DIP Episode 139 - Comprehensive USMLE Step 1 Pulm Review (Part 1)

Okay, welcome. My name is Divine, I am a resident. This is episode 139 of the Divine Intervention Podcast. And in today's episode I will be talking about, I will be beginning a series that relates to a comprehensive, like, Pomonology review for the USMLE Step 1 exam. I know some of you have been saying that, oh, Divine, but you just put out a podcast that fairly recently that related to Pomonology. That was more for Step 2 CK. Remember, the Step 2 CK exam is a lot more clinical than Step 1. So in that podcast, I talked about Pomonology, but I talked about it in a very heavy clinical context. So I would say that if you're, you know, like a first or a second-year medical student listening to this podcast, that podcast may actually not be a terrible one to take a look at. Because essentially we'll bring all these concepts that I'm going to explain to the to life. You know, they essentially just give you like, you know, like, almost like, oh, wait, all this basic science, whatever that I'm learning actually relates in some way shape or form to like things that will be as a, as a resident to us, a 30-year-old student. And the episode I'm referring to here is episode 129. That's also part of another series again that I'll be continuing in the future. But again, it'll be a clinically focused series because that is targeted more towards like Step 2 CK and Step 3.

But again, I promise you, if you're, if you're a person that's, you know, like, pre-step one or studying for step one, you will get a lot from that podcast. I think it's a very, thankfully, I feel like it's a fairly well-made podcast. So today, I'm going to begin a Pomonology series. And I will sort of take a somewhat a typical approach. I will because the thing is, Pom, Pom is one of those things that many people think they truly understand. Well, they actually unfortunately don't. And again, I'm not going to get into like a contest here. But the thing is, at the end of the day, Pom is probably one of the hardest bits of physiology for medical students to understand. So most people just, you know, memorize the big, you know, big picture things and say, okay, you know what, I'm going to, I'll be fine and set for my class exams or for Step 1 with this stuff. But I'm going to take the approach where I really want you to understand how to think about Pomonary physiology and Pomonary pathology. So I'll really try my very best to explain things the best way I can. Because I know sometimes people may say that, oh, diviner, Pom, Pom is heavily based on physics. So it's kind of hard for me to understand. The thing is, if by the end of these series of Pom podcasts, I will try to explain things so well that even if you're not a physics person, you have like a very detailed thorough understanding of the Pomonary system. So with that said, I'm going to get right into it.

And the thing is, to begin to learn poem, unfortunately, one thing you would have to, you know, kind of commit to memory, are your long volumes and long capacities. I'll encourage you if you listen to this podcast, maybe go ahead and pull up the first page that talks about these are long volumes. Kind of like the big ones to know, right? And it's kind of easy to remember there's four volumes, there's four capacities. And the thing is, capacities arise from the combination of, you know, like two or more long volumes. So let's talk about the long volumes first. And then we'll talk about the long capacities, right? So basically, if you're taking, you know, like just quiet breath, like you're inspiring and expiring without like, you know, like you wouldn't think about it. You're essentially breathing in and breathing out tidal volumes. Okay? Just like your regular inspiration, you're not like trying to force yourself to breathe in or force yourself to breathe out, you're just, you know, breathing like a, like a normal person that's not being chased by a lion, your tidal volume, you're essentially breathing in tidal volumes. And usually that volume tends to be right around like 500 C Cs or for people from other countries, like my chair, where I'm originally from 500 milliliters. Now, if for example, you say, you know what I want to breathe real hard inwards, I want to like inspire real hard, right?

Then if you're inspiring real hard, you're not going to only be breathing in those tidal volumes. You're going to breathe in something above that tidal volume baseline. Okay? You're going to be breathing in something above that tidal volume baseline. That thing you're breathing in above your tidal volume baseline is something called the inspiratory reserve volume. Okay? So that thing you're breathing in above your baseline is what's called the inspiratory reserve volume. And I guess I'll just slot in one of the capacities right now. If you actually add up that tidal volume and then the stuff you inspire above that tidal volume baseline, which is the inspiratory reserve volume, you essentially have something called the inspiratory capacity. Okay? So the inspiratory capacity is essentially the combination of the tidal volume and the inspiratory reserve volume. Now, let's take things from the other juncture, right? So let's say, you know, you're taking your quiet inspiration, inspiration, inspiration, inspiration. And you say, you know what? You know what? I've expired out my tidal volume, but let me try to expire some more. Let me see how much more air I can blow out. If you blow out more air beyond like your baseline, like air that you blow out with your tidal volumes, that's something called your expert expiratory reserve volume. Okay? That is something called your expiratory reserve volume.

And the thing is, if you actually go ahead and blow out that extra air beyond what you know, you don't really blow out in a tidal volume, there will always be some small amount of air left in your lungs. You can never truly blow out all the air in your lungs. You just can't. Right? So that small amount of air left in your lungs, you almost think of it as like, you know, your lung is like having your lungs essentially has like a like a savings account, right? Like a reserve fund. That's something called your reserve volume. Okay? So again, just to sort of summarize what I've said already, if you're breathing in and out like normal with a normal person, you know, being chased by a lion or anything like that, that's your tidal volume. If you inspire some more above that baseline tidal volume, that's something called the inspiratory reserve volume. The combination of your tidal volume and your inspiratory reserve volume is your inspiratory capacity. Now, if you decide that, okay, you know, that blown out air, you know, like normal, right? So I've blown out like a tidal volume's worth of air. You know what I want to blow out? So more, see how much I can get out of the lungs. That extra amount you can blow out or expire above your tidal volume is what's called your expatory reserve volume. But I said that your lungs is like a bank, right? You is, let's say your lung is a wise person. It doesn't want to like, you know, whistle. It's, it doesn't want to like spend all its money.

