all members of the kingdom animalianeed oxygen to make energy. oxygen is compulsory.without oxygen, we die. but as you know, thebyproduct of the process that keeps us all alive, cellularrespiration, is carbon dioxide, or co2, and it doesn'tdo our bodies a bit of good, so not only do we needto take in the oxygen, we also have toget rid of the co2. and that's why we have therespiratory and circulatory systems to bring in oxygen fromthe air with our lungs,
circulate it to all of our cellswith our heart and arteries, collect the co2 that wedon't need with our veins, and dispose of it withthe lungs when we exhale. now, when you think ofthe respiratory system, the first thing that youprobably think of is the lungs. but some animals can takein oxygen without lungs, by a process called simplediffusion, which allows gases to move into and passthrough wet membranes. for instance, arthropods havelittle pores all over their bodies
that just sort of let oxygenwander into their body, where it's absorbed byspecial respiratory structures. amphibians can take inoxygen through their skin, although they also haveeither lungs or gills to help them respire, because getting all youroxygen by way of diffusion takes freaking forever. so why do we have to havethese stupid lung things instead of just usingsimple diffusion?
well, a couple of reasons. for starters, the bigger theanimal, the more oxygen it needs. and a lot of mammals are prettybig, so we have to actively force air into our lungs inorder to get enough oxygen to run our bodies. also mammals and birdsare warm blooded, which means they have to regulatetheir body temperatures, and that takes many, many calories, and burning those caloriesrequires lots of oxygen.
finally, in order for oxygento pass through a membrane, the membrane has to be wet,so for a newt to take oxygen in through its skin, the skinhas 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 tobe constantly moist, do you? fish need oxygen, too, ofcourse, but they absorb oxygen that's already dissolved inthe water through their gills. if you've ever seen a fish gill,you'll remember that they're just sort of a bunch of filamentsof tissue layered together.
this gill tissueextracts dissolved oxygen and excretes the carbon dioxide. still, there are somefish that have lungs like lungfish, which we calllungfish because they have lungs. and that's actually wherelungs first appeared in the animal kingdom. all animals fromreptiles on up respire with lungs deep in their bodiesbasically right behind the heart. so while us more complexanimals can't use diffusion
to get oxygen directly,our lungs can. lungs are chock full ofoxygen-dissolving membranes that are kept moist with mucus. moist with mucus...another great band name. the key to these badboys is that lungs have a ton of surfacearea, so they can absorb a lot of oxygen at once. you wouldn't know from lookingat them, but human lungs contain about 75 square metersof oxygen-dissolving membrane.
that's bigger thanthe roof of my house! and the simple diffusionthat your lungs use is pretty freakin' simple. you and i breathe oxygen inthrough our nose and mouth. it passes down a pipe calledyour larynx which then splits off from your esophagusand turns into your trachea, which then branchesto form two bronchi, one of which goes into each lung. these bronchi branch offagain, forming narrower
and narrower tubescalled bronchioles. these bronchioles eventuallyend in tiny sacs called alveoli. each alveolus is about a fifthof a millimeter in diameter, but each of us has about300 million of them, and this, friends, iswhere the magic happens. alveoli are little bagsof thin, moist membranes, and they're totally covered in tiny, narrow blood-carrying capillaries. oxygen dissolves throughthe membrane and is absorbed
by the blood in these capillaries,which then goes off through the circulatorysystem to make cells all over your bodyhappy and healthy. but while the alveoli arehanding over the oxygen, the capillaries are switchingit out for carbon dioxide that the circulatory system justpicked up from all over the body. so the alveoli and capillariesbasically just swap one gas for another. from there, the alveoli takesthat co2 and squeezes it out
through the bronchioles,the bronchi, the trachea, and finally out of yournose and/or mouth. so inhale for me once!congratulations! oxygen is now inyour bloodstream! now exhale! wonderful! the co2 has nowleft the building! and you don't even have to thinkabout it, so you can think about something more importantlike how many cheetos you could realistically fit intoyour mouth at the same time!
so, now you're all,"yeah, that's great hank, but how do lungs actually work? like how do they do thething where they do where they get moved to comein and out and stuff?" well, eloquent question!well asked! lungs work like a pump,but they don't actually have any muscles in them thatcause them to contract and expand. for that we have thisbig, flat layer of muscles that sits right underneath thelungs called the thoracic diaphragm
at the end of an exhalation,your diaphragm is relaxed, so picture an arch pushingup on the bottom of your lungs and crowding them out so thatthey don't have very much volume. but when you breathe in,the diaphragm contracts and flattens out, allowingthe lungs to open up. and as we know fromphysics, as the volume of a container grows larger,the pressure inside it goes down. and the fluids, includingair, always flow down their pressure gradient, fromhigh pressure to low pressure.
so as the pressure in our lungsgoes down, air flows into them. when the diaphragm relaxes,the pressure inside the lungs becomes higher thanthe air outside, and the deoxygenatedair rushes out. and that is breathing! now, it just so happens thatthe circulatory system works on a pumping mechanismjust like the respiratory system. except, instead of moving airinto and out of the lungs, it moves blood intoand out of the lungs.
