Inhaling and exhaling | Respiratory system physiology | NCLEX-RN | Khan Academy
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Inhaling and exhaling | Respiratory system physiology | NCLEX-RN | Khan Academy


If you could have
a magical ability to actually see all the
air molecules in the air, you might see
something like this. It would be a lot
more crowded, but you could imagine it
might look like this. And let’s say that
you actually decide to do something a
little interesting, and that is to take a
jar and simply capture some of the air
molecules in your jar. So I’ve got my jar here. And I’m actually going to put
a little opening on my jar. So let’s say there’s a
little opening there. And I take that
opening, and I’m going to just make it kind of
a stretched-out neck. So this is my stretched-out
neck on my jar. And there’s the
opening to my jar. And on the other side,
what I want to do is actually kind
of compare what’s going on inside of
my jar to what’s going on outside of the jar. So to make it fair,
let me actually try to create a purple
box– kind of a dashed line around an equivalent volume. So this is going
to be, basically, a similarly-sized
part of the air. And of course, this
dashed line is just to show you which part
I’m talking about, because, of course, this
is an imaginary line. But let’s say we’re
comparing what’s going on inside of my
blue jar with what’s going on inside of this
purple dashed line. Now, we know that that
purple dashed line is kind of capturing
a certain amount of the air in the atmosphere. And that air is going
to have molecules that are bouncing
off of each other. Let’s say something like this. And you’ve got a bunch of
random collisions happening. And these collisions– the more
frequently the collisions are happening, the higher
the pressure in the air. And in fact, measured
pressure in the air is around 760
millimeters of mercury. So that’s how we think
about air pressure. So that’s the pressure
in the atmosphere. And if I was to measure
my jar pressure, it would be, of
course, the same thing. It would be 760
millimeters of mercury. And as a quick
aside, just thinking about what these molecules
are, if there are five of them, then you might say that this is
nitrogen, this one is nitrogen, this one is nitrogen,
this could be nitrogen, and this one might be oxygen. Because remember, oxygen
is about 21% of air. And so that might
be a fair estimation of what these five molecules
could be– mostly nitrogen. So in the air, we’ve
got nitrogen and oxygen. It’s bouncing around
in my jar, just as it is in the
atmosphere itself. And now let’s say
I decide to do kind of an interesting experiment. I decide to drop the floor–
just stay with me here– I drop the floor of my jar. So I actually expand
the bottom of my jar. For the moment, don’t
worry so much about how that could possibly happen. Let’s just assume that I
do creatively somehow kind of drop the floor. And now it looks a
little bit lower. So the volume has
gone up in my jar. And actually,
simultaneously, I should mention– I just
want to mention here that this door, or opening, of
my jar is closed at the moment. So my opening, I’ve
put a lid on it. So that’s closed, and my floor
just got a little bit lower. So the volume has gone up. That’s the big change, right? Actually, let me write
that up here in the corner. I’m just going to erase
some of these molecules to create some space. And the first thing
I want to mention is that the volume
has gone up in my jar. So all of the green stuff
I write in the corner is going to be from
the jar’s perspective. If the volume goes
up now– if that’s the case, then these
molecules inside the jar, they’re excited. They’ve got more room to kind
of run around and play and not bump into each other. So if they’re not bumping
into each other as much– because of course, they’ve
got all this extra space down here– then the pressure
on the inside of the jar is going to go down, right? Because there are less
collisions happening. So now we’ve got, let’s
say, a slight decrease. It went to 757. So a little bit less than
what’s on the outside. So because the volume went
up, the pressure went down. And again, that’s because
you have fewer collisions. And the new pressure is 757,
which is a positive number. But sometimes people refer
to this as negative pressure, or a vacuum. And the reason they’re saying
that is because they’re saying, well, relative to 760,
relative to this number, 757 is 3 points lower. And so in that
sense, it’s negative. So if you actually want to
compare them to each other, you’d say, well, 757
minus 760 is negative 3. And that would be
a negative number. But for the time
being, I’m just going to leave it in the numbers
we have, which is 757. Now, let’s say that
I open this door. This opening is now open. If I open up this
opening, what will happen? Well, we have all
this extra space down here I circled,
but I’m just going to remove
this for the time being– all this extra space. And molecules, of
course, are being knocked around all the time. So these collisions
are happening always. And some molecules are
going to get knocked perfectly so that they
actually move into the jar. Let’s say it goes in like this. So you’re going to get
some molecules that go in. And in fact, you might
have some molecules that get knocked right out. So it’s going to
happen constantly. But overall, what’s going
to be the net difference? Well, let’s say I leave
this and I walk away and do my own thing for
a minute and come back. I’m going to notice
that there are actually extra molecules on
the inside of my jar, because there’s more space,
less crowding in my jar because of all that
extra volume I created. So over time,
there’s going to be a few extra molecules in my jar. And maybe I got lucky, and
this one’s an oxygen molecule. So I’ve got extra
molecules on the inside. And these molecules–
so actually, that would be, I
guess, the next step, is that air molecules move in. And these molecules are now
going to do what molecules do, which is kind of bounce