I have nothing left. Your lung always has like some reserve, like some savings. That's what's known as the reserve volume. And it so happens that if you adopt that extra amount of air that you blow out beyond your tidal volume and that little amount of air that's left behind in the lungs that you can never truly blow out of the lungs. So basically like your expatory reserve volume plus your reserve volume, you have something called the functional reserve capacity. That functional reserve capacity is something that's very important. But I'll talk about what establishes it. Something down the line in this in this podcast. Alternatively, you can also see that you know what oh, if I take all those volumes that I've talked about already the inspector reserve volume, the title volume, the expatory reserve volume. And all I'm just left with if I essentially say oh, let me take all those volumes, but not include that savings of the lung, not include like there is zero volume of the lung. Then that's something called the vital capacity. The vital capacity is essentially every lung volume added together. That is what it does not include there is zero volume. I almost think of it as like the disposable income of the lungs like okay, the lung says you know what? Let me blow everything. We blow out everything. Again, like I said, there will still be savings.

If you don't include that savings, what you're just saying or what was I able to what were all those things I was able to blow out that's your vital capacity. And then if you're like oh, let me add up everything you know, let me take that vital capacity and include that savings. There is zero volume of the lung, right? Then that's the total lung capacity. Kind of like the total amount of stuff that you have when you talk about your disposable income plus your savings. Okay. So again, I talked about the inspector reserve volume or zero volume. I mean, inspector reserve volume, title volume, basically the way it builds this construct because it's kind of like the means I used to remember this. I studied with the title volume and then I added an inspector reserve volume on top of that. That's the inspectoric capacity. Those are three quantities. And then I talked about blowing out extra. That's your inspector reserve volume. That's for those are four quantities. And then I talked about the amount of oxygen that will be left in the lungs after you've blown out everything that reserve volume. Those are five quantities. And then I said if you add that inspector reserve volume to reserve volume, that's your FRC, that's the sixth quantity. And then I said if you add up all the long volumes with the exception of the reserve volume, that's your vital capacity. That's the seventh quantity.

And then if you just add up all the long volumes, basically your vital capacity plus your reserve volume. So like your lungs disposable income plus your lungs, savings, that's your total longer capacity. That's the if quantity. So again, those things I promise you, they're very high yield. They're very, very high yield to know for the USM list step one exam. And also if you're taking like a, you know, like a preclinical like poem exam, it's something you also want to know, right? It's something that is like easy for for tests. And the thing is, I said I'll talk about the functional reserve capacity. I'll talk about kind of talk about it in layers as I go along. But basically, your functional reserve capacity is the equilibrium point of your respiratory system. Okay? Is the equilibrium point of your respiratory system? You'll see what I mean in a second. Now, the next big thing I think I want to go ahead and talk about is something called the dead space. Okay? Something called the dead space. There are many types of dead species. But I think it's probably imperative to understand what is dead space in the first place. The thing is by definition, dead space is something that does not participate in gas exchange. Okay? Dead space means this thing does not participate in gas exchange because for gas exchange to happen, I kind of think of it as a marriage, right? Marriage, you have like an exchange of valves between the man and the woman, right?

So two people need to shop for you to have that interaction, right? For you to have that exchange of valves. The same thing happens in a respiratory system. You need blood to show up, but you need air to shop as well, right? So if, for example, there's a lot of blood, but there's no, I mean, there is a lot of air, but there's no blood around to exchange that air with. Then you essentially have dead space. Okay? So dead space is space that does not participate in gas exchange. Right? So what's like the first type of dead space? The first one is something that people call the anatomic dead space. Basically, the anatomic dead space constitutes like the non-avual parts of your airway, right? So basically, like all those tubes that connects you of your life ultimately, right? The parts of your airway, both the non-avual parts, essentially, right? So these things, right, that whenever you're taking a deep breath or whatever, these things all contain air, right? But because there are no, there's necessarily not, there are not necessarily blood vessels around them, right? And then like non-avual parts, they do not participate in gas exchange, right? Essentially, everything from your terminal bronchules upwards to like your mouth constitutes your anatomic dead space. And one easy thing to remember here is that your anatomic dead space is above like in, is above the same as your within pounds.

So if you have like, if you're like 100 pound per cent, your anatomic dead space is right around like, like, is right around like 100 cc's, like 100 milliliters. And really for the most part, if you want to think about it roughly, this anatomic dead space is essentially like the conducting zone of your airway. Now, if you then say, you know what, instead of talking about these tubes that connect to my airway line, let me talk about like just my airway line itself, but talk about our view line that are not participating in gas exchange. When you're talking about like the airway parts of your airway, so notice I said that the anatomic dead space are the non-avual parts of your airway that are not participating in gas exchange. That's your anatomic dead space. If you're talking about the, our view line, right, so the avial parts of your airways that are not participating in gas exchange, that's what's known as the avial dead space, right? So basically, you have your dead space, the avial light that are in the origin air or the kind of like, like, lux surrounding blood flow to any bogus exchange, right? So if you really think about it, these are avial light. I mean, I guess let's maybe give it like a, like a practical purpose, right? If a person has like a pulmonary embolus, right? You have an occlusion of a vessel that's around the avial light, right? If you have a pulmonary embolus, those are avial light that are around that pulmonary embolus.

