Before a baby is
born, there are a lot of adaptations that we
see that allow the baby to take nutrients
and oxygen from mom and successfully
get those nutrients and oxygen to the different
cells that need them in the body. So what I wanted to
do is kind of draw out for you in one diagram
all the kind of things that we see before birth. These are all the things
that are happening while the baby is
still in the uterus. Sometimes we say in utero. So before birth,
what do we notice? Well, this structure over
here, this is our placenta. This is partially mom
and partially fetus. So the placenta has mom's blood
kind of pooling in this area. And the baby actually sticks
its little capillaries inside of that pool of blood. And you can see that
the purplish blood is kind of going in, and the
reddish blood is coming out. And essentially
what I was trying to draw there is that
oxygen is getting picked up. So it's actually
getting oxygenated. And this blood, as
it's kind of reddish, is joining into this
blood vessel down here. So this is kind of the
smiley part of our face. And this is our umbilical vein. So this umbilical
vein is actually going to carry oxygen and blood
back towards the liver area. So let me actually just jot
that name down, umbilical vein. And this is actually the
first of the adaptations I was talking about. So I'm going to make a little
list of adaptations over here on the side, just so we can
keep track of what they are. And the first one will
be the umbilical vein. So once the blood goes
into the umbilical vein, it has kind of a branch point. You can see that it can either
go to the right or the left. And if it goes to the left,
it's going to enter the liver. So if it goes kind of this way,
it's going enter the liver. And it's going to take
a while for that blood to come out on the
other side, because it has to go through all the
little capillaries in the liver and then emerge
on the other side. But there is a shortcut. So the shortcut-- let
me just circle it here. The shortcut is actually
going to be right here. So let me just make sure
it's very clear what the shortcut is. This is called our
ductus venosus. And the ductus
venosus is basically going to allow blood to go from
the umbilical vein, through it. So it's like a little tube. So it is just like any
other blood vessel. It's going to go through it. And on the other side,
it hits and meets up with our inferior vena cava. So this is our
inferior vena cava. I'll write IVC just for short. And the IVC or the
inferior vena cava is a large vein picking up
blood from the right leg and also from the left leg. So this is our
left leg down here. So the interior
vena cava meets up with the blood coming from
the umbilical vein, which is very oxygenated. And so this blood I'm going
to draw is kind of purplish now, because it's kind
of got some oxygen, but it's not as rich as what
was coming out initially from the umbilical vein because
it mixed in with the IVC. And that blood dumps
into the right atrium. So this is our right
atrium on this side. And simultaneously,
you actually have blood from the superior
vena cava, or SVC. This is our head and arm
region, draining down this way. And this blood also kind of
ends up in the right atrium. So you've got this
blood kind of mixing. And now I'm going to draw it
as kind of a deeper purple, because it's mixed up blood. Now, the second
adaptation, then-- let me just make sure I
don't skip out on these. This is the first one. The second one would
be the ductus venosus, that I wrote out. Which is, as I said,
kind of a shortcut from the umbilical vein over
to the inferior vena cava. Now, the blood is
in the right atrium. So it has a couple of options. First, it could simply go
down into the right ventricle. And some of the blood does that. It just goes right down
into the right ventricle. And if goes into
the right ventricle, it's going to get squeezed. And once it gets squeezed, it
goes into the pulmonary artery. This is my pulmonary
artery over here. And we know the pulmonary
artery has a branch over to the lungs on both sides. So we've got some blood going
to one lung and some blood going to this other lung. But remember, once that blood
kind of approaches the lungs, we have to think about what's
going on inside of the lungs. So let me draw out
what's happening then inside the lungs. You've got these
sacs, air sacs, that actually are not full of air. Right? Because when the baby is
still inside of the uterus, or when the fetus is in the
uterus, it's full of fluid. So you've got these
sacs full of fluid. And going past them are
little blood vessels. So this is a little
blood vessel. And let's say this is
an arteriole over here. Now, if it's full of fluid, that
means there's not much oxygen. So what ends up
happening is that there's a process called hypoxic
pulmonary vasoconstriction. And what that means is
that the alveolus literally tries to help constrict
the arteriole. So the arteriole has some
smooth muscle like this. And because there's no
oxygen, the alveolus is going to cause that little
arteriole to basically contract down. So basically, it looks a
little bit more like this. And when it looks like that,
what we've essentially done is increased the resistance
of that arteriole. And if this is happening
millions of times in millions and
millions of alveoli, then the entire lung is going
to have a lot of resistance. A lot of resistance in
the lung at this point. So if that's the case, if
there's a lot of resistance, then a few things we have
to kind of deduce from that. The first is that if
