Welcome back. This is our first of several videos about the respiratory system. 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. Keep in mind, that because we're bringing in so many liters of air a day, roughly 10,000 liters a day. Our lungs have such a large surface area. That this is a major site of concern for our defense system, for our immune system. So, the respiratory system will have certain strategies that it uses to protect itself from pathogens. That's something that we're not really going to focus on, but keep in mind that it is an important factor that the respiratory system has to contend with. Now we're going to see many similarities between the respiratory system and the cardiovascular system. Because again, we're going to talk about flow. We will consider pressure gradients and the amplitude of that pressure gradient or the amount of the pressure gradient that will be important, as well as the resistance. Again, the resistance is going to be related to the radius of the tube. 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. Again, we'll be addressing that in similar ways that we did for the cardiovascular system. This is a very low magnification view of the anatomy of the respiratory system. It starts with the nose and the mouth, where air enters the body. Then the air enters the trachea which is a tube surrounded by rings of cartilage. The trachea branches into bronchi. They are also surrounded by cartilage. Then the bronchi continue to branch more and more until we get to what's called the respiratory part of the lung. We will be talking this region in a moment. Keep in mind also, that we have the diaphragm, which is located underneath the lungs. This is skeletal muscle. We can consider the respiratory system by dividing it into two parts. One is the conducting part, which contains about 150 mls of air. 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. That is certainly one of the roles of the conducting portion of the respiratory system, to distribute air into the rest of the lung. The conducting portion will also warm and moisten the air. We'll talk about that a little bit more in future sessions. It is also going to have a major role in cleansing the air. Again, this gets into the idea that the lung has got to be concerned with all of these liters and liters of air that it's bringing in. The conducting portion is going to cleanse the air using what's called mucociliary transport. This consists of some cells lining the conducting portion, the trachea, and the bronchis which produce mucus which will trap debris. Then there are other cells that have cilia. These cilia move the mucus, that 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 going to 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 control the flow through the system based on changing its diameter. The bronchioles also have smooth muscle around them which helps determine how easily air flows into the lung by changing their amount of contraction or relaxation of that smooth muscle. So those are the roles of conducting portion of the system. The largest volume is going to be in the respiratory portion of the lung. This is about three liters in each at rest. This is 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. Let's talk a little bit more about the structure of the alveoli. In this diagram, we have three alveolar sacs. Each one of these structures would be an alveolar sac. You can see that it's a spherical structure made up of smaller spheres. Each of these smaller spheres is an alveolus. The alveolar sac is made up of lots of different alveoli. The inner surface of these alveoli is what's in contact with the air. You can see how the outer surface is covered in capillaries. The inner surface of the alveoli makes up a large surface area, it's about 70 square meters in the lungs. This obviously is going to allow allow for a lot of gas exchange since we have a large surface area. That surface is comprised mostly of type one cells. They make up the surface of the alveoli that is in contact with the air. These cells are going are very flat thin cells. They are 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. They are the stem cells. They also produce surfactants. We will 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 going to move now to talk a little bit more about ventilation. This is how we get air in and out of our lungs. It's gonna be really important to remember that the way that we get air into our lungs is called negative pressure breathing. Basically, we will use our diaphragm, which is a dome shaped muscle, and contract it. It is skeletal muscle. If you contract it, that makes it shorter. It becomes flatter. As it becomes flatter it causes the chest wall to expand. As that chest wall expands then that will lower the pressure in the lungs. So you have negative pressure in the lungs. That means that air will flow into the lungs. So the diaphragm is going to be very important when we just doing normal activities like sitting and watching my lecture while your somewhat at rest. You will contract your diaphragm to inhale, and then relax your diaphragm to exhale. We'll be talking more about this. But if for instance, you're exercising, you're going to want to take a deeper breath. You are going to want to breathe more quickly. That's when you can bring in muscles of the rib cage. They're going to lift the ribs kind of like a handle on a bucket. As you lift it that will increase again the volume in the lungs. You can also use them 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 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 because of the elastic recoil of the lung. The lung wants to get smaller and that's what helps in that passive exhalation process. We'll be talking more about that in future sessions. So here's another view of this where if we contract the diaphragm, we will expand the chest wall. Because of Boyle's Law this causes the pressure in the lung to go down. That means that we're going to have a greater pressure out in the atmosphere. Air will flow into the lung. That's inhalation. Negative pressure breathing occurs when air enters because the pressure outside in the atmosphere is greater than the pressure in the alveoli. So, PA refers to the pressure in the alveoli. We will use 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. That's just when the thoracic cavity, the chest wall and the lung are gonna return to their normal dimensions. As that happens, the diaphragm will become dome shaped. As the diaphragm relaxes, that will increase the pressure in the lungs. That will make the lung's pressure greater than atmosphere pressure. Air will flow out. That's what shown here where air leaves when the atmospheric pressure is less than the alveolar pressure. Let's talk about ventilation, which is going to be this exchange of air between the atmosphere and the lungs. We can talk about a ventilation cycle which is one inhalation and one exhalation. We'll be talking some about the frequency of ventilation. How many breaths per minute are you taking? Often it's going to be between 10 to 18 breaths per minute. We'll also be talking about tidal volume which is how much air you're inhaling or exhaling. That will often be about 0.5 liter per breath. 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. 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, that we'll be talking about the sum and the depth, and the rate of breathing. This can change to dramatically increase airflow 20-fold. Blood flow can increase 3-fold relative to the respiratory system. So during heavy exercise we can make a huge adjustment in basically the minute ventilation. However, if you're going to do that, you're will need to have active exhalation. You don't have time to let everything slowly relax and let the recoil of the lungs happen. You will need to have to be able to squeeze out that air so that you can quickly take another breath in. That will require those abdominal muscles and the intercostal muscles between the ribs. Let's finish this up by just reviewing what we've talked. We will have this huge interface for gas exchange within the lung. We've talked about how it's going to contain a conducting zone, which is what we will call dead space. Because it can't exchange gas. Then there is alveolar space, which is the respiratory zone. We'll be talking more about that. We also 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 how we're going to have passive exhalation if we're at rest, but that during exercise, we will need active exhalation. That one reason why we can have passive exhalation is because the lungs want to recoil. That helps drive that process. We'll be talking more about that in future sessions.