They will have air in them, but they will not be capable of gas exchange because blood is not coming in that direction. That's an example of our viola dead space. And if you really wanted to think about it, those are avial light that constitute part of the avial dead space. What should be true of the P big ACO too? So I guess let me maybe backtrack here for a second. Whenever you see the term big A, it means you're dealing with the avialus. Whenever you see the term little A, it means that you're dealing with blood vessels, something that's happening like in the others, for example. So in our viola that constitutes our viola dead space, what should be true of the P big ACO too? So what should be true of the like the partial pressure of carbon dioxide in those avial light? I hope you're telling me that it's zero because yeah, it's not fully accurate, but it's technically accurate because the thing that happens is the primary source of carbon dioxide for your avial light is the carbon dioxide that's coming in from blood. So if blood is not flowing around around our violas, then that avialus will have no carbon dioxide in it. So the P big ACO too for the avial light that constitutes our viola dead space is zero. If you wanted to, you know what, maybe I shouldn't talk about this further, so I don't confuse you further, right? So that's what I will say. And essentially, if you decide to add you an atomic dead space to, well, you know what, let me go ahead and see this.

You may hear this sentence thrown around a lot, but it's a simple sentence, right? You say, oh, dead space, avialer dead space is like our viola that have been ventilated but not perfused. That's just essentially like a different way of stating what I have already just said, okay? I just wanted to throw that sentence in there because I know you pop up in many resources, but it's something people are like, what I exactly are they talking about? Now, here's the deal. When you add your anatomic dead space to your viola dead space, you have something called the physiologic dead space, okay? So your physiologic dead space is a combination of your anatomic and your viola dead spaces. So let me ask you a quick question. What should be the ideal situation in the respiratory system? The ideal situation should be that your physiologic dead space should equal your anatomic dead space because ideally, you would want all your viola to be participating in gas exchange, right? Because I mean, that's their job literally. That's literally their job, right? So what most I guess be true when the physiologic dead space exceeds the anatomic dead space? Well, it means that there must be parts of a person's lung that have been ventilated again, but they are not being perfused, right? Because again, normally, like I said, your physiologic dead space should be the same as the anatomic dead space. I may be seeing the van your repeating things over and over again.

I'm just doing this because again, pulmonary physiology is a touchy touchy subject for people. So I want to make sure that you really understand this stuff. Because the thing is the way I'm going to I'm trying to structure this podcast and I'm going to try to structure things to kind of build up on each other. Now, the next thing I guess I'll talk about here again, I'm just using this to lay a foundation for the future for the future pulmonary podcasts. So what is mini ventilation, right? You've probably heard this term mini ventilation, mini ventilation. What does it mean? Basically, mini ventilation just means your tidal volume multiplied by your respiratory rate. Okay? Your tidal volume multiplied by your respiratory rate is essentially like, oh, if you're taking in, you know, just normal breaths, normal breaths, those tidal volumes are referred to earlier. If you multiply that number by how many times you breathe per minute, that's your tidal volume, right? I mean, that's your mini ventilation. But the thing is, for purposes of the US analyst, one example, you want to be able to differentiate mini ventilation from our viola ventilation. Avual ventilation is like ventilation that is like true high-yield ventilation that gets oxygen into your actual body. So what do I mean by that? Essentially, I mean that in our viola ventilation, you're dealing with ventilation that you've removed dead space ventilation from. Okay?

So it's like if you're like, oh, mini ventilation, yes, that's great. That's like my total ventilation. If you subtract the component of that mini ventilation that is occupied by dead space ventilation, you essentially have our viola ventilation. So it should make sense already that your viola ventilation should be less than your mini ventilation, right? Because it's like mini ventilation is like, oh, tidal volume times respiratory rate. But your viola ventilation is like tidal volume. You subtract like your dead space volume, you get a number, and then you multiply that by your respiratory rate. And I guess it's kind of useful to know some values for tidal volume, already said earlier that tidal volume is about 500 cc's. Dead space volume for the most part is about let's say, you know, like the classic 150 pound male, let's say like 150 cc's. Remember, I told you that your dead space volume usually sits right around your weight in pounds, right? So that's why as a person, if a person gets more obese, their dead space volume actually goes up, right? That's why obesity ultimately causes hypoxia in a sense. If you really think about it in that, in that way. But there are many other factors, but that's probably like a simple constructs to use to understand that. So, and most people, you know, breathe out of the respiratory rate around like 10 or 12. But let me just use 10 as a convenient number, just to make the math, I'll talk about in a few seconds, easy.

So let me ask you a thinking question. What should be true? Of the our view of ventilation and minute ventilation when you increase tidal volume. So as that question again, what should be true of your view of ventilation and minute ventilation when you increase tidal volume? Well, think about it. The dead space is essentially remaining constant, right? Because it's not like, oh, you add more bronchi or whatever, no, no, no, no, no, right? The dead space remains constant. So whenever you increase tidal volume above baseline, that will increase your view of ventilation and your minute ventilation at the same, like basically at the same amount, right? So let me maybe use some numbers for this, right? So let's say your baseline tidal volume is 500, 500 C Cs, right? And your baseline respiratory rate is 10 breaths per minute. The minute ventilation for that person will be like 500 C Cs per minute multiplied by the 10 breaths per minute. So that means there are many ventilation is like 5,000 C Cs or like five liters, 5,000 milliliters. And the baseline of view of ventilation for those people, remember I said, our view of ventilation subtract dead space ventilation. So let's see, we take the tidal volume of 500 C Cs, subtract the 150 C Cs of dead space volume, right? We get 350. And then we multiply that 350 by the respiratory rate of 10, right? That's an overview of ventilation of like 3500 C Cs per minute, like 3.5 liters per minute.

