All members of the kingdom Animalia need oxygen to make energy. Oxygen is compulsory. Without oxygen, we die. But as you know, the by-product of the process that keeps us all alive, cellular respiration, is carbon dioxide or CO₂, and it doesn't do our bodies a bit of good. So not only do we need to take in the oxygen, we also gotta get rid of the CO₂. And that's why we have the respiratory and circulatory systems, to bring in oxygen from the air with our lungs, circulate it to all of our cells with our heart and arteries, and collect the CO₂ that we don't need with our veins and dispose of it with the lungs when we exhale.

(Intro)

Now when you think of the respiratory system, the first thing that you probably think of is the lungs. But some animals can take in oxygen without lungs by a process called simple diffusion, which allows gases to move into and pass through wet membranes. For instance, arthropods have little pores all over their bodies that just sort of let oxygen wander into their body where it's absorbed by special respiratory structures. Amphibians can take in oxygen through their skin, although they also have either lungs or gills to help them respire because getting all your oxygen by way of diffusion takes freaking forever.

So why do we have to have these stupid lung things instead of just using simple diffusion? Well, a couple of reasons. For starter, the bigger the animal, the more oxygen it needs and a lot of mammals are pretty big. So we have to actively force air into our lungs in order to get enough oxygen to run our bodies. Also, mammals and birds are warm-blooded, which means that they have to regulate their body temperatures, and that takes many, many calories, and burning those calories requires lots of oxygen. Finally, in order for oxygen to pass through a membrane that membrane has to be wet. So for a newt to take oxygen in through its skin, the skin has to be moist all the time, which you know, for a newt isn't a big deal. But you know, I don't particularly want to be constantly moist. Do you?

Fish need oxygen too of course, but they absorb oxygen that's already dissolved in the water through their gills. If you've ever seen a fish gill, you'll remember that they're just sort of a bunch of filaments and tissue layered together. This gill tissue extracts dissolved oxygen and excretes the carbon dioxide. Still, there are some fish that have lungs, like lungfish, which we call lungfish because they have lungs. And that's actually where lungs first appeared in the animal kingdom. All animals from reptiles on up respire with lungs deep in their bodies, basically right behind the heart.

So while us more complex animals can't use diffusion to get oxygen directly, our lungs can. Lungs are chock-full of oxygen-dissolving membranes that are kept moist with mucus. Moist with mucus: another great band name. The key to these bad boys is that lungs have a ton of surface area so they can absorb a lot of oxygen at once. You wouldn't know from looking at them, but human lungs contain about 75 square meters (807 square feet) of oxygen-dissolving membrane. That's bigger than the roof of my house. And the simple diffusion that your lungs use is pretty freaking simple.

You and I breathe oxygen in through our nose and mouth. It passes down a pipe called your larynx, which then splits off from your esophagus and turns into your trachea, which then branches to form two bronchi, one of which goes into each lung. These bronchi branch off again, forming narrower and narrower tubes called bronchioles. These bronchioles eventually end in tiny sacs called alveoli. Each alveolus is about a fifth of a millimeter (50 µm) in diameter, but each of us has about 300 million of them. And this friends, is where the magic happens.

Alveoli are little bags of thin, moist membranes, and they're totally covered in tiny narrow blood-carrying capillaries. Oxygen dissolves through the membrane and is absorbed by the blood in these capillaries, which then goes off through the circulatory system to make cells all over your body happy and healthy. But while the alveoli are handing over the oxygen, the capillaries are switching it out for carbon dioxide that the circulatory system just picked up from all over the body. So the alveoli and capillaries basically just swap one gas for another. From there, the alveoli takes that CO₂ and squeezes it out through the bronchioles, the bronchi, the trachea and finally, out of your nose and/or mouth. So inhale for me once - congratulations, oxygen is now in your bloodstream. Now exhale. Wonderful, the CO₂ has now left the building and you don't even have to think about it, so you can think about something more important, like how many Cheetos you could realistically fit into your mouth at the same time.

So now you're all like: "Yeah, that's great Hank, but how do lungs actually, like, work? Like, how do they do thing where they do where they get move to come in and out and stuff?" Well, eloquent question, well asked.

Lungs work like a pump, but they don't actually have any muscles in them that cause them to contract and expand. For that, we have this big flat layer of muscles that sits right underneath the lungs called the thoracic diaphragm. At the end of an exhalation, your diaphragm is relaxed. So picture an arc pushing up on the bottom of your lungs, and crowding them out so that they don't have very much volume. But when you breathe in, the diaphragm contracts and flattens out, allowing the lungs to open up. And as we know from physics, as the volume of a container grows larger, the pressure inside it goes down. And the fluids, including air, always flow down their pressure gradient from high-pressure to low-pressure. So as the pressure in our lungs goes down, air flows into them. When the diaphragm relaxes, the pressure inside the lungs becomes higher than the air outside, and the deoxygenated air rushes out. And that is breathing.

Now it just so happens that the circulatory system works on a pumping mechanism just like the respiratory system. Except, instead of moving air into and out of the lungs, it moves blood into and out of the lungs. The circulatory system moves oxygenated blood out of the lungs to the places in your body that needs it, and then brings the deoxygenated blood back to your lungs. And maybe you're thinking: "Well what about the heart? Isn't the heart the whole point of the circulatory system?" Well settle down, I'm going to explain.

