The Hidden Physics Behind Your Breath

Your right lung has three lobes because your heart rests in between your lungs, and below that is a weird-shaped, kinda-round, kinda-dome diaphragm muscle that's important for breathing. When the diaphragm contracts with the external intercostal muscles between your ribs, it expands the space inside of your chest, which decreases the pressure inside your lungs. Exhaling reduces the space inside your lungs and increases pressure outside, which then moves air outwards.

Breathing is one of the few things you do continuously, everyday, without thinking about it. Except now — now that I pointed it out, you’re definitely thinking about breathing. Our lives would be completely different if we couldn’t breathe air — more technically, if we couldn’t utilize oxygen. See, our bodies, like other vertebrates, depend on oxygen to run our normal aerobic metabolisms. Some of our bodily processes happen without oxygen, but to really take advantage of the food we eat, we need to burn some oxygen. That means at some point, we need to start plucking oxygen out of the air and shoving it down our throats and into our red blood cells. But here’s what I love about this topic. This biological process is really dependent on physics. Those ideal gas laws you may have learned in high school? Dalton’s law?

We see a beautiful example of them in the simple act of taking a breath. So breathe in and breathe out, today we’re talking about the respiratory system. We’ve been talking a lot about blood in this series so far, and with good reason. Multiple substances need to get to their target tissues so we can have raw materials to carry out some key physiology. Once those materials are in the bloodstream, the circulatory system delivers them to their destinations. And of course, one of the most important of those materials is oxygen. It’s everywhere around us, so all we have to do is pick it out from the air we breathe. That’s where the respiratory system comes in, all the hardware involved to breathe in and breathe out. You’re already familiar with the big players here, the lungs. You have two of these spongy pink air sacs that span from your stomach to your breastbone, and while they’re similar to each other, they’re not identical. Your right lung — your right, not the right of your screen — has three lobes, while your left lung has two. That’s because your heart rests in between your lungs, ever so slightly askew to the left in a little nook called the cardiac notch. And below all that, you’ll find a weird shaped, kinda round, kinda dome-shaped called the diaphragm. This muscle is a huge deal for breathing. See, your lungs don’t have any muscles of their own. They just go along for the ride with the rib cage. So when the diaphragm contracts along with the external intercostal muscles between the ribs, they expand the space, or volume, inside the chest. What that does is change the pressure inside the lungs since volume and pressure are inversely related. So as the volume of a container increases, the pressure on all those air molecules decreases. And vice versa — as the volume decreases, pressure increases. And yes, it may seem scary that physics is coming up in an anatomy video, but the movement of that vitally important oxygen depends on pressure differences. That’s because our lungs can be thought of as containers for gas. So when you contract your diaphragm and expand your chest’s volume, there’s less pressure on the air inside compared to the air outside your body, the stuff that you’re breathing in. This is where another physics law comes in. Whenever there’s a difference in pressure between two gases and they’re connected somehow, their pressures will tend to equalize. That means a gas will move from areas of high pressure to low pressure. It doesn’t matter if we’re talking about gas molecules in a weather system or in our physiology. Gases tend to flow from high pressure to low pressure. So with a reduced pressure inside the chest and constant pressure in the air around us, the lungs fill with air. The opposite happens when you exhale. Your diaphragm and intercostals relax, which decreases the space in your lungs, and with more pressure inside the lungs than outside, air flows outwards. Regular breathing really has nothing to do with sucking air in or squeezing air out. You’re just letting physics do its thing to your lungs. But all that depends on air actually getting into your lungs, so you have a few organs in place to get air from outside your body into your lungs. Despite starring roles in your ability to appreciate tacos, your mouth and nose are the big external interfaces for your respiratory system. And they both act as air-treatment centres, keeping the air warm and humid, and trapping any dust before it gets too far. Plus, the airway is lined with mucus membranes full of immune cells to make sure pathogens don’t creep in. Yep, the same mucus membranes that create boogers. After air comes in, it flows down cartilaginous tubes past the larynx, where we can find your vocal folds that make your beautiful voice. From here downward, your airway looks an awful lot like an upside down tree. In this case, the tree trunk, or trachea, is a thick tube of epithelial tissue surrounded by C-shaped cartilage rings. It traces the path of your sternum, right about here, where it splits. Then the trachea branches off into two bronchi. Those branches keep splitting off further and further throughout the lung until they become little twigs, or bronchioles. These twigs are only about a millimetre thick and at this point they’re not producing any mucus. Each of those tiny bronchial branches have anywhere from two to eleven leaves, or alveoli. Not to be confused with ravioli which is a delicious pasta dish and has nothing to do with breathing. These alveoli leaves interact with gases really similarly to how real leaves interact with the Carbon Dioxide around them. These alveoli, hundreds of millions of them are where air really starts interacting with our physiology. See, those alveoli have really thin walls, and they’re surrounded by extremely tiny blood vessels called capillaries which also have really thin walls. In order for us to get oxygen in and carbon dioxide out, those gas molecules need to cross this barrier.