off of each other. So they start bouncing
off of each other. And all of a sudden,
now you’ve got– let’s say this guy
collides over here as well, and maybe there’s some bouncing
and this collides over here. So now you’ve got–
because you’ve got six molecules on the
inside and the same volume, the pressure on the
inside has gone up. So pressure has gone up
on the inside of the jar, simply because there are
more molecules in there now. So even though you had
more volume initially, you’ve kind of filled it
up with more molecules. So the pressure
goes up, let’s say to 760 millimeters of mercury. So now it’s gone back up. So this is my new pressure. And this all happened– this
whole kind of series of events happened because I
decided to move the floor. Now, what would happen if
I decide to move it back? Let’s say I decide to go back
to the original floor size. And I get rid of
this lower line, and I raise the floor back up. And so now it looks
something like this. Well, now the
volume– this is kind of the new first step,
what’s going to happen. The volume has gone down. That’s obvious, because I just
moved the floor purposefully. And I’ve got six
molecules in my jar. And they’re thrashing around,
bumping into each other. But they’ve got less
space to do it in. So the pressure
is going to go up because there are
more collisions. They’re bumping into
each other more. So the pressure
is going to go up. Pressure is going to go
up now to, let’s say, 763 millimeters of mercury. Because it was 760. And at this point, let’s
say this is closed up. And so the pressure
on the inside is 763 millimeters of mercury. Let me erase this. And that’s because, again,
you have more molecules, but you reduced the volume. So then the pressure
on the inside is actually now higher
than the outside pressure. I mean, the outside pressure is
always going to be around 760. And that’s because the
atmosphere is just enormous, right? So the movement of a few
molecules this way or that way is really not going to change
the amount of collisions that are happening in the atmosphere. That’s always going
to stay the same. And so if I was to
open this up– open this door up– then some
molecules, of course, are going to be bouncing
around, bouncing around. And some of these things
might kind of bounce out. So some molecules might
kind of bounce out. And overall, again,
on the whole, you’re going to have more
molecules bouncing out than bouncing in because you
have more collisions happening on the inside. And, again, when I say more
collisions, in your mind, I want you to think
of higher pressure. So if there’s higher pressure on
the inside and more collisions happening on the
inside, you’re going to have more things
bouncing off each other, and molecules are going
to be sent outside. So the next step I could write
in would be air molecules. Air molecules move out. So the final point is that if
air molecules are moving out– let’s say just by random
chance, this oxygen molecule happened to be the one
that got sent away. So this one kind
of got knocked out. Then you have– let me try to
erase all this to clear it up– then you have five molecules,
again, on the inside. And you have the same
volume that we initially started out with. So the pressure
on the inside goes back to what it was
in the first place. The pressure falls to 760. And the reason that
I say exactly 760 is because this process
in step three will continue until the number
of collisions on the inside and outside of
the jar are equal. So this is kind of the
process– and actually, I forgot to mention. When we were back at
763, sometimes people call this positive pressure
for the same reason they called it negative before. Because all they’re
doing is they’re comparing 763 to atmospheric
pressure, which is 760, and saying, wow,
that’s a plus 3. That’s a little bit positive. And so when you compare
things relatively, you use words like
“positive” and “negative.” But if you’re using
just the total number in kind of absolute terms, then
you would stick to 757 or 763. Now, what does all of
this has to do with us? What does a jar and an opening
have to do with human beings? Well, let me just show you
that by simply changing my drawing a little
bit, you’ll see what this has to do with us. Now, instead of having all
the molecules inside the jar– I know that you
know that they’re there– I can actually
erase all this. And maybe I can change the
shape of this a little bit to help you see
what this could be. So let’s say I
make that like this and start drawing in like that. I’m going to keep all
of this kind of the same in terms of the way it looks. Maybe like that. And you can see now,
instead of a jar, what I’m creating for you
are a pair of lungs. So this is a pair of
lungs, left and right. This’ll be right
and this’ll be left. And it’ll look
something like this. And this one might go up like
that around that cardiac notch, and go like this. And then we have, of
course, the opening– which, if instead of calling it an
opening, I can call it a mouth, this would be my mouth. And I could erase the
word “opening” completely. And I think you’ll start
seeing how this is basically what happens in our body. So our head represents
the opening of this. And this could be the nose. And this could be
the head– kind of a flat head I’ve drawn here. But you get the idea, I think. So there’s your nose
and there’s your ear. And basically, air is
coming in the mouth and going into the lungs
and back out of the lungs. And what we call this
process is “inhalation.” So when you increase the
volume, we call this “inhaling.” So if you’ve ever wondered
exactly what happens when you inhale
air, there it is. And when you close up the lungs
and air molecules move out, we call that “exhaling.” Actually, I should probably
try to make it look the same. They’re both equally important. So I’ll draw them the same way. Exhaling. And now you can see how
inhaling and exhaling happen. So with every breath,
this is the process. You kind of subtly
change the volume, and all of a sudden,
the pressure changes. Air moves in and out.

