Placeholder Image

字幕列表 影片播放

  • Voiceover: There's a test that can tell us

  • how well diffusion is going in the lungs.

  • It's got one of those acronym names

  • that are really hard to remember.

  • It starts with a D for diffusion

  • and then L because it's in the lungs

  • and then CO, standing for carbon monoxide,

  • let me write that out,

  • carbon monoxide.

  • That's the gas we use to do this test.

  • And you're right,

  • it's the same scary gas that we're afraid of

  • of being in our homes because it can poison us.

  • We'll get to in just a second why we use this gas.

  • I guess to get really technical,

  • diffusion will be talking about moving

  • from a high concentration to a low concentration,

  • but for our purposes,

  • let's think of it as a gas moving across a barrier

  • from place A to place B.

  • In this case in the lungs,

  • we have an alveolus,

  • which is the end of the airway in the lungs.

  • This is where gas exchange will take place.

  • That is covered by this layer of blood vessels.

  • We care about how diffusion goes here

  • because usually, its job, this whole area,

  • is to have one gas diffuse from the airspace into the blood

  • and another one from the blood to the airspace.

  • Of course, this one is oxygen,

  • and this one going from blood into the lungs

  • is carbon dioxide.

  • This test is able to answer that question of

  • how well can the lungs move a gas into the bloodstream?

  • To do this test, we have our patient here.

  • Let's call him Mr. D for diffusion.

  • Mr. D here, if we look at his airway,

  • it's connected to his mouth,

  • and it's also connected to his nose up here,

  • so here through his nostrils.

  • Theoretically, that could go also down the airway.

  • To isolate all the numbers and data we're getting,

  • his nose is going to be plugged,

  • so he can only breathe through his mouth.

  • We put a mouthpiece into his mouth,

  • and I can't draw what the machine really looks like,

  • but just to get the idea here,

  • we have one reservoir there.

  • He breathes in through here,

  • and then when he blows it out,

  • the air goes to another machine.

  • Let's draw it like this.

  • In the first one where he's breathing from,

  • it's full of carbon monoxide,

  • the gas that we mentioned.

  • It's part of the name of this test.

  • He takes a big breath as much as he can breathe in,

  • so carbon monoxide goes down,

  • down his pipes into his lungs,

  • and it fills his lungs.

  • Now a certain amount gets absorbed here

  • into the bloodstream,

  • and then when he can't breathe in any more,

  • he holds it for a second, for just a split second,

  • and then he blows it all out as much as he can go,

  • keep going, keep going until he has no air left.

  • Now the computer is able to calculate two things for us.

  • One is how much carbon monoxide he breathed in

  • and then how much he breathed back out.

  • We care about these two numbers because essentially,

  • how much you took in minus how much came back out

  • equal however much went into his bloodstream.

  • That's the amount that was diffused across.

  • If we come back and look at this drawing here,

  • the carbon monoxide goes in here.

  • It fills this airspace.

  • A certain amount, if it crosses in the bloodstream

  • and then all that's still left in the airspace,

  • when he breathed out, it comes out here,

  • so there is nowhere else for the gas to go.

  • It either went in the bloodstream or it came back out.

  • Therefore, our equation here gives us an estimate

  • of how much gas diffused across.

  • The reason we use carbon monoxide

  • instead of the two gases that usually are in the lungs,

  • the oxygen, the carbon dioxide,

  • is because of hemoglobin.

  • Hemoglobin is something in our blood.

  • It's part of the red blood cell,

  • and its job is usually to carry these gases in our blood.

  • It can carry actually multiple gases.

  • First, it can carry carbon dioxide,

  • which, for our purposes, is a waste product.

  • The body makes carbon dioxide,

  • hemoglobin takes it up,

  • and then when it gets to the lungs,

  • it exchanges the carbon dioxide for oxygen.

  • Back here, when we talked about these two gases

  • being one going in, one going out,

  • the vehicle is hemoglobin carrying it.

  • Now a third gas it carries is, of course, carbon monoxide.

  • This is not usually part of what we breathe in.

  • We just so happen to know that hemoglobin

  • not only carries carbon monoxide

  • but actually has a huge preference for it.

  • It plays favorites.

  • This is like its favorite kid.

  • Here's why we use it in this test,

  • is because since it likes carbon monoxide so much,

  • we're able to maximize diffusion,

  • maximize diffusion.

  • Because when the hemoglobin in the blood

  • sees the carbon monoxide,

  • it grabs all of it up

  • and gives us that maximum value of how well

  • the diffusion is happening.

  • But the reason we're so afraid

  • of having carbon monoxide at home

  • is if you can imagine your air at home,

  • there is a leak and there's carbon monoxide in the air,

  • many, many molecules of that,

  • and there's also regular oxygen that we usually want.

  • If you're hemoglobin and you're picking out

  • of these gases to pick up,

  • you're going to choose all the carbon monoxide

  • because you just like it better.

