Welcome back. This is our first on several videos about the respiratory system. And so we are going to start off talking about the general anatomy of the lung and then some of the mechanics of breathing. How we get air in and out of our lung. Obviously, one of the main roles of the respiratory system is to bring in oxygen, so that we can do oxidative metabolism and then to remove the carbon dioxide that we produce. However, we're also gonna be talking about the very important role of the respiratory system in regulating our pH. And then also keep in mind, that because we're bringing in so many liters of air a day, roughly 10,000 liters a day. And our lungs have such a large surface area. That this is a major site of concern for our defense system, for our immune system. And so, the respiratory system is going to have certain strategies that it uses to protect itself from pathogens. And that's something that we're not really gonna focus on, but keep in mind that that is an important factor that the respiratory system has to keep in mind. Now we're gonna see many similarities of the respiratory system to the cardiovascular system. Because again, we're gonna be talking about flow. And so, we're gonna be talking about pressure gradients and the amplitude of that pressure gradient or the amount of the pressure gradient is gonna be important, as well as the resistance. And again, the resistance is gonna be related to the radius of the tube and the radius is going to be to the fourth power. So the radius is going to make a huge difference in the resistance of the system. And again, we'll be addressing that in similar ways that we did for the cardiovascular system. So just in a very low mag view of the anatomy of the respiratory system where it's going to start with the nose and the mouth, where air enters the body. And then, it's the air is gonna enter the trachea which is gonna be a tube that is surrounded by rings of cartilage. And then the trachea will branch into bronchi. That are also surrounded by cartilage. And then those will continue to branch more and more until we get to what's called the respiratory part of the lung, which we'll be talking about in the moment. And then keep in mind also, we're gonna be talking about the diaphragm, which is gonna be sitting underneath the lungs and which is going to be skeletal muscle. So we will be talking about the respiratory system by dividing it into two parts. One is going to be the conducting part, which is going to contain about 150 mls of air. And as its name suggests, one of its main roles is to conduct or bring air into the main part of the lung that is going to be responsible for exchanging gas with the blood. So that is certainly one of the roles of the conducting portion of the respiratory system is to distribute air into the rest of the lung. The conducting portion is also going to warm and moisten the air, and we'll talk about that a little bit more in future sessions. And it's also gonna have a major role in cleansing the air. So again, this gets into the idea that the lung has got to be concerned about all of these liters and liters of air that it's bringing in, and so the conducting portion is going to cleanse the air using what's called mucociliary transport. Which is going to be when some cells lining the conducting portion, the trachea, and the bronchis are going to be producing the mucus, and then that will trap debris. And then there will be other cells that have cilia that then take that mucus, that it contains the debris, and beats it up towards the entryway of the respiratory system. So that's the mucociliary transport system, where we have mucus and cilia that are gonna work together to remove particulate matter and pathogens. The other aspect of the conducting system is that in particular the bronchioles, very similar to the way the arterioles controlled the flow through the system based on changing its diameter. The bronchioles are going to also have smooth muscle around them which is going to help determine how easily air flows into the lung through changing their amount of contraction or relaxation of that smooth muscle. So those are the roles of conducting portion of the system. Then the largest volume is going to be in the respiratory portion of the respiratory tract, which is going to be about three liters kind of at rest. Where we're gonna have gas exchange. And it's gonna be made up of alveoli that have a very thin epithelium that allows for diffusion of gases between the alveoli and the capillaries that are gonna receive and dump off gases. So let's talk a little bit more about the structure of the alveoli. So in this diagram, we have three alveolar sacs. So each one of these structures would be an alveolar sac and you can see that it's a spherical structure that is then made up of smaller spheres and each of these smaller spheres is an alveolis. So, the alveolar sac is made up of lots of different alveoli, and the inner surface of these alveoli is what's in contact with the air. And then you can see how the outer surface is just covered in capillaries. The inner surface of the alveoli makes up a large surface area, so it's about 70 square meters in the lungs, which is obviously gonna allow for a lot of gas exchanges since we have a large surface area. And that surface is going to be comprised mostly of type one cells that are gonna make the surface of the alveoli that is in contact with the air. And these cells are going to be very flat cells. So they're gonna be the epithelial cells of the alveoli. They are very flat and thin so that the air does not have to go a very long distance to cross the type one cells. In between the type one cells will be type two cells that don't cover as much of the surface area of the alveoli but they're there as stem cells and as cells that produce surfactants and we'll be talking more about surfactant in the future. So you can see how we have