So let's assume with then increase the tidal volume above baseline, right? We increase it by 100, right? So we take the tidal volume from like 500 to 600, right? If you calculate the minute ventilation, you notice that the tidal volume is going up by 100, right? I mean if you calculate that, I guess the increase in, let me make this a little simpler. If you calculate that, the increase in mini ventilation, you're essentially taking the bump top tidal volume of 100 C Cs and multiply by the respiratory rate of 10, right? So that means the person's mini ventilation is going up by like 1,000 C Cs. You are going from like 5,000 C Cs to 6,000 C Cs. If you did the same calculation for the person's Avular Ventilation, right? You should also go up by 1,000 C Cs because you're bumping the tidal volume by 100 C Cs. You're bumping the, I mean the respiratory rate is staying the same, right? So it's like again, still the same 100 times 10, that's 1,000 C Cs extra. So you're going from like 3,500 C Cs to 4,500 C Cs. So whenever the tidal volume, you may see, define this, this makes obvious sense. I promise you, your friends at the NBNC can make up questions where this thing that makes obvious sense may look very confusing. So just stick with me here for a few minutes. So when you increase the tidal volume, the mini ventilation and the Avular Ventilation go up by the same amount, okay?

They go up by the same amount and the reason they go up by the same amount, if you're increasing tidal volume is because the death speed volume stays constant. But now, let me go into the width a little. Let me ask you a follow-up question. What is true of the mini ventilation and the Avular Ventilation when you bump up the respiratory rate? What is true of the mini ventilation and the Avular Ventilation when you bump up the respiratory rate? Let's do some math here. I mean, the Cliffs Notes answer is that they both go up or they don't go up by the same amount. So you may see, why does that make any sense? Notice, now I'm not changing the tidal volume. The feeling of changing is the respiratory rate. So let me explain what happens here. So let's do the baseline calculations again. Mini ventilation, 500 C Cs times 10, breaths per minute, that's 5,000 C Cs per minute. Avular Ventilation, again, 500 minus 150, which is the death speed volume. So you get 350 times 10 breaths per minute. That's an Avular Ventilation of, you know, 3500 C Cs per minute. Now, let's assume we increase the respiratory rate to 15. If you calculate the mini ventilation, it's like 500 C Cs for your tidal volume times the new respiratory rate of 15 breaths per minute. That is 7500 C Cs per minute. So essentially, the mini ventilation has increased from the baseline 5,000 to the new value of 7500. So that's like an increase of 2500 C Cs per minute.

Now, if you calculate the Avular Ventilation of the new respiratory rate, you have like the 350, right? So 500 minus 150, the 350 multiplied by 15. That is 5,250 C Cs per minute, right? So if you combine that to the baseline 35,500, you want by 1750. So do you see that if you increase the respiratory rate, the Avular Ventilation does not increase to the same amount as the mini ventilation. It does not increase as fast as the mini ventilation. So it may see divine. Why? If you really think about it, if you increase respiratory rate, you are also increasing the ventilation of the dead space, right? Because it's like you breathe more, you use your dead space. If you breathe faster, you use your dead space more, right? Again, I promise you this is a critical concept you want to understand. It's something that's actually simple, but it's something that people kind of lose side of. Okay? So if you increase your respiratory rate, you're also increasing the ventilation of your dead space, right? So an increasing respiratory rate does not increase the Avular Ventilation as much as it increases your mini ventilation. There is in your mini ventilation, increasing more is that by increasing your respiratory rate, your Avular Ventilation is increasing, but your dead space ventilation is also increasing as well, right? So that's again, critical concept to keep at the back of your mind.

So if for example, they give you like a ventilator style question on the USMLE step one exam, or if again, let's say you become like a critical care physician in the future, if you're managing a ventilator, a more high-yield means of increasing Avular Ventilation is to actually bump the title volume up as again, bumping the respiratory rate up, right? Because the Avular Ventilation just raises a lot more with increases in title volume than increases in respiratory rate. So I hope this makes sense. If it doesn't make sense, I will strongly encourage you rewind this and listen to it. Again, it's a very critical concept to understand. So let me give you an, let me award this, let me ask you like a slightly different question. What if you have two people that have identical mini-ventilations? What do you have different respiratory rates? Who have the higher Avular Ventilation? I'll say this again, let's assume you have two people that have identical mini-ventilations, what they have different respiratory rates? Who should have the higher Avular Ventilation? I would hope that you are telling me it's the person that has the, the person that has the lower respiratory rate, right? So why does that make any sense? So let's use again a simple number, let's say 5,000 cc's. So let's say we're comparing person X and person Y, person X has a mini-ventilation of 5,000 cc's, person Y has a mini-ventilation of 5,000 cc's. And then let's make the thing that we veer the respiratory rate.

So let's say person A has a respiratory rate of, I don't know, 10, and person B has the respiratory rate of 20, right? So person A having the respiratory rate of 10 means that his title volume has to be 500 cc's, right? Person B that has the respiratory rate of 20 means that his title volume has to be 250 cc's. So if you think the first person, person X, title volume, sorry I think I may have said it or B, but person X, title volume 500 cc's, person Y, title volume 250 cc's, right? Remember the dead space, let's say it's the same human being, the dead space is roughly the same between both people. It is a track that 150 cc's from 500 cc's. Do you see that you're left with like an ovula, like you're left with a value of 354% X and a value of 100% Y. So if you think that value of 354% X, 100% Y, and then you multiply by the respiratory rates to get the ovula of intuition. Do you see that the person that has the lower respiratory rate has an ovula of intuition of essentially like 35? So because the person is breathing at like 10 breaths per minute, the person has an ovula of intuition of like 350 times 10, that's 3500 cc's per minute. Well the person that's breathing faster, right? Oh, the person is breathing real fast. You think 100 and multiply by their faster rate, their faster breathing rate like 20 breaths per minute, that's just 2000 cc's per minute. So when you breathe faster, your ovula actually do not get us get oxygenated better, okay?

