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了解肌肉收缩和肺的弹性回缩帮助肺容量随着呼吸变化的.里希(Dr.RIshi Desai)是儿童传染病医师,他是可汗学院的成员之一. Rishi Desai 创建




so we talked about inhaling and exhaling and I had mentioned that the key first step for both of them is kind of this change in volume right going up in volume or going down in volume but I didn't really talk about how that happens exactly so I thought you kind of jump into that now and let me begin by telling you that in the middle of your chest you have this an 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 breast bone or the 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 ball in this sternum bone so in white these are the ribs and between the ribs you actually have muscles so I'm gonna 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 body or your brain and their name let me just jot down here on the side is intercostal muscle an 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're gonna start moving outwards when your brain says hey I want to take a deep breath so these muscles are going to contract and 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 you also have another muscle that kind of rides down here and has kind of a 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 it's gonna go instead of going out it's gonna go down so it's gonna kind of flatten out and I can actually draw this if you can now just stick with me for a moment I'm gonna erase this dome-like shape and I'm gonna draw it what it looks like as it contracts so when it contracts it's actually gonna 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 this is our 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're gonna also move out as the as the intercostal muscle moves 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 know 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 out be here and drawing forever right it would take forever to kind of 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 out words so they are actually gonna 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 kind of thinking of elastin as a rubber band and when the muscles move down and out and the alveoli are pulled open let me actually now kind of 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 muscles contract 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 alveoli are stretched open so those rubber bands those elastin proteins are literally physically being stretched open so the alveoli are stretched open and keep that in mind because what's gonna happen then is when the muscles relaxed which is what happens when you exhale when the muscles relax what do you think is gonna happen to that elastin well if it's like a rubber band if that's what I'm saying it's gonna be like then the alveoli are going to recoil they're gonna 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 you know stop contracting those muscles now that you have a nice big volume what's gonna happen well all these elastin molecules are gonna snap it back let me do it with a different color let's do this color they're gonna snap back like this all that protein is going to want to snap back into into kind of the original sighs and when they do this thing gets smaller so my alveoli kind of goats back to its original size which was much smaller than 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 kind of 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 right 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 elastic potential energy so this process of kind 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