Baby circulation right after birth | Circulatory system physiology | NCLEX-RN | Khan Academy
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Baby circulation right after birth | Circulatory system physiology | NCLEX-RN | Khan Academy


We’ve talked about
fetal circulation, and I’ve talked about all
the different interesting adaptations that the
fetus has to make sure it can adjust to life
within the uterus, within mom. But when the baby
comes out– let’s say the baby is just
delivered– there’s got be a lot of
changes that happen. In fact, these
adaptations, each of them plays a role in the first few
minutes, hours, days of life. And so what I wanted to do is
go through all the adaptations, think through them,
and see what’s happening actually after birth. So we know what
happens before birth and how the baby
adjusts there, but how does this now
translate into what’s going to happen
after birth and what the baby has to do now
that it’s separated from mom and
breathing on its own? And the first two things
I want to point out are the idea of– what are the
big things that are changing? And one big thing is, of
course, that the placenta, which the baby’s been using for 40
weeks, or nine months or so, is no longer around. The placenta is removed
from the baby’s circulation. We’re going to cut it away. And the second big thing
that’s going to happen is that the lungs
get used to bring in air for the first time. So the lungs take in air. So these are the two huge
things that are going to change. And these two things
are going to end up affecting a whole lot of
other things, as well. So let’s get started. Let’s see what happens when
the placenta gets removed and when the lungs take in air. Let’s start with the placenta. So let’s say that you decide
that the baby is now delivered, and you want to cut the
cord, cut the umbilical cord, and put an umbilical
clamp right there. And this is often done. You’ll see this done in
movies, or if you’ve ever gone to a delivery, you’ll see
this done pretty routinely. So this is a little
umbilical clamp, and it’s clamping the cord. And if you’re ever worried about
whether that hurts the baby or the mother, it doesn’t. Because the umbilical
cord does not have nerves. So that’s kind of the first
interesting thing about it. But this stuff, remember–
this pale yellowish stuff that’s kind of jelly-like–
we call this Wharton’s jelly. Wharton’s jelly. And one of the
things that I always thought was really cool
about Wharton’s jelly is that it’s a really
interesting Mother Nature-type idea, that the Wharton’s
jelly starts contracting. It gets kind of tighter around
the three vessels– the two eyes and the smiley face
that I’ve drawn here, which are the two umbilical
arteries in the vein. The Wharton’s jelly starts
squeezing around those vessels as soon as the
temperature falls. So temperature
falls– and remember, the temperature in
the mom is going to be much warmer than it is
outside in the delivery room, so immediately that Wharton’s
jelly is exposed to cold air. And when the temperature
falls, the Wharton’s jelly starts to contract,
causes contraction. And of course, that’s going to
squeeze down on all the vessels inside. It’s going to basically
clamp down on them. And so it’s almost like we have
this man-made clamp that I drew in orange, but the
Wharton’s jelly is kind of a natural clamp
that we already have. So we’re taking this very
low-resistance placenta– remember, it used to be very
low-resistance, a lot of blood liked to flow in
that direction– and creating really
high-resistance. So this is the
first big change, is that the placenta
gets removed and you go from low-resistance
to high-resistance. So that’s a key idea. Now as a result of the
high-resistance– remember, there used to be blood flowing
through the umbilical vein, but now in the first
few days, there’s really no blood
flowing through here. All the blood
starts clotting off. And that’s true even
of the ductus venosus. You get some blood
clots in there. So you don’t really
have any flow anymore, and in the first few days,
you really completely lose any flow
through those things. So this becomes
non-used, or unused, over the first few days of life. Now you still have blood
flowing from the portal vein into the liver, and you
still have blood going up the inferior vena cava, and
this is all deoxygenated blood, so that is still
the same as before. And this deoxygenated blood
now has no new fresh oxygen to mix with. So I’m not going
to color it purple. I’m going to leave it
the same blue color. So deoxygenated blood
comes up from the legs and it comes down from
the head and the arms, from the superior vena cava. And now all this
deoxygenated blood fills in the right atrium,
and some of that blood is going to now go into the
right ventricle, so let’s color that in blue. And it’s going to
get squeezed out into the pulmonary arteries
from the right ventricle, so let me color that in the
same deoxygenated blue color. And this is headed
toward the lungs. Now in the lungs,
what was happening? Well, initially, remember
we had these little alveoli. And they’re full of fluid. And that fluid now is going
to get replaced by air. So air is going to
push the fluid out. Air is going to push
all that fluid out. And what’s on the outside? Well, we’ve got
little capillaries. So we’ve got these
capillaries, and the fluid will enter the capillary. But remember, right before the
capillary is the arteriole. Let me actually sketch
it a little bit smaller, the arteriole. Because it used to
be very constricted. Remember, there was that hypoxic
pulmonary vasoconstriction. But now that you
have air in there, the oxygen levels are
rising in the alveolus. And what that’s
going to do is that’s going to send a signal over
to the arteriole– this is our arteriole– to say,
hey, it’s time to open up now. It’s time to finally dilate. So this arteriole is excited. It’s never really been very
dilated before in its life. So it finally says,
yay, it’s my chance. So it dilates. It dilates like this. And it’s nice and plump and big. And when it gets big, what
does that really mean? It means that the
resistance has fallen. Resistance has fallen. So remember, the lungs used
to have high resistance. And now, millions of alveoli
are causing the arterioles to open up and resistance falls. And this happens, of
course, on both sides. So on the left lung
and the right lung, the resistance is falling. And that deoxygenated