Especially if you're dealing with the same if you're holding the mini ventilations a constant. Again, you may say this is low yield, but these are those kinds of questions that the vast majority of people taking step one end up getting wrong, right? They will just essentially describe it as an experiment from the exam, and then you have to think through all this respiratory physiology, right? But again, it's simple. If you really try to buckle down and understand it, they could very easily make this a multiple choice question and then mix a much thing, they may say like, oh, so assuming you have two people who are identical many ventilations, but different respiratory rates, who has the higher available ventilation, and then they will start changing all the variables like, oh, the person with the respiratory rate of 20 and tidal volume of this, the person having the respiratory rate of this and tidal volume of that. So you can already see how in the heat of a USMLE step one example of that pressure, that can very effectively mess with your head. Okay, so the next concept I guess I'll go ahead and jump to is that is to talk about the types of respiration, right? The types of respiration. So you may say divine, what do you mean by types of respiration? Basically, I mean, I guess this is almost like a meetup concept, but I think of respiration as being one of three types. You either inspire or you expire or you forced expire, right? So inspire, right?

means you're inspiring, you're bringing in air to your lungs, right? The things that help you accomplish those goals are your diaphragm and your external intercostals. That's how you to know. Now, expiration in and of itself, like regular, just regular expiration is a passive process. So essentially, if you're diaphragm and your external intercostals, stop contracting, things will return right back to normal, right? Expiration, normal, quiet expiration is a passive process, but let's see, you want to do something called forced expiration. So you want to blow out that expatory reserve volume I talked about earlier. You're going to need to get some additional help from your abdominal muscles, right? And the abdominal muscles that you get extra help from are things like your your external and internal bleak muscles or your retus and your transversals abdominis muscles. And I guess your internal intercostals also help with like forced expiration. So I actually learned this numonic or trick from my med school professor that fought me uponary physiology. Basically, he said that like to remember like which of the intercostals helps with expiration versus expiration is just to remember that for inspiration, you use your internal intercostals and for expiration, you use your external intercostals. So you just basically let me take that mantra with into your brain like, oh, the eyes match, the eyes match. And then remember that that is wrong. Okay.

So like, oh, for inspiration, I use my internal intercostals, for expiration, I use my external intercostals. And they remember that that is wrong. It's usually take the opposite and then you should be squared away because the truth is that for inspiration, you use your external intercostals for expiration, you use your internal intercostals. Or you can just say, let me just remember one concept and take the opposites like, oh, for the eye activity, for the inspiratory activity, I'm using the e-word, your external intercostals or for the e-activity, for expiration, I'm using the i-word, internal intercostals. Then that can whichever method floats your boat that can help you keep keep things straight. Now, for the next phase of what I want to talk about, I just want to talk about almost like a series of thought experiments. To again, establish the mechanisms behind a person inspiring, like getting into the lungs and a person inspiring that's getting air out of the lungs. So this is one area where you just want to try to follow my series of statements. If I were you, I'll listen to a short part of this pause, run it through your head and make sure it makes sense to you. Okay? Now, my first statement is that the lung likes to recoil in word. Okay, the lung likes to recoil in word, right? Essentially, you have your like contains elastic tissues, those elastic tissues make the lungs recoil in word. So the natural tendency of the lungs is to recoil in word. Okay?

So it's kind of like a balloon, right? If you blow up a balloon, right? The natural thing the balloon wants to do is to deflate, right? It wants to deflate, it wants to recoil, right? Back, it wants to recoil in words. Now, at higher lung volumes, the desire of your lungs to recoil in words increases. I'll say that again, at higher lung volumes, the desire of your lungs to recoil in word increases. Again, just kind of compare this to a balloon. If a balloon is, if you compare like a balloon that's just a little inflated to one that's maximally inflated, the maximally inflated balloon, right? Well, want to recoil in word more, right? You have a bigger desire to recoil in word. So remember that statement and then you can essentially like tick the corollary to that, right? If the desire to recoil in word is less for a lung that is not maximally inflated, right? So let me essentially just again explain this a different way. Recoil or I guess introduce a new concept with this recoil is synonymous to elastance, okay? So if a lung is at high volumes, right? So if again, think of this as like a maximally inflated balloon, the elastance is very high. So elastance is highest when the lung is at its highest volume, when the lung is maximally distended, right? So there is a great propensity to snap back when the lung is at its highest volume. So like your total lung capacity, for example.

And the second concept I want you to keep in mind is that compliance is inversely proportional to elastance. The thing is, if I were you, I'll just try to understand and be because I've essentially introduced this concept as elastance talking about like, oh, the lung wanting to recoil in words. Just remember that direction and then just take the opposites and you can derive any concept you need for compliance. So I've talked about the lungs. Now what about the chest wall? The thing is the chest wall loves to recoil outwards, okay? The chest wall loves to recoil outwards. But the problem there is that yes, the lungs wants to recoil in words, the chest wall wants to recoil outwards. The problem is that the chest wall is feathered to the lung itself by plural, okay? The chest wall is feathered to the lung itself by plural. So you essentially have like two forces that are competing against each other, right? Like the chest wall wants to recoil out the, of your like, one like to recoil in, right? So the thing is when you have these two forces counterbalance in each other, right? So you're essentially like, it has still made like this outward recoil of the chest wall inward recoil of the lungs. When you have these two forces counterbalance in each other, the lung will be at FRC. The lung will be at the functional residual capacity. So essentially the equilibrium volume of the lung, basically again like I said, the FRC is determined by these two forces.