Remember, these aren’t thick concrete walls, they’re squishy, mobile cells that readily let certain substances cross. But these are gas molecules, they’re not actively swimming through fluid. So how do they get across the membrane? It happens thanks to diffusion, a physics concept you’re familiar with if you use a perfume or spray deodorant. At first, the concentration is greatest around the spray bottle, so the smell is the strongest around that area. But then, as the odorant molecule spread throughout the room, even people far away from the source can smell it. And the smell is weaker at the source. Those molecules spread out evenly throughout the room. In the case of diffusing oxygen, it diffuses through the membrane. That same principle of diffusion is at work allowing oxygen into our bloodstream but of course, that comes with some asterisks. One of those is because air is transferring from a gas, the atmosphere, to a liquid, your blood. So any given air molecule has to be soluble, or able to dissolve in your blood, if you want it to travel through your circulation. For example, carbon dioxide is very soluble in liquids, while Nitrogen, literally eighty percent of the air we breathe, is not very soluble. Now, oxygen isn’t very soluble either, but it takes advantage of another reason that gases move — pressure differences. If you had equal levels of oxygen in your alveoli and in the capillaries around them, oxygen wouldn’t move across the barrier. But when we study the movement of dissolved particles between a liquid and a gas like in this instance, we have to compare the pressures of individual gases. I’ll explain. Air pressure itself is a thing because a bunch of different gas particles collide and bump into the walls of their container. When we measure those forces for a given sample of gas, we call that pressure. When they collide faster and harder, that’s a greater pressure — when there are less and weaker collisions, that’s less pressure. Now, if you were to take some of the gas particles away from a sample, you would change the overall pressure. After all, those molecules were contributing to all the bumps and collisions. And through the beautiful and strange magic of math, as long as we know the concentration of the gases in a sample, and the volume doesn’t change, we can calculate how much each of those gases contributes to overall pressure. This is called partial pressure, how much pressure each gas exerts by itself. And in the real world, the air in our alveoli is a mixture of gases — mostly Nitrogen but also Oxygen, water vapor, and carbon dioxide. The partial pressure of these gases drives gas exchange all over the body. This is why I spent so much of your time talking about partial pressure.

Oxygen isn’t a person, it can’t move across membranes just because it feels like it. It just goes along for the ride. Speaking of a ride, buckle up, we’re about to follow an oxygen molecule around the body. Starting in the alveoli, the partial pressure of oxygen is higher than in the capillaries around it, and with that partial pressure difference, oxygen flows into the blood. Those oxygen molecules bind to the haemoglobin in red blood cells and get transported around the body to oxygen hungry tissues. Once it gets to the tissues, we see partial pressure differences again! Its partial pressure in the tissues is lower than the blood, so it flows into the tissues. The tissues consume that oxygen as part of their aerobic metabolism and produce carbon dioxide as a byproduct. Then to get rid of that CO2, they dump it back into the bloodstream. It’s not as crude as it sounds, but either way your body now has carbon dioxide heading back to the lungs via blood. When that blood makes its way back to the lungs, the partial pressure of carbon dioxide is higher in the blood than the alveoli, so it flows out. And just like that, oxygen comes in, carbon dioxide goes out and we keep on living. So at this point we know how we get oxygen into our bodies and how we deliver it to different tissues.