I’m starting a new series about babies called Woven Together because the development of a little person is fascinating. For the most part, this series will deviate from the main content of the blog – how things go wrong – to tell how things go right. I hope you enjoy!
Babies in the womb have to make some adjustments to account for the fact that they’re in the womb: there’s no air to breathe and no food to eat. Today, we’re going to learn how their adjusted circulatory system works.
First off, mother and baby don’t share blood. They have two completely separate circulatory systems. ((That’s how mother and baby can have two separate blood types with relatively few problems.)) Baby’s heart starts beating about 22 days after conception so that he can pump his blood to deliver nutrients to all the parts of his rapidly-growing body. The heart is baby’s first organ to completely form!
There is one point of interaction where mom and baby exchange stuff: the placenta. Mom gives baby oxygen, food, and antibodies against disease – everything the baby needs to survive and grow. Baby returns this favor by giving mom carbon dioxide and waste. (Thanks, mom!) In the placenta, mom’s blood vessels surround baby’s blood vessels. No blood cells can get through this barrier, but small things like oxygen and nutrients can.
This system works because baby’s hemoglobin (fetal hemoglobin, HbF) binds oxygen more tightly than mom’s hemoglobin (adult hemoglobin, HbA) does.
Mom’s hemoglobin floats by the placenta, carrying oxygen. Baby’s hemoglobin pulls the oxygen across the placental barrier and races back up the umbilical cord to deliver it to baby.
Then what?
Well, first, let’s briefly review adult circulation so that you can see what’s so special about baby’s circulation.
Human hearts have four chambers: a right atrium, a right ventricle, a left atrium, and a left ventricle.
De-oxygenated blood ((deoxygenated blood is actually just dark red, but the scattering properties of your skin make veins appear blue from the outside – see the link in Further Reading)) comes from your body into the right atrium, which pumps it into the right ventricle. The right ventricle pumps the blood to your lungs, where the blood picks up oxygen and turns bright red. Then, it’s back to the heart into the left atrium (blood always enters through the atria). The left atrium pumps it into your left ventricle, which pumps blood to the rest of your body.
Got it? Good.
Onto babies in the womb.
The one key difference between fetal and adult circulation is this: fetuses don’t breathe.
The baby’s lungs are filled with fluid while they develop and aren’t functional, so the capillaries in the lungs are constricted. This means there’s much higher capillary pressure in a fetus’s lungs than in adult lungs, where the capillaries are wide open to allow for oxygen intake.
The right side of a human heart isn’t strong enough to force blood through such constricted lungs. Instead, the fetus has an elegant solution – two holes that allow most of the blood to bypass the lungs. These are the foramen ovale (“oval-shaped hole”) and ductus arteriosis (“passageway between vessels”).
In the fetus, oxygenated blood from the placenta and deoxygenated blood from the baby’s body enter the right atrium of the heart, just like normal.
Here’s where things are different from an adult. This blood has three options in the fetus:
1) Travel to the right ventricle and then into the lungs.
2) Travel through the foramen ovale into the left atrium.
3) Travel to the right ventricle, but detour through the ductus arteriosis into the aorta.
These two additional options (#2 and #3) reduce the amount of blood going through the baby’s uninflated lungs but still allow sufficient oxygen to get to the baby’s growing body.
So, what happens when the baby is born? The one key difference changes: the baby breathes for the very first time.
Let’s look at each hole in turn. First, the ductus arteriosis, the passageway to the aorta.
In the womb, the baby’s lung pressure is much higher than the pressure in the aorta. Fluid flows from high pressure to low pressure, so the blood tends to flow towards the aorta through the ductus arteriosis ((Enough blood still gets to the lungs to supply them with oxygen.)).
Once the baby breathes, the pressure in the lungs starts to drop, and eventually becomes less than the pressure in the aorta. This slows or reverses blood flow through the ductus arteriosis, and the newly open lungs release chemicals to encourage the ductus arteriosis to contract and close. In a matter of weeks, the opening has turned into a ligament.
Next, the foramen ovale, the opening between the right and left atria.
In the womb, the pressure in the right atrium is a little bit higher than that in the left atrium. The left atrium pressure is low because it’s not getting much blood flow from the lungs, and the right atrium pressure is high because it’s getting a lot of blood from the placenta. At birth, the lungs open up and get more blood, sending more blood to the left atrium and increasing its pressure. Also, the umbilical cord is cut and the right atrium doesn’t get blood from the placenta ((It’s still getting blood from the baby’s body.)), so its pressure drops. Though the pressure difference between the atria is small, it’s enough to push the flaps of the foramen ovale closed within about an hour after birth, and they knit closed within a few days.
Sometimes, these two holes don’t close.
Actually, the foramen ovale remains open in about 25% of adults (this condition is called patent foramen ovale). It usually doesn’t cause any problems, but if necessary, a surgeon can fix it. In one awesome version of the procedure, a surgeon snakes a catheter into the heart and deploys a small plug to close the hole.
When the ductus arteriosis remains open, it can cause big problems. The high pressure in the aorta forces some blood back to the lungs through the ductus arteriosis (in the reverse direction from in the fetus). The lungs weren’t designed to handle all the deoxygenated blood from the right heart AND oxygenated blood from the aorta, and they start to get full of fluid. Extra fluid makes it hard for the baby to breathe, which causes all sorts of problems. To help the DA close, doctors can give a synthetic version of the chemical the lungs should have used to help close the DA, or they can go in surgically and sew the DA closed.
Think babies are cool? Did I miss something? Curious about how some other disease works? Let me know in the comments!
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Sources and Further Reading
Related plush toys you just might need (you help support Pathology Storybook at the same time!):
For the video inclined: Why you didn’t die at birth from Smarter Every Day (an awesome blog!!)
Why veins appear blue from Kienle et al.
Interactive adult heart showing flow, pressure, ECG, and heart sounds! from University of Utah
Fetal Circulation from the American Heart Association
Fetal Circulation from University of Rochester
On changing heart pressures from the Merck Manuals
Changes at birth from Berkeley
Pathology Storybook 
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