Respiratory system, an introduction: To begin with, let me introduce you to one of the bravest pioneers in the history of life on planet Earth. An organism that blazed the trail for every single vertebrate that lives on land today — and many that don’t. It’s one of your most important ancestors. Meet…well, it doesn’t have a name. And we don’t know exactly what it looked like, either. But we do know that about 380 million years ago, this fishy-looking thing with big, fleshy fins achieved one of the animal kingdom’s greatest milestones: breathing air. Sounds simple enough, but believe me it wasn’t. Because, for billions of years before this fishy ancestor came around, basically all of life evolved in water. From the very beginning, the earliest, simplest forms of life — like bacteria — extracted oxygen they needed right from the water, through their membranes.
And they did it through simple diffusion — when a material automatically flows from where it is concentrated, to where it is less concentrated, so it balances out. Diffusion works really well, and it requires zero effort, but it wasn’t gonna cut it in the big leagues. Anything larger than a small worm is simply too big and needs too much oxygen for diffusion to work. So in order to get bigger, early life forms needed a circulatory system that could move bulk amounts of oxygen around faster inside their bodies, and a respiratory system to bring more oxygen in contact with their wet membranes. So their respiratory surfaces moved from their outer surfaces to the insides of their bodies. First, there were gills. But gills, of course, still only work inside of the water. And a little over 380 million years ago, this was starting to lose some of its charms. Earth was getting warmer, the seas were getting shallower, and much of the planet’s surface water had lower concentrations of oxygen than it used to. Finally, a humble little lobe-finned fish got fed up, swam up to the water’s surface, and started breathing air.
It could do this because it had evolved a fancy new interface to move gases between the air and its cell membranes. I’m talking about lungs. Wet lungs. With an efficient new way to take in nearly limitless amounts of oxygen from the air, animals were eventually able to get bigger and more diverse over the ages, and now all of us lung-having vertebrates share that common ancestor. For lots of animals, including humans, those lungs come with a bunch of other equipment, like protective ribs, a stiff trachea, and in mammals a strong diaphragm. And together, they form your respiratory system. Which happens to be best friends and business partners with your circulatory system. It’s only by working together and using both the bulk flow and simple diffusion of oxygen that they can make possible the process of cellular respiration. In other words: life itself. So, a lot of improvements have been made to it over the eons, but the respiratory system that you are using right now is your inheritance from that ancient, ambitious fish — leader of one of the most important anatomical revolutions of the past half-billion years.
Pretend for a minute that you can’t breathe. Like, you just don’t have lungs anymore. You are some bizarre evolutionary oddity — a huge, human-shaped organism that doesn’t have a respiratory system. Instead, you get all of your oxygen the way that your oldest, smallest evolutionary ancestors did — by simple diffusion. Or at least, you try to get your oxygen that way. How would it work? Well, poorly. And that’s partly because one of the keys to efficient diffusion of any material is distance. If you want a molecule to diffuse across a particular space quickly, you want it to be as close to its destination as possible, with the fewest obstacles in the way.
But, for a single molecule of oxygen to diffuse from the air through, say, your scalp and then go to a neuron deep inside your brain, it would have to move through your skin, and then your skull, and then your connective tissue and all sorts of things. It would eventually get there, like maybe a month later, but at that point, the cell that needed the oxygen in the first place would have, you know, suffocated to death. Basically, obtaining oxygen through diffusion alone is like wanting to go to a party at your friend’s place across town, and then walking 20 miles to get there. You could do it, but it would take forever, and by the time you arrived, you’d be all haggard and the party would be over. So, diffusion alone isn’t enough to get the job done. We do use it, but only when a whole bunch of the materials we need is right up against the tissues that can absorb them.
So you know what else we need? Bulk flow. Bulk flow is like public transportation — it moves large numbers of molecules, quickly. Rather than walk the whole way across town, you can hop on a bus with a bunch of other people, and get there in twenty minutes. Every time you take a deep breath, you’re bringing a hundred quintillion oxygen molecules into your lungs all at once — they’re on a bulk-flow bus ride. And once those oxygen molecules filter down into the cells in your lungs, they’re suddenly very close to the blood they’re trying to reach.
All they have to do is diffuse across four layers of cell membranes to get from the lung cell into the blood. It’s like just hopping off the bus, and then walking half a block to your friend’s apartment. That’s why your respiratory system is the way it is: It’s set up to take full advantage of both bulk flow and simple diffusion. The bulk flow part of things is handled by some of your system’s biggest and most obvious moving parts. Starting with your lungs, which basically operate like a pump or a bellows. They don’t have any contractible muscle tissue, because they need to be able to expand, so they require outside help in order to move.
