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找到空气在肺部的进出口.里希(Dr. Rishi Desai)是一名儿科传染病医师,是可汗学院团队一员. Rishi Desai 创建




if you could have a magical ability to actually see all the air molecules in the air you might see something like this it would be a lot more crowded but you could imagine it might look like this and let's say that you actually decide to do something a little interesting and that is to take a jar and simply capture some of the air molecules in your jar so I've got my jar here and I'm actually gonna put a little opening on my jar so let's say there's a little opening there and I take that opening and I I'm gonna just make it kind of a stretched out neck so this is my stretched out neck on my jar and there's the opening to my jar and on the other side what I want to do is actually kind of compare what's going on inside of my jar to what's going on outside of the jar so to make it fair let me actually try to create a purple box kind of a dashed line around an equivalent volume so this is gonna be basically a similarly sized part of the air and of course this dashed line is just to show you which part I'm talking about because of course this is an imaginary line but let's say we're comparing what's going on inside of my blue jar and with what's going on inside of this purple dashed line now we know that that purple dashed line is kind of capturing a certain amount of the air in the atmosphere and that air is gonna have molecules that are bouncing off of each other let's say something like this and you've got a bunch of kind of random collisions happening and these collisions the more frequently the collisions were happening the higher the pressure in the air and in fact measure measured pressure in the airs around 760 millimeters of mercury so that's how we think about air pressure so that's the pressure in the atmosphere and if I was to measure my Jar pressure it would be of course the same thing it would be 760 millimeters of mercury and as a quick aside just thinking about what these molecules are if there are five of them then you might say that this is nitrogen this one is nitrogen this one is nitrogen this could be nitrogen and this one might be oxygen because remember oxygen is about 21% of air and so that might be a fair estimation of what these five molecules could be mostly nitrogen so in the air we've got a nitrogen and oxygen it's bouncing around in my jar just as it is in the atmosphere itself and now let's say I decided to do kind of an interesting experiment I decide to drop the floor just stay with me here I dropped the floor of my jar so I actually expand the bottom of my jar and for the moment don't worry so much about how that could possibly happen let's just assume that I do creatively somehow kind of drop the floor and now it looks a little bit lower so the volume has gone up in my jar and actually simultaneously I should mention I just want to mention here that this door or opening of my jar is closed at the moment so my opening I've put a lid on it so that's closed and my floor just got a little bit lower so the volume has gone up that's the big change I actually let me write that up here in the corner I'm just gonna erase some of these molecules to create some space and the first thing I want to mention is that the volume has gone up in my jar so all the green stuff I write in the corner is going to be from the jars perspective if the volume goes up now if that's the case then these molecules inside the jar they're excited they've got more room to kind of run around and play and not bump into each other so if they're not bumping into each other as much because of course they've got all this extra space down here then the pressure on the inside of the jar is going to go down right because there are less collisions happening so now we've got let's say a slight decrease it went to 757 so a little bit less than what's on the outside so because the volume went up the pressure went down and again that's because you have fewer collisions and the new pressure is 757 which is a positive number but sometimes people refer to this as negative pressure or a vacuum and the reason they're saying that is the as they're saying well relative to 760 relative to this number 757 is three points lower and so in that sense it's negative so if you actually want to look at them compare them to each other you'd say well 757 - 760 is negative three and that would be a negative number but for the time being I'm just gonna leave it in in the numbers we have which is 757 now let's say that I open this door this opening is now open if I open up this this opening what will happen well you we have all this extra space down here I circled but I'm just gonna kind of remove this for the time being all this extra space and molecules of course are being knocked around all the time so there's these collisions are happening always and some molecules are gonna get knocked perfectly so that they actually kind of move into the jar let's say it goes in like this so you're gonna get some molecules it going and in fact you might have some molecules I kind of get knocked right out so it's gonna happen constantly but overall what's gonna be the net difference well let's say I leave this and I kind of walk away and do my own thing for a minute and come back I'm gonna notice that there are actually extra molecules on the inside of my jar because there's more space less crowding in my jar because of all that extra volume I created so over time there's going to be a few extra molecules in my jar and maybe maybe I got lucky and this one's an oxygen molecule so I've got extra molecules on the inside and these molecules so actually that would be I guess the next step is that air molecules move in air molecules move in and these molecules are now going to do what molecules do which is kind of bounce off of each other so they start bouncing off each other