About James Carlton

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65 thoughts on “Inhaling and exhaling | Respiratory system physiology | NCLEX-RN | Khan Academy

  1. Never will i cease in shouting out my thankyous😊💜 KhanAcademy has been my own personal tutor in soo many ways!!! Thankyou for posting such helpful material for Respiratory Therapy students! I appreciate every single video….I really do! Thankyou for making such a complex physiological action simplified and explained in much detail…THANKYOU! Blessings to your team, your family and all that you do!!!!

  2. Excellent!  Such a great explanation.  I use your videos to reinforce material learned in lectures for A&P.  Always helpful.  Thanks! 🙂

  3. Thank you so much for your videos. I am a physiotherapy student currently studying Cardio-Respiratory and this has been of so much help to me. Your videos are always broken down into easy to digest segments. I would recommend them fully!
    Thanks again Khan Academy! From A-Levels to Masters!

  4. The difference between pleural and aveolor pressure should be mentioned first. It makes the whole topic quite confusing if someone doesn't get it. This video makes it look like that the lung is actually used for the breathing and not the alveoli. The lung has always a negative pressure, the aveoli don't.

  5. Absolutely incredible video. It's oversimplified, but necessary to simplify in order to get the main concepts. Great great video.

  6. I'm a beginning doctor in my first year and this has really helped me thank and subscribe to this channel it's my sons.

  7. What a great class with the febulous hand drawing and very easy understanding explanation. A worker from China. Thank KhanAcademy. Thank the teacher.

  8. You described Boyle's Law in this lecture without recognizing it 🙂 (For all of you to know!!!: P1V1=P2V2) Pressure and volume are inversely proportional. That is the main takeaway message 🙂

  9. Awe. Thank thank you a million times. I did not understand this until now. Totally make sense.

  10. yeahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh……………………………………

  11. Would I be correct in saying inhaling is creating a positive system, and that when I expel I am moving from a positive air system toward a negative? or do I have it backwards?

  12. Wow, awesome! This video helped me understand the concept of pressure. I also really needed to understand the concept of negative pressure to understand respiration. This video helped, and also this definition of negative pressure I found on Quora:

    "Negative pressure is simply… when the enclosed area has lower pressure than the area around it."

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