blood now can flow in. Because initially, it
wasn’t really wanting to flow in because the
pressures had to be so great. But now the pulmonary artery
pressures are falling. It’s easier to actually get
the blood into the lungs. And that means, of course,
the right ventricular pressures are falling. And the right atrial
pressures are falling. So the entire right
side of the heart now is working under
lower pressures because the resistance in
the lungs has gone down. And now the resistance
in the lungs going down, that means that more
blood is going to go in, and if it goes in,
it’s going to go into all the little
thousands of capillaries and it’s going to
get oxygenated. And those capillaries are going
to send all that blood back and it’s going to flow
into the left atrium. So you have all this fantastic
oxygenated blood coming in from both sides, coming
into those pulmonary veins. So now tons of
oxygenated blood is dumping into the left atrium,
which is different than before, because you didn’t have
much flow through the lung. So now you’ve got lots of
blood kind of flowing in here. And at the same time, the
pressures on the right side have fallen. So if pressures on the
right side have fallen, think about what’s happening
to our foramen ovale. Before, blood was actually
kind of gushing through there. But now, because the pressures
on the right side are so low, this little flap of tissue,
like a little valve, closes off. And so now you can actually
see that this flap of tissue will do this. It’ll close off. Because you basically
have more pressure on the left side
than the right side, and it pushes that
flap of tissue over. And now the foramen ovale
is basically closed. And this happens, actually,
in the first few minutes– first few minutes after
a baby is out of the mom, you actually see this foramen
ovale close, which is amazing. Now blood continues
to go down, it likes to go into
the left ventricle. So it’s going to go
down here and get squeezed into the aorta. So let me show– now oxygenated
blood for the first time kind of getting into
the aorta this way. And then you have the question
of the ductus arteriosus. Remember, initially
the reason that blood was moving from the pulmonary
artery into the aorta was because the pressures in the
pulmonary artery were so high. But now the pressures
are pretty low, the pressures are much lower. If anything, you would actually
have flow going this way because the aortic
pressures are higher than what the pulmonary
pressures are now. But it turns out, interestingly,
that in the first few hours of life, you actually
have some constriction of the muscles in that
ductus arteriosus. So that ductus arteriosus has
smooth muscle in the walls. And those smooth
muscles are going to sense that now
oxygen levels are high. They’re going to
sense the increase in oxygen levels in the blood. And they’re going to
start getting twitchy, they’re going to want
to start constricting. The other thing that the
ductus arteriosus senses is that the placenta is removed. How would you sense
something like that? And how would the ductus– which
is over here– how would it sense that the placenta– which
is over here– how would it know that it’s been removed? Well, it turns out
the placenta actually makes a little chemical
called prostaglandin. And when prostaglandin
levels fall, when prostaglandin
levels go down, then the ductus arteriosus
also is more willing or able to close down. So those little muscles inside
of the ductus arteriosus– remember, it’s like a
little artery, in a sense. It’s got smooth
muscle around it. Those muscles are
going to constrict, they’re going to tighten down
when the oxygen levels go up and when the prostaglandin
levels go down. It’s going to sense that. And so it’s going
to know that hey, it’s time for me to close
up shop and tighten down. And over time– and I’ll
say over a course of hours– this is going to happen. So let me actually just jot
down the time frame for you. So over the course
of a few hours, the beginning of
constriction will happen. So over time, this
will actually get kind of tighter and
tighter and tighter. Let me sketch it
out, getting smaller and smaller and smaller. You actually have on
the inside of it maybe a tiny little opening, and then
over time, a smaller opening, and over time, no
opening at all. So that’s going to
happen at the beginning of the first few hours of life. Now, following the
blood all the way down, you actually have aortic blood
with oxygen flowing down here into the, let’s say, the right
leg and into the left leg, over here. And there are these
branches, these big branches, called the internal iliac
branches, and off of them, where the umbilical
arteries, right? The umbilical arteries
where branches off of the internal iliac. And what’s going to
happen is that you’re going to still get blood
flowing to other branches off the internal iliac, like
this little branch might go to the bladder. But that last little
bit, really, there will be no flow through
there because the resistance is so darn high. Because the resistance
over here is so darn high, no blood is going to want
to go in that direction. In addition, the
umbilical arteries, just like the ductus arteriosus,
have smooth muscle in them. And so that smooth
muscle is going to respond to the very
high levels of oxygen that, for the first time,
these arteries are seeing, and low prostaglandins that
are kind of circulating. And they’re also going to
kind of start constricting. So just as the ductus
arteriosus started constricting, these arteries also
start constricting. They get tighter and
tighter and tighter until really there’s almost
no space in the middle left. And that’s how I’m going
to draw it fizzling out. So initially, they get
kind of more narrow and they get even more narrow
as the muscles in the walls tighten and tighten and tighten. And they finally get
something like this. And you still have
blood, of course, going to other branches,
which is what I’ve drawn here. But that last little
bit going just to the umbilicus, that part
is going to constrict down. And this process happens over
the course of a few hours. So now you have it. You have all the
five adaptations and how things change over
the course of minutes, hours, and days. And of course, it’s not exact
and each baby is different, but these amazing changes are
happening soon after birth.

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