and all of a sudden now you've got let's say this guy collides over here as well and maybe there's some bouncing and this collides over here so now you've got because you've got six molecules on the inside and you know kind of the same volume the pressure on the inside has gone up so pressure has gone up on the inside of the jar simply because there are more molecules in there now so even though you had more volume initially you kinda filled it up with more molecules so the pressure goes up let's say to 760 millimeters of mercury so now it's gone back up so this is my new pressure and this all happened this whole kind of series of events happened because I decided to move the floor now what would happen if I decide to move it back let's say I decide to kind of go back to the original floor size and I kind of get rid of this lower line and I raise the floor back up and so now it looks something like this well now the volume this is kind of the new first step what's gonna happen the volume has gone down that's obvious because I just moved the floor purposefully and I've got six molecules in my jar and they're smashing around bumping into each other but they've got less space to do it in so the pressure is gonna go up because there are more collisions they're bumping into each other more so the pressure is gonna go up pressure is gonna go up now so let's say 763 millimeters of mercury because it was 760 and at this point let's say this is closed up and so the pressure on the inside is 763 millimeters of mercury let me erase this and that's because again you have more molecules but you reduce the volume so then the pressure on the inside is actually now higher than the outside pressure when the outside pressure is always gonna be around 760 and that's because the atmosphere is just enormous right so the movement of a few molecules this where that way is really not gonna change the amount of collisions that are happening in the atmosphere that's always going to stay the same and so if I was to open this up open this door up then some molecules of course are gonna be bouncing around bouncing around and some of these things might kind of bounce out so some molecules might kind of bounce out and over all again on the whole you're gonna have more molecules bouncing out than bouncing in because you have more collisions happening on the inside and again when I say more collisions in your mind I want you to think of higher pressure so if there's higher pressure on the inside and more collisions happening on the inside you're going to have more things bouncing off each other and molecules are going to be sent outside so the next step I could write in would be air molecules air molecules move out air molecules move out so the final point is that if air molecules are moving out let's say just by random chance this oxygen molecule happen to be the one that got sent away so this one kind of got knocked out then you have I'm trying to erase all this to clear it up then you have five molecules again on the inside and you have the same kind of volume that we initially start out with so the pressure on the inside goes back to what it was in the first place the pressure Falls pressure Falls to 760 and the reason that I say exactly seven or sixty is because this process in step three this step will continue until the number of collisions on the inside and outside of the jar are equal so this is kind of the process and actually I forgot to mention when we were back at 763 sometimes people call this positive pressure positive pressure for the same reason they called it negative before because all they're doing is they're comparing some 163 to atmospheric pressure which is 760 and saying wow that's a plus 3 that's a little bit positive and so when you compare things relatively you use words like positive and negative but if you're using just the total number in kind of absolute terms then you would stick to 757 or 763 now what is all of this have to do with us you know what is a jar and an opening have to do with with human beings well let me just show you that by simply changing my drawing a little bit you'll see what this has to do with us now instead of having all the molecules inside the jar I know that you know they're there I can actually erase all this and maybe I can change the shape of this a little bit to help you see what this could be so let's say I make that like this and start drawing in like that I'm gonna keep all this kind of the same in terms of the way it looks maybe like that and you can see now instead of a jar but I'm creating for you or a pair of lungs so this is a pair of lungs left and right this will be right and this will be left and it'll look something like this and this one might go up like that around that cardiac notch and go like this and then we have of course the opening which if instead of calling it an opening I call it a mouth this would be my mouth and I could erase the word opening completely and I think you'll start seeing how this is basically what happens in our body so our head represents the opening of this and this could be the nose and this could be the head kind of a flat head I have drawn here but you get the idea I think so there's your nose and there's your ear and basically air is coming in the mouth and going into the lungs and back out of the lungs and what we call this process is inhalation so when you increase the volume we call this inhaling so if you've ever wondered it is exactly what happens when you inhale air there it is and when you close up the lungs and air molecules move out we call that exhaling now Josh might try to make it look the same they're both equally important so I'll draw them the same way exhaling and now you can see how inhaling and exhaling happens so with every breath this is the process you kind of subtly change the volume and all of a sudden the pressure changes air moves in and out