the circulatory system movesoxygenated blood out of the lungs to the places in yourbody that needs it and then brings the deoxygenatedblood back to your lungs. and maybe you're thinking,"whoa, what about the heart?! isn't the heart the wholepoint of the circulatory system?" well settle down!i'm going to explain. we're used to talkingabout the heart as the head honcho ofthe circulatory system. and yeah, you would be in serioustrouble if you didn't have a heart!
but the heart's job is to basicallypower the circulatory system, move the blood all aroundyour body and get it back to the lungs so that it can pick upmore oxygen and get rid of the co2. as a result, the circulatorysystem of mammals essentially makes a figure-8: oxygenated blood is pumped fromthe heart to the rest of the body, and then when it makes itsway back to the heart again, it's then pumped on ashorter circuit to the lungs to pick up more oxygen andunload co2 before it goes back
to the heart and startsthe whole cycle over again. so even though the heartdoes all the heavy lifting in the circulatory system,the lungs are the home base for the red blood cells, the postal workers thatcarry the oxygen and co2. now, the way that yourcirculatory system moves the blood around is pretty nifty. remember when i wastalking about air moving from high pressure to low pressure?
well, so does blood. a four chambered heart, whichis just one big honkin' beast of a muscle, is set upso that one chamber, the left ventricle,has very high pressure. in fact, the reason it seemslike the heart is situated a little bit to the left of centeris because the left ventricle is so freakingenormous and muscley. it has to be that way inorder to keep the pressure high enough that the oxygenatedblood will get out of there.
from the left ventricle,the blood moves through the aorta, a giant tube, and thenthrough the arteries, blood vessels that carryblood 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 offto go to different places, they form smaller arteriolesand finally very fine little capillary beds, which,through their huge surface area,
facilitate the delivery of oxygen to all of the cells inthe body that need it. now the capillary beds arealso where blood picks up co2, so from there the bloodkeeps moving down the pressure gradientthrough a series of veins. these do the opposite ofwhat the arteries did: instead of splitting offfrom each other to become smaller and smaller,little ones flow together to make bigger and bigger veins
to carry the deoxygenatedblood back to the heart. the big difference betweenmost veins and most arteries is that instead of beingthick-walled and squeezy, veins have thinner walls,and have valves that keep the blood from flowing backwards. which would be bad. this is necessarybecause the pressure in the circulatory systemkeeps dropping lower and lower, until the blood flowsinto two major veins:
the first is the inferior venacava, which runs pretty much down the center of thebody and handles blood coming from the lowerpart of your body. the second is the superiorvena cava, which sits on top of the heart and collects theblood from the upper body. together they run into theright atrium of the heart, which is the point of the lowestpressure in the circulatory system. so, all this deoxygenated bloodis now back in the heart. and it needs to sopup some more oxygen,
so it flows intothe right ventricle, and then intothe pulmonary artery now arteries, remember,flow away from the heart, even though in this caseit contains deoxygenated blood, and pulmonary means "of the lungs," so you know this isthe path to the lungs. after the blood makesits way to the alveoli and picks up some fresh oxygen,it flows to the pulmonary vein, remember it's a vein becauseit's flowing to the heart,
even though it containsoxygenated blood and from there itenters the heart again, where it flows intothe left atrium and then intothe left ventricle, where it does the wholebody circuit again. and again and again and again.and that is the way that we work! our hearts are reallyefficient and awesome, and they have to be, becausewe're endotherms, or warm-blooded, meaning that we maintain asteady internal temperature.
having an endothermicmetabolism is really great because you're lessvulnerable to fluctuations in external temperature thanectotherms, or cold-blooded animals also, the enzymes that doall the work in our bodies operate over a very narrowrange of temperatures. in humans that range is between36 and 37 degrees celsius. but the trade-off is thatendotherms need to eat constantly to maintain our highmetabolisms and also create heat. and for that we needa lot of oxygen.
hence, the amazing,efficient 4-chambered heart and our gigantic freakin' lungs. ectotherms, on the otherhand, have slow metabolisms and don't need as muchin the way of food. a snake is totally pumped ifit gets a meal once a month. so, since ectotherms aren't doingmuch in the way of metabolizing, they don't need muchin the way of oxygen. so their circulatorysystems can be, you know, a little bit janky andinefficient: it's still cool.
remember back when we were trackingthe development of chordates? one of the signs of complexitywas the number of chambers in an animal's heart. fish only have two chambers,one ventricle and one atrium. the blood gets oxygenated asit moves through the gills, and then carries oxygenthrough the rest of the body, back to the heart where it'smoved through the gills again. but reptiles and amphibianshave three-chambered hearts: they've got two atriabut only one ventricle.
and what that means isthat not all the blood gets oxygenated every time itmakes a full pass around the body. so oxygenated blood getspumped through the body and mixed up with alittle deoxygenated blood. not super efficient, but again,it doesn't really have to be. so there you have it. the how and why behindhow 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 fromthe digestive system. and that's what we're goingto be talking about next time. thanks for watching this episodeof crash course biology. if you want to go review any ofthe stuff we talked about today, click over there. it's all annotated up for you.
thanks to everyone who helpedput this episode together. if you have any questions,ideas or thoughts, please leave those in the commentsbelow or on facebook or twitter. and we will do our best. see you next time.
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