 Hey everybody, Dr. O here. In this video, we're going to talk about the mechanics of breathing. So this is, I cannot make this easy, but I'm going to try to make it as simple as possible. We start talking about gas laws and pressures, and students often get confused. But let's just start with, this is Boyle's Law. I'm not big on asking you to know Boyle's Law, Henry's Law, the other gas laws that are at play here, but I need you to understand this concept. If you take the same amount of substance, whether it's an air or anywhere, and you try to cram it into a smaller space, the pressure is going to increase. So as you can see here, as the volume increases, the pressure decreases. That's what happens when you inhale, your diaphragm drops and your rib cage expands. So now the volume inside your thoracic cavity has gone up, so the pressure has gone down. So keep that in mind for later. On the flip side, as volume decreases, pressure increases. So when you exhale, diaphragm pops back up, your rib cage drops back down. The volume in your thoracic cavity has gone down, so the pressure has gone up. And the reason this is so important is because you breathe in when there's room in your thoracic cavity and the pressure inside your thoracic cavity is lower than atmospheric pressure, the pressure that's on you constantly. You have a column of air above you putting pressure on you. That's atmospheric pressure. Then you exhale when the volume in your thoracic cavity goes down, the pressure goes up, and now the pressure inside your lungs is higher than atmospheric pressure and it pushes it out. Okay, so this is the reason this gas law is so important. Let's actually see it in action. So here we see the pressures involved in breathing, and we'll cover inhalation and exhalation separately. Atmospheric pressure, that's that column of air that's above you right now, putting pressure on you. It is 760 millimeters of mercury at sea level, so that's going to obviously will change depending on where you're at, but that's what's known as one ATM or one atmosphere. So that's atmospheric pressure. Just remember, if atmospheric pressure is higher than the pressure inside your lungs, you will breathe in. If atmospheric pressure is lower than the pressure inside your lungs, you will breathe out or exhale. The other pressures here, intra alveolar pressure or intrapulmonary pressure, that's the pressure inside your lungs, and that's going to change based on whether you're inhaling or exhaling. Then you do have the intraplural pressure, that's always going to be right there about negative 4 millimeters of mercury. That's because your lungs want to recoil and deflate, but they're being held in place by the tension created by your visceral and parietal pleura. I know this is complicated, but unless your lungs are deflating and you're having a new, because of a pneumothorax, that's generally not in play here. So just remember, the two key pressures we care about are the pressure inside your lungs, your intra alveolar or intrapulmonary pressure, and the pressure in the atmosphere, your atmospheric pressure. So let's go ahead, again, I don't want to give too bogged down the details. Let's look at what happens when you inspire or breathe in or inhale or expiration or exhaling, and then we're going to talk about the different types of breathing here as well. So on the left-hand side here, you see this person is inspiring or inhaling. So when that happens, normally, normal quiet breathing, the diaphragm will contract, and that will cause it to drop down. The external intercostals will contract, and that's going to elevate your ribs. So your diaphragm has dropped down, your ribs have expanded. This has increased the volume of your thoracic cavity, which has decreased the pressure and air is going to rush in because of that pressure change. That's how we inhale. Exhalation, normal quiet exhalation, is really going to be a passive process. There aren't any muscles that have to contract for you to exhale. The diaphragm will relax and pop back up. Those external intercostal muscles will relax and your rib cage will pop back down. Now you're literally squeezing air out of your lungs back into the atmosphere because now the pressure in the thoracic cavity is higher than atmospheric pressure, and you're going to exhale. Now don't be confused by these arrows. I wish they weren't there, but they're actually showing. They're talking about the ribs going up and going down, not air. When you inhale, air is coming in. When you exhale, air is going out. OK, types of breathing. I've already mentioned quiet breathing a few times. So quiet breathing, normal quiet breathing like I'm doing here, hopefully you're doing there. It is a very simple process. About 75% of a normal quiet breath in is caused by the diaphragm dropping. The other 25% is caused by the external intercostal muscles contracting. Normal quiet breath out. When you exhale, requires no muscle activity, just the relaxation of those muscles and then the lungs, which are elastic, when they snap back into shape, that's going to be exhaling. That is normal quiet breathing. Forced breathing is when you're literally sucking air in and blowing air out, like when you're exercising or playing a sport or something. This is going to involve a lot more muscle activity. So forced inhalation will still involve the diaphragm and the external intercostals, but it's also going to involve the muscles that raise the ribs, a lot of like your neck muscles. I think of like the scalene muscles, your trapezius muscles. These muscles will play a role in helping to pull your rib cage open. So those will be some muscles involved in forced inhalation, forced inspiration. Forced expiration, think of you, hands on your hips, literally blowing air out. That's going to involve your abdominal muscles. So forced exhalation, when you're exhaling as hard as you can, it will involve your abdominal muscles, your rectus abdominis, your internal intercostal muscles as well as your oblique muscles. So your abdominal muscles will contract, which will push the diaphragm. So basically, it's going to push the organs up into the diaphragm, which is going to push the diaphragm up even harder. And that's how you're going to squeeze out some more air. So normal quiet breathing only involves the diaphragm and the external intercostals. Forced breathing involves all of these accessory muscles of respiration, like your scalenes, like your internal intercostals, and like your abdominal muscles. OK, one last thing to say here, and then we'll be done. We've been talking about quiet breathing versus forced breathing. There is some more terms you might see with breathing. If you've ever been maybe in band or music, maybe you've heard this idea of diaphragmatic breathing versus chest or costal breathing. I think this is a pretty big deal. So diaphragmatic breathing involves the diaphragm, taking a big, deep, satisfying breath. Costal breathing, if you're a chest breather, a lot of your breathing is going to come from the chest. And so clinically, the reason this is so important is diaphragmatic breathing is generally a lot easier on the body and involves less of these muscles. So I've had lots of patients that had a lot of neck and shoulder tension, primarily caused by postural issues. So usually, poor posture can lead to you becoming more of a chest breather than a diaphragmatic breather. The other thing is maybe you're holding in your stomach, trying to hold in your gut, whatever. I think that's common as well. And that can impact your ability to use your diaphragm. So you might become a chest breather. So if you get a lot of headaches, have a lot of neck tension, have poor posture, you should really be thinking about this. Simplest way to tell if you're a costal or chest breather or a diaphragmatic breather would be lay on your back, put a hand on your chest and a hand on your stomach. When you breathe, your lower hand that's on your stomach should move about twice as much as your upper hand. And if that's not true, maybe you should start to, maybe try some relaxation and breathing exercises where you focus on using your diaphragm. A little bit of a tip here. Sorry, my hi-h's. If you want to learn how to use your diaphragm to breathe better, try breathing through a straw. You really, you have to use your diaphragm if you're sucking air in through a straw. So just a little tip for you. Okay, so that is the mechanics associated with breathing and the pressures involved. I hope this helps. Have a wonderful day. Be blessed.