  • Instead of carrying oxygen, it's going to carry

  • carbon monoxide like this.

  • Now this is a problem because hemoglobin's job

  • is to take the oxygen all over.

  • It can take the carbon monoxide to the same places,

  • but our body can't use carbon monoxide.

  • It's useless to us.

  • Now it's OK for Mr. D to breathe in one breath of this,

  • but if you do this for a couple of minutes

  • and you're breathing carbon monoxide

  • instead of oxygen,

  • then this person is quickly,

  • their oxygen level is going to drop,

  • so they're basically suffocating

  • even though they're still moving air in and out.

  • That's why carbon monoxide poisoning is so serious.

  • Coming back to talk about diffusion,

  • what on earth exactly are we testing for in the lungs?

  • So what, it can diffuse well,

  • but what does it mean if it does or does not diffuse well?

  • Let's look an equation of how gas behaves

  • and what are the things that affect diffusion.

  • The volume of gas that can move across a barrier

  • is equal to the area, surface area that's going across

  • divided by thickness of the membrane,

  • or I'm sorry, the barrier.

  • Once applied to a constant,

  • this constant, as experimentally found,

  • is related to the gas,

  • so we're not going to worry too much about that.

  • But it's also related to the partial pressure

  • of the first place minus the partial pressure

  • of the second place, with respect to whatever gas.

  • OK, let's tackle this one thing at a time.

  • This first glob here, the area over thickness,

  • that's really talking about the nature of this septum,

  • the nature of the barrier that we have to move across.

  • In our case, assuming the blood vessels are OK,

  • we're really looking at the membrane of the alveolus here.

  • The tissue of the lungs, what is the condition of that?

  • For the gas to go through here, what is the surface area?

  • What is the thickness?

  • Now remember, for fractions,

  • whatever is in the top of the fraction,

  • if area goes up, then volume goes up,

  • but if thickness goes up, volume goes down.

  • Let's say something that can affect the surface area

  • would be a disease such as emphysema,

  • where the lung tissue is being destroyed

  • and you literally get less surface area for diffusion.

  • In emphysema, the area goes down.

  • And area goes down, the volume is going to go down too

  • because this is at the top of the equation.

  • Another example, something that might affect that thickness

  • would be, let's say, fibrosis,

  • where the lung tissue gets scarred and thickened,

  • there's too much connective tissue.

  • The thickness would go up.

  • And because that's at the bottom of the equation,

  • that actually drives the volume down too.

  • You see how both these diseases would drive down

  • the amount of the gas that goes across,

  • and that's how they both impair the lung function.

  • Now for partial pressures,

  • I find the concept a little confusing.

  • Let's try to look at it this way.

  • Say there's a barrier,

  • that you're a certain gas and you want to go

  • from area one to area two.

  • Now how willing this gas is to make its way over here

  • depends on how much of it is on either side of this barrier.

  • Say, in the first scenario,

  • if there is a ton of particles in the first area

  • and only one or two or here,

  • let's say four particles there,

  • that's going to have a huge drive to push it over this way

  • because the partial pressure of P1 is really high,

  • and the P2 is really low.

  • This absolute difference between the two is a driving force.

  • In the other case, if we have about this much P1 here

  • and then P2 is about the same,

  • then this drive is going to be very small.

  • There is not that much force pushing it over.

  • That's the same concept here, P1 minus P2.

  • In asking what's the difference in the gas

  • in the airspace versus in the blood,

  • really, the question is how much gas

  • did we get into the area?

  • How much gas was Mr. D able to breathe

  • into his alveolus?

  • P2 here should be about zero.

  • There should be no carbon monoxide in your blood,

  • so if you breathe a lot of it in,

  • then the P1 minus P2 will be large.

  • The more of a difference between P1 and P2,

  • the higher the volume for diffusion.

  • The diseases that affect this part of the equation

  • are the diseases that make it hard for air

  • to get into the lungs,

  • let's say chronic bronchitis.

  • There's all this mucus and blockage,

  • so the air can't get into the alveolus.

  • The lack of that air pressure in the airways,

  • that's going to have a lower P1 minus P2.

  • Actually, also with fibrosis and also with emphysema,

  • they're also bad for the partial pressure difference

  • because it's hard to get air in.

  • As you can see, for this test for diffusion,

  • many different diseases in many different mechanisms

  • can affect the diffusion.

  • This is not really a diagnostic test as much as it tells us

  • how severe somebody's disease is.

Voiceover: There's a test that can tell us

字幕與單字

單字即點即查 點擊單字可以查詢單字解釋

B1 中級 美國腔

肺部對一氧化碳的擴散能力(DLCO)|NCLEX-RN|可汗學院 (Diffusing capacity of the lung for carbon monoxide (DLCO) | NCLEX-RN | Khan Academy)

  • 18 1
    yao 發佈於 2021 年 01 月 14 日
影片單字