And again, you see how these concepts all relate when I begin to talk about the pulmonary pathologies. Now the next concept I want to establish is the whole concept of flow, right? There's such a thing as flow, right? But there's also such a thing as pressure. And the thing is for respiratory physiology, always think about pressure as a, as a relative term. Essentially, you're typically trying to compare like two pressures, like you're typically comparing like, oh, one pressure that is bigger than the other, right? You're typically comparing like a side with a pressure that is higher than another side. Now, for flow to happen, you need to have some kind of pressure differential. A pressure differential must exist for flow to happen, right? Because I mean, if you think about it logically, right? Things flow from higher pressures to lower pressures, right? The thing that typically flows is essentially like a fluid of some sort. Remember the word fluid literally means you have the ability, you have the capacity to flow, right? So the things that flow are either like gases or liquids, right? A fluid just means either a gas or a liquid. So now that I've sort of talked about these, let me introduce the, because I kind of wanted to use that as a foundation to talk about the concept of positive versus negative pressure, okay?

The thing is a system that a system being a positive pressure means that the pressure within that system exceeds the pressure in the surroundings around that system. So let me talk about that again. So a system that is a positive pressure means that the pressure inside that system exceeds the pressures outside that system. Conversely, a system that is at negative pressure means that the pressure inside the system, so the pressure within the system is exceeded by the pressures that exist outside that system. So if you're kind of flowing along with all these statements have been making, it should make sense, right? That flow should happen away from positive pressure and towards negative pressure, right? I mean, if you want to maybe find that, yes, a more convenient way of staying stating this is that nature abhors a vacuum, right? So you always want to bring things back to equilibrium, or you can bring things back to equilibrium by having pressure, having like flow happen away from the positive pressure side of things and towards the negative pressure side of things. So let me give you an example, right? Like a real life example, right? If you blew into a straw, right? The pressure within that straw will go up above atmospheric pressure. And you'll notice on the other end of the straw, and air will flow out of that straw, right?

Because by blowing into the straw, you're making that the lumen, the inside of that straw, have be a positive pressure system because the pressure inside it will be higher than the pressures of the surrounding atmosphere. But on the other hand, let's assume you suck out a straw, right? When you suck out a straw, you let all the air out of that straw. That the lumen, like the inside of that straw becomes a negative pressure system, because the pressure on the outside will be higher than the pressure on the inside, okay? That is why flow from your drink will go into the straw under those circumstances because your drink that surrounds that straw lumen that you've sucked air out of, that straw lumen will be at lower pressures on the inside relative to the drink that is kind of like an atmospheric pressure, so you'll have higher pressures, right? So that's why when you sip on a drink, that drink flows into the straw, because by that initial sip, you get all the air out of the straw. So the straw, the lumen of the straw becomes a negative pressure system. Now, when you decide to essentially take, like, you know, like make your own standard and you say, you know what? I want to compare pressures on the inside to pressures on the outside. So again, we're dealing with like a system, like a close system, if you may. If you say, you know what? Let me make my standard to compare pressures on the inside, to pressures on the outside.

We essentially delve into the realm of something called a transmural pressure, okay? Basically, the transmural pressure for any system is essentially the pressure difference from taking, subtracting the pressure on the outside from the pressure on the inside. So if you want to like express it as like a, you know, like a logical equation, transmural pressure is inside pressure, minus outside pressure. I mean, stated conveniently, like a transmural pressure is like the pressure that's pushing on the walls of an enclosure, okay? Is the pressure that is pushing on the walls of an enclosure? So for a positive pressure system, right? You should kind of be able to deduce that the transmural pressure has to be some positive number, right? Because the pressure inside the system exceeds the pressure outside the system, right? And I mean, the reverse should also be the case, right? If you have like a negative transmural pressure, it should mean that the pressure on the outside of the system exceeds the pressure on the inside of the system, right? So that essentially makes that system collapse inwards. Now, so I guess let me again, let me summarize. If you have a positive transmural pressure, that will cause outward expansion of a system, right? Because again, the pressure within the system exceeds the pressure outside. Conversely, if you have a negative transmural pressure, you will cause that system to collapse because the pressure within that system is less than the pressure outside.

Again, you may see the findings are all logical things. These are all simple things, but you'll see how the all quarter cross pulmonary pathology when we begin to talk about those things. And one thing I've noticed is if you have a deep understanding of things like at this level, when you see NV Me questions, they become very easy because you essentially see through what the examiner is trying to lay in front of you. That's why again, I always tell people in everything you do, always try to seek understanding, understanding bits memorization every day of the week. So now that I've talked about all these statements, let me begin to make parallels with the respiratory system, right? But before I start making those parallels, I want you to keep a construct at the back of your mind. Think of your lungs as being the insights of your respiratory system, and then think of the plural space as being the outside of your respiratory system. You'll see what I mean going forward. So think of your lungs like your ovuli, like the inside of your ovuli as like the inside, and then think of your plural space as being the outside. Now, let me talk about the, I've been saying purr purr purr purr purr. What does purr mean, right? Basically, the purr is a sheet of tissue that's essentially like folded back on itself. Remember, I told you that the purr threaders the lungs to the chest one. So the purr is a sheet of tissue that's, you know, essentially folded back on itself.