this thin epithelium and then lots of capillaries on the other side of it to exchange the gas between the air and the blood. We're gonna move now to talking a little bit more about ventilation, and how we get air in and out of our lungs. And it's gonna be really important to remember that the way that we get air into our lungs is called negative pressure breathing. So basically, what we're going to do is we are going to take our diaphragm, which is a dome shaped muscle, and contract it. And so, its skeletal muscle and if you contract it, that's going to make it shorter and it's gonna become flatter. So as it becomes flatter then that is gonna cause the chest wall to expand. And as that chest wall expands then that's gonna lower the pressure in the lungs. So you have negative pressure in the lungs and that means that air is gonna flow into the lungs. So the diaphragm is what's going to be very important when we're just doing normal activities like sitting and watching me lecture where your somewhat at rest. You're going to contract your diaphragm to inhale, and then you're just gonna relax your diaphragm to exhale and we'll be talking more about this. But if for instance, you're exercising, you're gonna want to take a deeper breath and you're gonna want to breathe more quickly and that's when you can bring in muscles of the rib cage, they're gonna kind of lift the ribs kind of like a handle on a bucket and you lift it. And that's gonna increase again the volume in the lungs. And then you can also use when you're trying to exhale quickly, many other muscles of the chest wall, as well as the abdomen. So, depending on the type of breathing you're doing, you might use more muscles, but very often just in normal, rest breathing, you're using primarily the diaphragm. The other thing that to keep in mind and we'll be talking much more about this is the reason why you can kind of just relax your diaphragm and everything just compresses, the chest wall gets smaller on its own, is because of the elastic recoil of the lung. That makes the lung want to get smaller and that's what helps and that passive exhalation process. And we'll be talking more about that in future sessions. So here's another view of this where if we contract the diaphragm, we're going to expand the chest wall, that's going to because of Boyle's law caused the pressure in the lung to go down. And that means that we're gonna have a greater pressure out in the atmosphere, and that means that air is gonna flow in. So that's inhalation. Negative pressure breathing, and air is gonna enter when the pressure outside in the atmosphere is greater than the pressure in the alveoli. So, PA refers to the pressure in the alveoli. We're gonna be using this abbreviation a lot, so it's good to get used to it. When you inhale, and then you often kind of pause before you exhale, then very rapidly, the pressure will equilibrate between the alveolar pressure and the atmosphere pressure, and so flow will become 0. So in between breaths, that's what's going to happen under most circumstances. When we expire, then that's gonna be usually passive under normal conditions. And that's just when the thoracic cavity, the chest wall and the lung are gonna return to their normal dimensions. And as that happens, so the diaphragm is gonna become dome shaped, as it relaxes, that's gonna increase the pressure in the lungs. And that will make it's pressure greater than atmosphere pressure and so air will flow out. So that's what's here where air leaves when the atmospheric pressure is less than the alveolar pressure. So let's talk about ventilation, which is gonna be this exchange of air between the atmosphere and the lungs and we can talk about a ventilation cycle which is gonna be one inhalation and one exhalation. And we'll be talking some about the frequency of ventilation. How many breaths per minute are you taking? Often it's gonna be between 10 to 18 breath a minute. We'll also be talking about tidal volume which is how much air you're inhaling or exhaling. And that's going to often be about 0.5 liter per breath. And so using the tidal volume and multiplying it by the frequency of breathing, then we can determine a minute ventilation, how much air are you bringing into your respiratory system each minute. And so if you're breathing 0.5 a liter minute, 10x a minute, then that's gonna be about 5 liters a minute. Keep in mind, and we'll be talking about the sum that the depth, and the rate of breathing can change, and this can dramatically increase airflow 20-fold. And then blood flow can increase 3-fold to the respiratory system, so during heavy exercise we can make a huge adjustment in basically the minute ventilation. However, if you're gonna do that, you're gonna need to have active exhalation. You don't have time to let everything slowly relax and let the recoil of the lungs happen. You're gonna need to have to be able to squeeze out that air so that you can quickly take another breath in. And so that will require those abdominal muscles and the intercostal muscles between the ribs. So let's finish this up by just reviewing what we've talked about where we're gonna have this huge interface for gas exchange with the lung. And we've talked about how it's gonna contain a conducting zone, which is what we're gonna call dead space. Because it can't exchange gas. And then it's gonna have an alveolar space, which is the respiratory zone. And we'll be talking more about that. We've talked about some of the basics of how we use negative pressure breathing to get air into the lung. And then how we exhale. And then, how we're going to have passive exhalation if we're at rest, but then during exercise, that we're gonna need active exhalation. And that one reason why we can have passive exhalation is because the lungs want to recoil, and so that's going to help drive that process, and we'll be talking more about that in future sessions.
