let me introduce you to one of the bravestpioneers in the history of life on planet earth. an organism that blazed the trail for everysingle vertebrate that lives on land today -- and many that donâ€™t. itâ€™s one of your most important ancestors. meetâ€¦well, it doesnâ€™t have a name. and we donâ€™t know exactly what it lookedlike, either. but we do know that about 380 million yearsago, this fishy-looking thing with big, fleshy fins achieved one of the animal kingdomâ€™sgreatest milestones: breathing air. sounds simple enough, but believe me it wasnâ€™t.
because, for billions of years before thisfishy ancestor came around, basically all of life evolved in water. from the very beginning, the earliest, simplestforms of life -- like bacteria -- extracted oxygen they needed right from the water, throughtheir membranes. and they did it through simple diffusion -- whena 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 zeroeffort, but it wasnâ€™t gonna cut it in the big leagues. anything larger than a small worm is simply toobig and needs too much oxygen for diffusion to work. so in order to get bigger, early life formsneeded a circulatory system that could move
bulk amounts of oxygen around faster insidetheir bodies, and a respiratory system to bring more oxygen in contact with their wetmembranes. so their respiratory surfaces moved from theirouter surfaces to the insides of their bodies. first, there were gills. but gills, of course,still only work inside of water. and a little over 380 million years ago, thiswas starting to lose some of its charm. earth was getting warmer, the seas were gettingshallower, and much of the planetâ€™s surface water had lower concentrations of oxygen thanit used to. finally, a humble little lobe-finned fish got fed up, swamup to the waterâ€™s surface, and started breathing air. it could do this because it had evolved afancy new interface to move gases between
the air and its cell membranes. iâ€™m talkinâ€™ about lungs. wet lungs. with an efficient new way to take in nearlylimitless amounts of oxygen from air, animals were eventually able to get bigger and morediverse over the ages, and now all of us lung-having vertebrates share that common ancestor. for lots of animals, including humans, thoselungs come with a bunch of other equipment, like protective ribs, a stiff trachea, andin mammals a strong diaphragm. and together, they form your respiratory system. which happens to be best friends and businesspartners with your circulatory system.
itâ€™s only by working together and usingboth the bulk flow and simple diffusion of oxygen that they can make possible the processof cellular respiration. in other words: life itself. so, a lot of improvements have been made toit over the eons, but the respiratory system that you are using right now is your inheritancefrom that ancient, ambitious fish -- leader of one of the most important anatomical revolutionsof the past half-billion years. pretend for a minute that you canâ€™t breathe.like, you just donâ€™t have lungs anymore. you are some bizarre evolutionary oddity -- ahuge, human-shaped organism that doesnâ€™t have a respiratory system.
instead, you get all of your oxygen the waythat your oldest, smallest evolutionary ancestors did -- by simple diffusion. or at least, you try to get your oxygen thatway. how would it work?well, poorly. and thatâ€™s partly because one of the keys toefficient diffusion of any material is distance. if you want a molecule to diffuse across aspace quickly, you want it to be as close to its destination as possible, with the fewestobstacles in the way. but, for a single molecule of oxygen to diffusefrom 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 connectivetissue and all sorts of things. it would eventually get there, like maybea month later, but at that point, the cell that needed the oxygen in the first placewould have, you know, suffocated to death. basically, obtaining oxygen through diffusionalone is like wanting to go to a party at your friendâ€™s place across town, and thenwalking 20 miles to get there. you could do it, but it would take forever, and by the time you arrived,youâ€™d be all haggard and the party would be over. so, diffusion alone isnâ€™t enough to get the job done.we do use it, but only when a whole bunch of the materials we need are right up againstthe tissues that can absorb them. so you know what else we need? bulk flow.
bulk flow is like public transportation -- itmoves 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, youâ€™rebringing a hundred quintillion oxygen molecules into your lungs all at once -- theyâ€™re ona bulk-flow bus ride. and once those oxygen molecules filter downinto the cells in your lungs, theyâ€™re suddenly very close to the blood theyâ€™re trying toreach. all they have to do is diffuse across four layers of cell membranes to get fromthe lung cell into the blood. itâ€™s like just hopping off the bus, and thenwalking half a block to your friendâ€™s apartment.
