I want to start us
out by orienting us to what you see here. We've got a couple
of lungs here-- the left lung and
the right lung. And we have a heart
at the bottom, right? And specifically I
actually divided this up into the four
chambers of the heart. And I'm going to show
you the four chambers. This is the right atrium. This is the right ventricle. Then we have the left atrium
and the left ventricle. So these are the four
chambers of the heart. And I kind of cut away
a lot of the stuff that comes into and out of the
heart, a lot of the vessels, because I want to highlight
one particular vessel. I'm actually just going
to label it for you. And it's the pulmonary artery. And I've drawn it in blue just
to kind of point out the fact that it's full of blood
that has no oxygen in it. But it's called an
artery, you remember, because arteries take
blood away from the heart. So this is the pulmonary
artery right here. And of course there's a left
and right pulmonary artery. This would be the
right pulmonary artery. And this would be the
left pulmonary artery. It's actually kind of
difficult to say quickly. You can see I'm tripping
over the words a little bit. In any case, so that's
the way the blood goes out of the right ventricle. And let's say
about five liters-- I'm just going to label it right
here, five liters per minute. So a lot of blood
is kind of gushing through that
pulmonary artery going to the right and left lung. And let's say we have some
blood kind of going up this way into the left
long and some blood going this way into
the right lung. And this is kind of a normal
thing that's happening. Now, let's say you're
eating some food. Let's say you're
eating some peanuts, and you accidentally choke
on one of the peanuts. So obviously this would
be a terrible thing that would happen, but let's
say you choke on a peanut. And that peanut kind
of goes down this way. And it has to either go down to
the right lung or the left lung through what we call the
main bronchus, right? So is it going to go down
the left main bronchus or the right main bronchus? And just by looking
at it you might remember that gravity
is going to push more towards the right main bronchus. So things kind of
have a tendency of getting stuck on the right
main bronchus a little bit more just because it's more vertical. You can see the
shape-- it's going to attract more
things like food. And so if a peanut
get stuck there, our question or my question
is, what would happen next? So let me actually have you
put on your x-ray goggles, and let's see if
you can actually-- I'm going to kind of just
clear up some of this stuff, and see if we can kind of see
what would happen in our lung. I'm actually just going
to clear out both sides and reveal to you
what things might look like if you could
look inside of them. So you can see there's a
little alveoli here, right? That's the first thing
I want you to notice. This is a little alveoli. And I'm just going to
label it on this side. But you can see both pictures
are kind of the same. And we have a pulmonary
arterial and a capillary. So this purple one
is a capillary. And I drew it in purple
just to kind of let you know that gas
exchange is happening. So some of the carbon dioxide is
leaving, and some of the oxygen is kind of getting into
the blood at that point. So it's kind of a
purplish color or that's kind of how we
think of it anyway. And right before the
capillary, the blood again is kind of
coming this way. I should do it
with a white line. Blood is going that way. Right before the capillary
is the arterials. Let me actually
write that in here. This is the arterial
or pulmonary arterial-- you might hear that
phrase as well. And all that means is kind
of the arterial in the lungs. So this is the arterial
and the capillary that are coming up
very near an alveoli. And in our peanut
situation, what's happening? Well, our left lung is actually
doing pretty well, right? It's pretty happy. This little alveoli
is really happy because it's full of oxygen. And that's kind
of the key idea I want to present
today is that there's a difference in the
amount of oxygen that's getting into the lungs
and, of course, all the alveoli within the lungs, right? So in the right lung,
what's happening? Well, this alveoli is
not too happy at all. Not too happy because
there's very little oxygen getting in there. And when little oxygen gets into
the alveoli, when there's not too much oxygen there, an
interesting thing happens. And I'm actually just
going to kind of show you using this arterial. This arterial has a
lot of smooth muscle, and this smooth muscle,
it can tighten down. Like any muscle, it
can actually contract. What happens is that instead of
being this nice large arterial, because the smooth
muscle starts to contract down-- and remember, the
reason that's contracting down, I should point this out,
is that there's actually a little signal that gets sent
from the alveoli's low oxygen. Because there's low oxygen
in there, a signal gets sent. And this is actually a
signal that is heavily researched upon
exactly how it works. So suffice to say,
there is a signal. And this little arterial
gets a little smaller. So the size of the tube, if
you think of it as a tube, is now kind of tinier
than it was before. And so blood is
still going through, but obviously there's
a lot more resistance. So really the big change
is that the alveoli had very little oxygen,
it sent a signal, and as a result of the signal,
the size of that arterial got smaller. And because we know
that when size goes down resistance goes up-- I'm going
to write increase resistance here. So basically, the
amount of resistance goes way up as a
result of having very little oxygen in that area. So you might be thinking, well,
that's not a huge deal, right? Because this is just
one little alveoli and who cares if a little
resistance goes up. Will that really
affect anything? And the truth is that it does. It really does. Because remember there
isn't just one alveoli having this problem, you have
about 250 million alveoli-- let's say about that
many in the right lung. And let's say a
very similar number of alveoli in the left lung. So you have these large
numbers of alveoli all having kind of
similar problems. And as a result, what
happens is that it's not just one little unhappy face
on this right lung. You actually have
millions of them. I can't really draw millions. But you get the idea that
this entire lung is really without oxygen. It's really not doing so well. And on the other side,
things are actually really, really awesome, right? This side, the
alveoli are really happy because they're
full of oxygen. They're doing really well. So things are good on the left
side, but not on the right. And if all these alveoli are
doing the exact same kind of trick, then the resistance is
going to go up in this vessel. So this vessel right here,
the right pulmonary artery, that vessel is actually going
to have lots of resistance, lots and lots of resistance. And as a result, if blood
has a choice-- and of course, it does, right? In a sense, it's not
thinking but, of course, it has a choice in
terms of whether to go to the right or the left. Now, a lot more blood is
going to go to the left because it's going
to say, why the heck would I go to the
right when there's all that resistance over there? It's going to go to the left. So you have a lot
more blood coming out of the pulmonary artery on
the left and a lot less blood going to the right
pulmonary artery. So if you were to think about
it in terms of blood flow, flow goes up. In this lung,
blood flow goes up. And similarly you could
also say, well, obviously it's not like the
amount of tissue on the left or
right lung changed. So if there's more
blood flow, there's also going to be more perfusion. So you'll often hear
this word, perfusion. And that really refers to the
idea that there's more blood, you could say,
perfusing the left lung. Now, this whole trick, the
idea of oxygen going down and blood kind of as a result
going to the opposite lung-- there's a name for this trick. I'm going to write it out here. It's called hypoxic, which
just means low oxygen. Hypoxic-- pulmonary, which of
course just refers to the lungs because this trick is
happening in the lungs, hypoxic pulmonary
vasoconstriction. Remember, we said
vasoconstriction just means kind of making the
blood vessels smaller. So it's kind of a fancy
name, hypoxic pulmonary vasoconstriction,
but all it means is kind of what we described
happening in this side where the alveoli has
very little oxygen and as a result it sends a
signal out to the arterioles to tighten down. Resistance goes up and blood
goes flowing the other way. So an easy way to
remember this is I always think of
blood chasing oxygen. You can think of it that way. And it makes it
kind of an easy idea to remember if you think
of it in these terms.