We used to talk about the heart as the head honcho of the circulatory system. And yeah, you would be in serious trouble if you didn't have a heart. But the heart's job is to basically power the circulatory system, move the blood all around your body, and get it back to the lungs. So that it can pick up more oxygen and get rid of the CO₂. As a result, the circulatory system of mammals essentially makes a figure-eight. Oxygenated blood is pumped from the heart to the rest of the body, and then when it makes its way back to the heart again it's then pumped on a shorter circuit to the lungs to pick up more oxygen and unload CO₂, before it goes back to the heart and starts the whole cycle over again. So even though the heart does all the heavy lifting in the circulatory system, the lungs are the home base for the red blood cells: the postal workers that carry the oxygen and the CO₂.

Now the way that your circulatory system moves the blood around is pretty nifty. Remember when I was talking about air moving from high pressure to low pressure? Well so does blood.

A four-chambered heart, which is just one big honkin' beast of muscle is set up so that one chamber, the left ventricle, has very high pressure. In fact, the reason is seems like the heart is situated a little bit to the left of center is because the left ventricle is so freaking enormous and muscly. It has to be that way in order to keep the pressure high enough that the oxygenated blood will get out of there. From the left ventricle, the blood moves through the aorta - a giant tube - and then through the arteries and blood vessels that carry the blood away from the heart to the rest of the body.

Arteries are muscular and thick-walled to maintain high pressure as the blood travels along. As arteries branch off to go to different places, they form smaller arterioles and finally, the very little capillary beds which, through their huge surface area facilitate the delivery of oxygen to all of the cells in the body that need it. Now the capillary beds are also where the blood picks up CO₂. So from there, the blood keeps moving down the pressure gradient through a series of veins. These do the opposite of what the arteries did: instead of splitting off from each other to become smaller and smaller, little ones flow together to make bigger and bigger veins to carry the deoxygenated blood back to the heart.

The big difference between most veins and most arteries is that, instead of being thick walled and squeezy, veins have thinner walls and have valves that keep the blood from flowing backwards, which would be bad. This is necessary because the pressure in the circulatory system keeps dropping lower and lower until the blood flows into two major veins. The first is the inferior vena cava, which runs pretty much down the center of the body and handles blood coming from the lower part of your body. And the second is the superior vena cava, which sits on top of the heart and collects the blood from the upper body. Together, they run into the right atrium of the heart, which is the point of the lowest pressure in the circulatory system. 

So all this deoxygenated blood is now back in the heart and it needs to sop up some more oxygen. So, it flows into the right ventricle and then into the pulmonary artery. Now arteries, remember, flow away from the heart, even though in this case it contains deoxygenated blood. And pulmonary means of the lungs. So you know that this is the path to the lungs. After the blood makes its way to the alveoli and picks up some fresh oxygen, it flows to the pulmonary vein, remember it's a vein because it's flowing to the heart, even though it contains oxygenated blood, and from there it enters the heart again where it flows into the left atrium and then into the left ventricle, where it does the whole body circuit again and again and again and again.

And that is the way that we work. Our hearts are really efficient and and awesome, and they have to be because we're endotherms, or warm-blooded, meaning that we maintain a steady internal temperature. Having an endothermic metabolism is really great because you're less vulnerable to fluctuations in external temperature than ectotherms, or cold-blooded animals. Also, the enzymes that do all of the work in our bodies operate over a very narrow range of temperatures. In humans, that ranges between 36 and 36 °C (97-99 °F). The trade-off is that endotherms need to eat constantly to maintain our high metabolisms and also create heat. And for that, we need a lot of oxygen, hence the amazing efficient four-chambered heart and our gigantic frackin' lungs.

Ectotherms, on the other hand, have slow metabolisms and don't need as much in the way of food. A snake is totally pumped if it gets a meal once a month. So since ectotherms aren't doing much in the way of metabolizing, they don't need much in the way of oxygen. So their circulatory systems can be, you know, a little bit janky and inefficient, still cool. Remember back when we were tracking the development of chordates? One of the signs of complexity was the number of chambers in an animal's heart. Fish only have two chambers - one ventricle and one atrium - the blood gets oxygenated as it moves through the gills and then carries oxygen through the rest of the body back to the heart, where it's moved through the gills again. But reptiles and amphibians have three-chambered hearts. They've got two atria, but only one ventricle. And what that means is that not all of the blood gets oxygenated every time it makes a full pass around the body. So oxygenated blood gets pumped through the body and mixed with a little deoxygenated blood; not super efficient but again, it doesn't really have to be.

So there you have it, the how and why behind how oxygen gets to all the places it needs to be. The question is: What powers the diaphragm? What powers the heart? Where does that energy come from? Well, it comes from the digestive system. And that's what we're gonna be talking about next time.

Thanks for watching this episode of Crash Course Biology. If you want to go review any of the stuff we talked about today, click over there. It's all annotated up for you. Thanks to everyone who helped put this episode together. If you have any questions or ideas or thoughts, please leave those in the comments below, or on Facebook or twitter, and we will do our best. See you next time.