there's a lot of resistance, then the pressure in the
pulmonary artery-- remember, this is our pulmonary artery
right here, these two. I'll actually draw a little
arrow to both of them. The pressure in the
pulmonary artery is going to go very high. So these pressures
are going to be high. And that's simply because
you've got a lot of resistance that you have to try
to fight against. So they have a lot of pressure. And if there's a lot of pressure
in the pulmonary artery, just think back,
and think, well, where did that pulmonary
artery come from? It came from the
right ventricle. So for there to be
forward flow of blood, you better have
a lot of pressure in the right ventricle. And then I could take the
argument back and say, well, if you have a lot of pressure
in the right ventricle, then you must have a lot of
pressure in the right atrium. So you have a lot of
pressure, basically, on the right side of
the heart, because of the fact that you've got a
lot of resistance in the lungs. So these pressures, especially
the right atrial pressure, starts getting so high that
it starts getting higher than the pressure
in the left atrium. And so you get a little
bit of blood flow that starts going
from the right atrium, across that foramen ovale,
that allows-- right here-- that allows blood to
actually go across it. So this is our foramen ovale. Foramen ovale allows blood
to go from one atrium over to the other. And since blood
can now go across, you're going to see
some of the blood continue down in
the right ventricle. But some of the blood will also
kind of go across into here. And that's actually
quite useful, because at the same time that
you have blood going across, you actually don't have
too much blood coming back through the pulmonary veins. And the reason for
that, again, is because it's hard to get
blood flow through the lungs because there's so
much resistance there. So you have a little bit
of blood kind of coming in through the pulmonary veins. And you get some blood
coming from the right atrium. Now, from the left
atrium, blood is going to go down into
the left ventricle. And on its going to get
squeezed around into the aorta. So now you get
blood in the aorta. That gets squeezed there or
sent there from the aorta-- or from the left ventricle. I apologize. So the left ventricle
is squeezing blood down into the aorta. And the aorta is distributing
blood all the way down. Now, before I finish
off showing you where the aortic blood goes,
let's actually make sure I don't forget my
list over here. My third adaptation, then,
should be the foramen ovale, foramen ovale sending
blood from the right atrium to the left atrium. And a fourth adaptation,
actually, I've just kind of sketched out, but
I haven't talked about yet, is right here. So you actually have this
little guy right here. A little connection,
a little vessel-- you can think of it as a
vessel because blood flows through there-- between the
pulmonary artery and the aorta. So this thing right here is
called the ductus arteriosus. So the ductus
arteriosus allows blood to go from the pulmonary
artery to the aorta. And why would blood go in
that direction in particular? Well, remember the
pulmonary artery, again, has very high pressures. And the high pressures are
because of the high resistance in the lungs. So because of those high
pressures, blood, of course, goes from high
pressure to a place where there's lower
pressure usually. And in this case, it's going
to go from the pulmonary artery over to the aorta. So it's actually going to
flow in this direction. Let me just draw a little arrow. It's going to go flowing
in that direction. So ductus arteriosus is
another fetal adaptation. So we've got four so far. Ductus arteriosus. And this actually
explains, then, why you don't get too
much blood coming back through the pulmonary veins. Because a lot of the blood
goes into the pulmonary artery trunk, ends up going
into the aorta. It actually doesn't
even go into the lungs because the
resistance is so high. So now let's kind
of wrap this up. Let's say blood is
now down in the aorta. As I said, it's going
to go into the legs. And it's also
going to kind of go into these internal
iliac arteries. So I've drawn these
arteries here. These are the internal
iliac arteries. And there are, of
course, lots of branches off the internal iliac. But the important branch that
I want to point out right now is this one. This branch, this
major one that I'm kind of sketching in,
this is actually-- we have a name for it. We call this the
umbilical artery. So this is actually
bringing blood back towards the placenta. Now, why would so much
blood go to the placenta? I mean, that's a fair question. Why doesn't it go--
there's a branch here that goes to the bladder. There's a branch that
goes to other places. Why is blood going into
the placental branch or the umbilical artery? Why so much? Well, it turns out that
the placenta-- and this is very clever-- actually
has a very low resistance, very low resistance. So just as the lungs
have a high resistance and they're kind of making
blood divert away from them, the placenta has
a low resistance, and it makes blood
divert towards it. So you can see now that this
is a really ingenious kind of system. We have these five adaptations--
the umbilical vein, the umbilical artery now,
we have the ductus venosus, and we have the foramen ovale,
and the ductus arteriosus. I don't want to miss
out on any of them. So we have five important
adaptations here. And this is how blood
flows in the fetus.