Enter the diaphragm — a big, thin set of muscles that separates your thorax from your abdomen. When your lungs empty, your diaphragm relaxes and looks kinda like an arc pushing up to squish your lungs. You also have the weight of your rib cage, pushing on your lungs from the top and sides, and together these forces decrease the volume of your lungs. When you breathe in, your diaphragm contracts, pulling itself flat, and your external intercostal muscles between your ribs contract.
They lift the ribs up and out, causing the chest cavity to expand. This makes the pressure inside the lungs lower than the air outside your body, and — since fluids like gases move from areas of high pressure to low pressure — the lungs fill up with outside air. Then the diaphragm relaxes again, and the weight of the ribs settles in, and the pressure inside the lungs becomes higher than the outside air, and the air rushes out. And that, my friends, is breathing 101. Now, your respiratory system contains a lot of parts besides your lungs — some prominently displayed on your face, others are hidden deep within your chest. And functionally, all of these organs fall into one of two physiological zones. The upper parts that funnel the air in, make up what’s known as the conducting zone, and it starts with this thing.
Your nose is supported by bone and cartilage, and the bristly hairs and mucus inside it that help filter out dust and other particles. But it, along with your sinuses, performs another important function: It warms and moistens incoming air, so it doesn’t dry out those sensitive lung cells that must remain wet. Remember, moisture is key. We evolved from organisms that lived in water. So, just like with our aquatic bacterial ancestors, we need water for oxygen to dissolve into, before it can diffuse across the phospholipid bilayer membrane of our cells. Now, if you’ve ever choked on a poorly timed sip of water, you’ve noticed that you breathe through the same tube that you also move foods and liquids through. This is yet another leftover from those first fish lungs, which evolved as a branch of the esophagus.
Looking back, it was not ideal. But we are stuck with it. So, the stuff that you swallow soon encounters the epiglottis — a little trap door of tissue — which covers the larynx, and directs bites of sandwich and sips of cola toward your esophagus and keeps them out of your lungs. And you’ll notice that the esophagus, which heads to your stomach, is nice and flexible, while your trachea, or windpipe, is rigid and has prominent rings. That’s because your trachea is basically built like a vacuum hose — since the lungs create negative pressure with every breath, the trachea needs those rings to keep it open. If it were soft and floppy, it would collapse every time the pressure dropped, and you wouldn’t be able to breathe.
From there, the trachea splits in two, forming the right and left main bronchi. You can imagine these inner lung parts as sort of an upside-down tree. Now we are in the lung tissue and have entered what we call the respiratory zone. This is where the actual gas exchange occurs, and everything you find here has a form to suit that function. So the smaller branches of the upside-down tree are bronchioles, which taper down into progressively narrower tubes until they empty into the alveolar ducts and then dead-end into tiny alveolar sacs, where the bulk of the gas exchange finally occurs.
Because that’s because each sac contains a cluster of alveoli, these tiny cavities lined with super-thin, wet membranes made of simple squamous epithelium tissue. It’s here that oxygen molecules dissolve in the wet mucous, diffuse across the epithelial cells, and then cross the single layer of endothelial cells lining the capillaries to enter the bloodstream. And of course, it’s also where carbon dioxide diffuses out of the blood and then follows the same route back up to the nose and mouth, where it’s exhaled. So it’s your alveoli where diffusion meets bulk flow. Because while you’re picking up oxygen and dispensing with CO2 one molecule at a time, you’re doing it in enormous quantities at any given second. Both of your lungs contain about 700 million alveoli, which together provide an amazing 75 square meters of moist membrane surface area. So, the principles that make respiration possible are relatively simple — diffusion and bulk flow.
And so are the mechanisms in your body that use them. It just took us about 400 million years to figure out how to make it all work. But today you learned how it does work — including the mechanics of both simple diffusion and bulk flow, and the physiology of breathing, and the anatomy of the conducting zone, and the respiratory zone, of your respiratory system. Thank you to all of our patrons who help make Crash Course possible for themselves and for everyone in the world for free with their monthly contributions.
If you like Crash Course and you want to help us keep making videos like this one, you can go to patreon.com/crashcourse. This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed and edited by Nicholas Jenkins; the script supervisor was Nicole Sweeney; our sound designer is Michael Aranda, and the graphics team is Thought Cafe.
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