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  • All members of the kingdom Animalia need oxygen to make energy.

  • Oxygen is compulsory. Without oxygen, we die.

  • But as you know, the byproduct 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 have to 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,

  • collect the CO2 that we don't need with our veins,

  • and dispose of it with the lungs when we exhale.

  • 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 starters, 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 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,

  • the 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 of 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 of oxygen-dissolving membrane.

  • That's bigger than the roof of my house!

  • And the simple diffusion that your lungs use

  • is pretty freakin' 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 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, "Yeah, that's great Hank,

  • but how do lungs actually work?

  • Like how do they do the thing where they do

  • where they get moved 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 arch 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, "Whoa, what about the heart?!

  • Isn't the heart the whole point of the circulatory system?"

  • Well settle down! I'm going to explain.

  • We're used to talking 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-8:

  • 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 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 a muscle, is set up so that one chamber,

  • the left ventricle, has very high pressure.

  • In fact, the reason it seems like the heart is situated

  • a little bit to the left of center is because the left ventricle

  • is so freaking enormous and muscley.

  • 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,

  • blood vessels that carry 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 very fine

  • little capillary beds, which, through their huge surface area,

  • facilitate the delivery of oxygen