. So this process of inhaling and exhaling is really a little different from each other. On the one hand, you're using ATP. You're actually burning through these molecules. And then when you exhale, you're actually not using chemical energy anymore. You're just using that elastic potential energy, kind of the same sort of energy that you can imagine you would have if you snap a rubber band. So let's stop there, and we'll pick up in the next video.