The inward part of this fold is what is threaded to the lung, that's like your visceral purr. The outward part of this fold is threaded to the chest one. That's essentially your preropulur. The space between this inward fold and the outward fold is your plural space. Okay? And many times you'll hear people throw around the term that, oh, this space is at negative pressure. Basically, what they mean is that the surroundings of the plural space generally have higher pressures than the plural space itself. So the pressure within the plural space is always in general is always less than the pressures of surrounding spaces. So, I mean, this should kind of make logical sense, right? Think again, sort of think about this in the context of transmural pressures. At rest for the lungs, we know that our viola are not collapsed, right? We know that our viola are open. For those of you like to be open, right? Because essentially, they're expanding outwards. It should kind of make sense that the pressure within those of your life, right? Should be higher than the pressures outside those of your life, essentially like in the plural space, right? So the pressures on the outside of your view like, again, the plural space pressure should be less than the pressures inside the of your life. Now, another thing I guess I want you to realize is that remember I said that again, everyone says that, oh, your plural space is at negative pressure. Your plural space is at negative pressure.

Plural space is at negative pressure. It means that again, surroundings of the plural space tend to be at higher pressures than the plural space itself, right? So I already said that, oh, your viola at rest is usually kind of like held open. So that means the pressures within your viola exceed your plural pressures. The same is also true for the atmosphere. The atmospheric pressure is usually greater than the pressure within the plural space, okay? So in other words, again, relative to the surrounding atmosphere, your plural space is at negative pressure. So let me summarize all these things again. Let me summarize these high-yield relative pressures. The pressure in the out of your life is higher than the pressure in the plural space. That's concept one. Concept two is that the pressure in the atmosphere is also higher than the pressure in the plural space. And in respiratory physiology, we set the atmospheric pressure to be zero. Typically, we set the plural pressure to be around like negative five, okay? So if you look at the situation when the lung is at rest, if the lung is at rest, so basically like there's no flow of air into the lung is just, you know, at equilibrium, there's no air flow into the lungs, like basically like what happens when you inspire, right? So again, like just lung at rest, air is not coming in, air is not going out.

It should make sense that the pressure within the out of your life should kind of be the same as the pressure in the atmosphere, right? Because if the lung again at equilibrium at FRC, at functionary is the raw capacity, air is not coming in, air is not going out, there is no flow between the atmosphere and the inside of your out of your life. That means that there is no pressure differential, right? So that means at that point, the atmospheric pressure is zero, the out of your pressure is also zero. So at that rest end point, just sort of think of it like time lapse, at that rest end point, if you completed the transmural pressure of the out of your life relative to the plural, if you get a value of five, right? Because the pressure inside the out of your life is zero, the pressure outside the out of your life, right? Within the plural space is negative five, as I said, so if you take zero minus negative five, that is plus five, right? And again, because it's a positive transmural pressure, that means it's a pressure that is tend to keep the out of your life open, which is what you want? If you have your life collapsed, you have a big problem because you will not be able to get, you have a hard time performing a gas exchange. So now that we've talked about all these things, let's sort of talk about the process of getting air into the lungs, the process of inspiration. When you're diaphragm and your external intercostal muscles contracts, right?

Your plural pressure actually gets more negative, so essentially it becomes like a value that's more negative than negative five. So let's see like negative seven, for example, if you think about it, if the plural pressures become like your interplural pressures become more negative, what should happen to the transmural pressure between the out of your and the plural space? It should increase, right? Because if you think that all you're like, oh my aviolize at zero, and my introplural pressure is negative five initially, that the spread is positive five. What if you know your external intercostal muscles contract your diaphragm contracts? Introplural pressure can go from like negative five to like negative seven, for example. If you take that zero within the aviolize minus negative seven, you come up with a new transfer pressure of positive seven. So that transmural pressure goes up when your introplural pressure, like your plural pressure gets more negative. And again, like I said, as transmural pressure is increasing, right? That means you exerting more force on the walls of like the surrounding container. So that means the aviolize should expand, right? Because again, if a transmural pressure again is becoming more positive, it means that enclosure, that system is expanding, okay? And think about it, when the aviolize expands, what is intrinsically happening to the aviolium? The aviolums are going up, right?

And if the volume is increasing, what is happening to the pressure within the aviolize? If you're thinking about like boils, low, the pressure within the aviolize should be decreasing, right? Because as volume goes up, right? Gas molecules are farther apart. So the pressure within that system should go down, right? So the pressure should decrease. So with the aviolium increasing, the aviolize pressure is decreasing, right? So for a moment in time, your aviolize pressure transiently like dips below the atmospheric pressure. If the pressure within your aviolize dips below the atmospheric pressure, then that will cause air to flow into the lungs, right? The air will rush into the lungs because again, like I said, nature wants equilibrium. So the air rushing in will essentially equalize the pressures between the inside of the aviolize and the atmosphere, so basically when you have a negative intraplural, like a more negative intraplural pressure, the transfer pressure between the aviolize and the plural space will increase, right? So the aviolize will expand. When the aviolize expands, the aviolum volume goes up. If the aviolum volume goes up according to boils, low, the aviolus pressure will come down. When the aviolus pressure comes down, we essentially have like a negative pressure system between the atmosphere and the lungs because the atmosphere, like because the atmosphere itself will have higher pressures than the lungs, so air will flow down, right?

