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 to all of the cells in
the body that need it. Now the capillary beds are
also where 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. 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 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 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 the work in our bodies operate over a very narrow
range of temperatures. In humans that range is between
36 and 37 degrees Celsius. But 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 4-chambered heart and our gigantic freakin' 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: it's 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 the blood gets oxygenated every time it
makes a full pass around the body. So oxygenated blood gets
pumped through the body and mixed up 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 going
to 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,
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.