thatâ€™s why your respiratory system is theway it is: itâ€™s set up to take full advantage of both bulk flow and simple diffusion. the bulk flow part of things is handled by some ofyour systemâ€™s biggest and most obvious moving parts. starting with your lungs, which basicallyoperate like a pump, or a bellows. they donâ€™t have any contractible muscletissue, 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 musclesthat separates your thorax from your abdomen. when your lungs empty, your diaphragm relaxes andlooks 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 volumeof your lungs. when you breathe in, your diaphragm contracts,pulling itself flat, and your external intercostal muscles between your ribs contract. they lift theribs up and out, causing the chest cavity to expand. this makes the pressure inside the lungs lowerthan the air outside your body, and -- since fluids like gases move from areas of high pressureto low pressure -- the lungs fill up with outside air. then the diaphragm relaxes again, and theweight of the ribs settles in, and the pressure inside the lungs becomes higher than the outsideair, and the air rushes out. and that, my friends, is breathing 101. now, your respiratory system contains a lotof parts besides your lungs -- some prominently
displayed on your face, others hidden deepwithin your chest. and functionally, all of these organs fall into one of two physiologicalzones. the upper parts that funnel the air in, makeup whatâ€™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, performsanother important function: it warms and moistens incoming air, so it doesnâ€™t dry out thosesensitive lung cells that must remain wet. remember, moisture is key. we evolved fromorganisms that lived in water. so, just like
with our aquatic bacterial ancestors, we needwater for oxygen to dissolve into, before it can diffuse across the phospholipid bilayermembrane of our cells. now, if youâ€™ve ever choked on a poorly timedsip of water, youâ€™ve noticed that you breathe through the same tube that you also move foodsand liquids through. this is yet another leftover from those firstfish lungs, which evolved as a branch off the esophagus. looking back, it was notideal. but we are stuck with it. so, the stuff that you swallow soon encountersthe epiglottis -- a little trap door of tissue -- which covers the larynx, and directs bitesof sandwich and sips of cola toward your esophagus and keeps them out of your lungs.
and youâ€™ll notice that the esophagus, whichheads to your stomach, is nice and flexible, while your trachea, or windpipe, is rigidand has prominent rings. thatâ€™s because your trachea is basically builtlike a vacuum hose -- since the lungs create negative pressure with every breath, the trachea needsthose rings to keep it open. if it were soft and floppy, it would collapse every time the pressure dropped,and you wouldnâ€™t be able to breathe. from there, the trachea splits in two, formingthe right and left main bronchi. you can imagine these inner lung parts as sort of an upside-downtree. now we are in the lung tissue, and have enteredwhat we call the respiratory zone. this is where the actual gas exchange occurs, and everythingyou find here has a form to suit that function.
so the smaller branches of the upside-downtree are bronchioles, which taper down into progressively narrower tubes, until they emptyinto the alveolar ducts and then dead end into tiny alveolar sacs, where the bulk ofthe gas exchange finally occurs. because thatâ€™s because each sac containsa cluster of alveoli, these tiny cavities lined with super thin, wet membranes madeof simple squamous epithelium tissue. itâ€™s here that oxygen molecules dissolvein the wet mucous, diffuse across the epithelial cells, and then cross the single layer of endothelialcells lining the capillaries to enter the bloodstream. and of course itâ€™s also where carbon dioxidediffuses out of the blood, and then follows the same route back up to the nose and mouth,where itâ€™s exhaled.
so itâ€™s your alveoli where diffusion meets bulkflow. because while youâ€™re picking up oxygen and dispensing with co2 one molecule at a time, you'redoing it in enormous quantities at any given second. both of your lungs contain about 700 millionalveoli, which together provide an amazing 75 square meters of moist membrane surfacearea. so, the principles that make respiration possibleare relatively simple -- diffusion and bulk flow. and so are the mechanisms in your body thatuse them. it just took us about 400 million years tofigure out how to make it all work. but today you learned how it does work -- includingthe 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 respiratorysystem. thank you to all of our patreon patrons whohelp make crash course possible for themselves and for everyone in the world for free with their monthlycontributions. 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 cherylc. kinney crash course studio, it was written by kathleen yale, the script was edited byblake de pastino, and our consultant is dr. brandon jackson. it was directed and editedby nicholas jenkins; the script supervisor was nicole sweeney; our sound designer is michaelaranda, and the graphics team is thought cafe.