From high pressure to low pressure to fill up the lungs. That's essentially what happens in inspiration. If you take the negative, if you take the opposite of everything I've just described, you already understand the mechanism behind expiration. So I know this is kind of going on for a long time. I'm going to go ahead and pause here. And again, you may see divine this podcast is very typical, but again, I promise you, if you understand everything that I have described in this podcast, most of the other podcasts that will come after this way can begin to go more direct, like I'm a little more like direct and less less like abstract, where begin to go like more direct into like talking about like this physiology and this pathology and this pathology and that pathology, they will all just flown nicely in your brain because you already have this foundation down path. So as I do at the end of every podcast, I don't for again, one on one tutoring for essentially every exam you take as a medical student. So I offer tutoring for you, the US Emily step one, step two CK step two CS and step three exams. I tutor for the preclinical, med school exams, the third year, clinical shelf exams. And then if you're a medicine resident, I tutor to the medicine board exams and the internal medicine in the training exam. And then if you're a college student, I actually tutor for like a tutor like Gen CAM, O-CAM, physiology, histology, biochemistry, all that stuff.

And then I also do like one on one like advice in slash, I guess, uh, uh, consulting for like med students applying to a residency. So like an era's up or like a college student applying to med school. So an AMCA's up. Again, I have like admission committee experience. So I've, uh, and I've worked with lots and lots of people that have applied for either of these things. And again, the vast majority of people have worked with have been, have been very successful. And, uh, also this whole concept of longitudinal tutoring, I also offer that where your study med school, I tutor you through your, um, your preclinical, a tutor for essentially your preclinical exams. But as I'm tutoring you for your preclinical exams, I'm infusing step one knowledge with each of those blocks. Uh, so that when you get your dedicated period, you feel like very well prepared, super ready for, for step one. Because unfortunately, there is, at most med schools, there's a dissonance between what is thought in med school for your cost work and what you expect that to know ultimately for step one. So this longitudinal tutoring essentially bridges that gap. And it helps you like in two ways. It helps you crush your preclinical exams, but it also helps you be ready for step one when you rose along. Usually those people also tutor them during their dedicated periods. I do the same thing for 30 med students. Again, longitudinal tutoring, I tutor you for each of your shelf exams.

As I'm doing that again, I'm infusing knowledge that is pertinent to step two CK. So that again, when you hit your step two CK dedicated period, you're like super ready. You're very well prepared. Right? So, um, if you need any of those things, again, reach out to me through the website or reach out to me, um, through like an email, like divine intervention podcasts with an SAB in that gmail dot at gmail.com. Um, and I guess as an update to the website, like over the last like one or two weeks, there is a section you may see at the top called podcast topics. It essentially breaks things down by tag, right? So it's just much easier to navigate that way. So if you go to the main website and you click on podcast topics, let's see, I tried to study step one nephrology, right? You'll see like a section that says you're a semilistep one. You'll see a like a tab down that says nephrology. You click on that and you'll see like everything that is written, like every podcast that is renal related that I have made for the US Emily step one example. Um, hopefully over the next couple of days, two weeks, um, I get to a point where, um, the these podcasts are moved to or not necessarily move, but accessible through like a podcast app. So like Spotify, for example. So I would try really hard to make that happen.

And also like again, kind of long term over the next few weeks, I would try to get the podcasts to my You Tube channel because right now my You Tube channel, I just have my videos on there. I would try to put like these podcasts on You Tube so that you can again access these things across multiple platforms like Spotify, the You Tube channel, and the website, whichever floats your boat. So have a wonderful rest of your day. God bless you. I'll see you in the next podcast. Thank you.

The balance between two opposing elastic forces: the outward recoil tendency of the chest wall and the inward recoil tendency of the lung parenchyma.

Obesity increases dead space volume because increased weight leads to greater airway resistance and larger conducting airways, which are non-alveolar.

$\dot{V}_E$ is simply Tidal Volume $\times$ Respiratory Rate (total air moved). $\dot{V}_A$ is calculated by subtracting dead space volume from TV before multiplying by RR, representing only the air that participates in gas exchange.

Flow always occurs from an area of higher pressure to an area of lower pressure (pressure differential).

Alveolar dead space, because the PE causes ventilation/perfusion mismatch ($\dot{V}/\dot{Q}$ mismatch), leading to ventilated but unperfused alveoli.

The abdominal muscles (internal and external oblique, transversus abdominis) and internal intercostals.

Any volume of air that does not participate in gas exchange.

$\text{Transmural Pressure} = \text{Pressure}_{\text{inside}} - \text{Pressure}_{\text{outside}}$.

VC = TV + IRV + ERV.

It creates a positive transmural pressure ($\text{P}_{\text{alveolar}} - \text{P}_{\text{intrapleural}}$), which forces the alveoli to expand (inspiration).

Anatomic dead space.

The person with the lower respiratory rate, because a slower rate allows for greater proportion of air to reach the alveoli (less proportional increase in dead space).

Any volume of air that does not participate in gas exchange.

$\text{Transmural Pressure} = \text{Pressure}_{\text{inside}} - \text{Pressure}_{\text{outside}}$.

VC = TV + IRV + ERV.

It creates a positive transmural pressure ($\text{P}_{\text{alveolar}} - \text{P}_{\text{intrapleural}}$), which forces the alveoli to expand (inspiration).

Anatomic dead space.

The person with the lower respiratory rate, because a slower rate allows for greater proportion of air to reach the alveoli (less proportional increase in dead space).