Respiratory System, Part 1: Crash Course A&P #31
29
August

By Adem Lewis / in , , , , , , , , , , , , , , , , /


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 water. And a little over 380 million years ago, this
was starting to lose some of its charm. 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 talkin’ about lungs. Wet lungs. With an efficient new way to take in nearly
limitless amounts of oxygen from 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
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 are 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 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 off 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 Patreon 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.


100 thoughts on “Respiratory System, Part 1: Crash Course A&P #31

  1. Does anybody else get it when they think about how they breath and try do it manually, you forget how to breath?

    Just me…..

  2. I really enjoy your videos. I am getting back into school after almost a 20 year break and your animations are Awesome!!

  3. Thanks respiratory system
    Fact: If we still only had simple diffusion it would take almost 2 years for the brain to get oxygen and the brain only last 6 minutes with no oxygen
    so thank evolution

  4. Took us 400 million years to be able to breathe air. Took us 8:20 minutes to understand how this all happened and how it works. Life logic.

  5. thumbs up if you were aware of your breathing throughout the entire video, an amazing and entertaining way to gain knowledge by the way. thank you!

  6. While it is true that these "proto-fish" were the pioneers of breathing air among the chordates, some arthropods, annelids and molluscs had started living a terrestrial life long before. In fact, their presence was a great lure for the vertebrates to come out of water in the first place, as they looked quite yummy…

  7. This video is interesting as our breathing evolved from a fish , that, now, breathes using gills. Also im interested how long did it take for the fish to evolve from bacteria or other fish

  8. Interesting but had to turn off – Slow down man when you talk!! Its not a race to the finish line … You must have big lungs to talk that fast! 🙂

  9. so useful and thanks a lot crash course and i request to too many videos like this to get useful of this thnx thnx a lot im very much satisfied on crash course

  10. man man man man man mnan man man mna man ama mna man man man mnan mna man man am man man man man man man man man m
    were were were were were were were were were were were were were were were were were were were were were were were were were were

    than than than than than than than than than than than than than thsan than than than thab=

  11. GUYS – can any one point me to a good video for this please?! "explain the need for transport and ventialltion systems in a multi-cellular organism" please?! Any help would be amazing. Level 3 M criteria!

  12. My brain can’t handle the way he says ‘alveoli’. My teachers have always taught us ‘al-vee-oh-lee’ rather than ‘alvee-oh-lie’. Probably because we’re British, tho…

  13. hank is so funny in these videos I love it 🙂 lectures at uni are cool but I alwaysalways have to come home and check out crash course to get me really into it 🙂

  14. We learned how the respiratory system works today, but our bodies knew how to do it before that. As well as other systems, where the brain would send neurons to trigger hormones or molecules to create action, but we (our brain) didn't how things were taking action…

  15. honestly u are pathetic. what a useless fairy tale to start a video with so much potential.. but no u had to tell a bed time story and wish us all sweet dreams. nice religion.

  16. He was so happy about our WET LUNGSSSSS…..why so happy about wet lungs, bloody breath mints, angler fish and such….

  17. A sweet fast and good 9 minutes video teaching amazingly the very boring chapter of human physiology. Thanks! ❤

  18. We are not related to a fishy creature. God created us in his own image. As man. We did not evolve over millions of years.

  19. I love the knowledge we learn from these videos, but man the sense of humour is one of the best qualities

  20. The human body was beautifully designed by God, not a chance of a hundred million years you say, the introduction of your video is pure speculation

  21. contrast between bulk flow and simple diffusion kinda reminds me of bulk transport (involving membrane-bound vesicles) within a cell

  22. omg thanks
    I have a test tomorrow and I was panicking so bad but after watching this I feel soo confident
    but like he was speaking so fast that I had to slow the video down by 0.75 just to understand everything but still thanks

  23. Why do people still teach science as if evolution is a fact when it is a theory at best and when the majority of the world's population believes in creation??

  24. This video is the most racist thing that I have ever seen. It implies that Asian military look like fish. This is disgusting.

  25. my question about the fish 380 trillion years ago. how did it breed? and how did its original DNA change from extracting oxygen from liquid to land?

  26. y does this channel infer that evolution is a fact? it's just a theory, its concept doesn't even follow the scientific method. and if you dig deep into ul see most scientists who support the idea of evolution are atheists/agnostics. the whole idea of evolution was created to replace the belief in the existence of a god… these atheists say they don't believe in miracles but really they do, which is evolution, more specifically the beginning of evolution where chemistry becomes biology. they've clearly succeeded in infecting the west with this belief because apparently if you don't agree with it then you're a caveman. and btw I'm not against science.

  27. I never cease to be amused at the people that still cling to the unproven, antiquated theory of Darwinian evolution, born in the mind of one that was no scientist. Check the literature, mathematically, and genetically, it is dead in the water.

  28. This course is both concise and helpful. I must say though that the idea of a fish coming on land and "growing" a respiratory system seems more like religious superstition. How does a system this complex, just grow? Especially if it was supposed to grow gradually.

  29. What wonderful graphics and such an amazing way to explain the difficult concepts about the human body. Thankyou for your video.
    Just a small request –
    I wish you could speak a tad bit slower though. The information was moving too fast to take it all in.

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