Okay, so in your basic mama/baby package, you have this umbilical cord connecting the two. At the mama end you have the placenta, which basically puts good blood into the umbilical and takes the used blood out of it. So what is the equivalent on the baby end? Does the blood go straight from the umbilical to the heart? Does the blood take an on-ramp onto an artery, or does it just sort of diffuse into the bloodstream?
The placenta is not part of the mother but is made up of both maternal and fetal tissue. Blood from the fetus goes to the placenta via the umbilical arteries, and returns via the umbilical vein. These vessels connect to the fetal circulatory system.
The maternal and fetal blood supplies are not actually connected (at least not in a major way), but just come very close within the placenta so that gases and nutrients can be exchanged.
Now this is pretty weird. I would have expected the baby’s cardiovascular system to behave before birth like it behaves after birth, in that the arteries carry O2-rich blood and the veins take away the O2-poor blood. (I suppose I imagined some mysterious umbilical cord terminal organ in the baby to work somewhat like a lung.) But it looks like it actually works in the reverse: the veins carry O2-rich blood to the liver and heart, and the arteries carry away the O2-poor blood from the heart and shunt some of of it into the umbilical cord.
When the baby is born and starts breathing on its own, is there some specific process where the blood vessel types reverse roles, or does that not matter so much?
The definition of artery is a vessel that carries blood away from the heart, and likewise, a vein carries blood back to the heart, regardless of oxygenation of the blood. For instance, the pulmonary artery carries oxygen-poor blood to the lungs to enrich the blood’s oxygen levels, and the pulmonary vein carries oxygen-rich blood back to the heart in order to be recirculated throughout the body.
So, I have zero experience with fetal circulation, but my WAG is that once the baby is born, the nomenclature changes, and “veins” from before are now “arteries” and vice-versa.
Most of the fetal vasculatur works in the normal way. A small series of special application shunts brings blood to and from the placenta, then shrivels up after birth.
When the infant breathes for the first time the pressures in the lungs, and subsiquently, the heart change, closing the foramen ovale, an opening between the right and left artia. In a few cases, it doesn’t close, causing an atrial septial defect, requiring open heart surgery.
The ductus arteriosus is a small vessel that also closes due to the pressure changes in the lungs. It takes a little longer to close, because it’s dependent on blood flow to remain open. When it remains open, beyond the expected period,the surgery is less invasive, since the PDA is on the outside of the heart.
Both structures might stay open for several days to weeks then close on their own, or with medication, so surgery is often delayed for a few weeks.
Yes, a series of coordinated changes in the circulatory system begins with the baby’s first breath. They are explained here . It may take several months after birth until all circulatory changes are complete.
If this does not happen correctly what is known as a “blue baby” - one unable to get enough oxygen into its system - is born. In particular, you may hear of a baby being born with a “hole in its heart.” This usually refers to cases in which the foramen ovale has failed to close, permitting oxygenated and unoxygenated blood to mix in the heart. Sometimes a small hole can even persist undetected until adulthood.
I thought “blue babies” were the result of an atrial or ventricular septal defect, not a PFO (patent foramen ovale). I’ve heard estimates from 10-25% of the adult population has a PFO. I’ve heard they’re trying to link adult PFOs to migranes, and PFOs can pose an increased stroke risk.
One nifty thing about fetal circulation is that due to some subtle shunting of blood flow, the upper extremities (including the brain, most importantly) tend to get a slightly more oxygen-rich blood flow than the rest of systemic circulation. This is because the ductus arteriousus (which dumps medium-oxygenated blood from the pulmonary artery into the aorta) is located downstream of the vessels serving the upper extremities (brachiocephalic artery, left common carotid, and left subclavian artery). Also, the inferior vena cava tends to shunt most of its blood flow across the foramen ovale into the left atrium, before it can mix with more deoxygenated blood from the superior vena cava.
Children with Tetralogy of Fallot are usually referred to as “blue babies”
ASD usually doesn’t cause cyanosis. VSD can.
What Picunurse said.
I have always thought that the moment when the baby switches over from fetal circulation to normal circulation was dramatic, and cool, and neat. And it’s a matter of hydraulics - simple enough engineering for me to understand - which has to be simple.
When you’re a fetus, you don’t use your lungs to breathe. You breathe (that is, get oxygen) through your bellybutton. Because the placenta supplies all oxygen to you through the blood - as if you were a fish, using gills to extract oxygen from water. Damn, that’s cool. Like most fish, you have a lower oxygen in your blood than you would if you were outside the water breathing air. That’s because your mom can’t transfer 100% of the oxygen in her blood to your blood; transfer’s not that efficient, and anyway, it would be bad for her. Fetal blood oyxgen saturation is always lower than mother’s.
If you are born with your umbilical cord wrapped tightly around your neck (for a picture of a dummy in a dummy birth situation, not real, with the cord around the baby’s neck, go to here; scroll down, the first page is blank/ads, in case you feel this is not work safe), until the baby is born and needs to take that first breath, the cord doesn’t strangle the baby. Because the baby doesn’t breathe through its throat until after it’s been born. To put it in terms of imaginary homicide, if you wanted to strangle a fetus, you wouldn’t put your hands around its throat; you’d pinch off its umbilical cord.
While your lungs aren’t being used to breathe, they are a high pressure zone for the blood that tries to move through them. I imagine this as a dish sponge that has been allowed to dry out so it’s stiff. When water first tries to move through it, the sponge resists. In the lungs’ case, it’s because they’re - collapsed - that’s not the right word, because they’ve never been blown up. They are like a balloon that’s never been inflated.
So most of the blood that in you and me makes its way through the lungs has a detour to drain it off. The blood comes out of the heart through the right vessel, and heads towards the lungs, but just before it makes the left or right turn it has a nice big channel to drain off into the aorta, and because the lungs are high pressure zones and the aorta’s somewhat lower, that’s where most of it goes. The channel is named the ductus arteriosus.
Also, blood in the heart goes through the foramen ovale… gonna leave that part out for now.
So you’re born. Somebody slap your ass. Take that first breath. You’re blowing up the lung balloons. Suddenly the pressure drops, like the amount of work you have to do to force air into a balloon decreases as the balloon gets larger (Laplace’s Law). Presto, it’s a lower pressure zone. The vast majority of the blood turns right or left and goes into the lungs instead of trying to cross the ductus to the aorta, which is now a higher-pressure zone because of the heart’s contractions. The flap of tissue over the foramen ovale that swung open freely when blood was coming from its back side slaps closed now that blood is pressing on its front side. Brilliant piece of engineering and design. (Design by evolution - if you’re asking me - which you’re not.)
The flap seals closed after awhile in most people, but it’s closed whether it seals or not, which is why probe patent foramen ovale is an incidental finding in most people (leaving out certain complex situations and diseases). The ductus has two brilliant solutions to its disuse. First, there isn’t much blood going through it, so it withers up and goes away. Second, what blood comes through it is coming from the aorta, which is now getting real adult oxygen straight from the air through the lungs through the heart. The ductus is set up with a chemo-electrical trigger to close when it experiences high oxygen levels. Closing takes a week to complete, but functional closing (more than 50%, approaching 90%) takes only a few hours.
Lots of things go wrong with this design. But that’s true of any design. Let’s not think of all the problems. Let’s applaud the millions of times it works as designed.
I forgot to bold picunurse’s name.
I had a friend who was about six month’s pregnant when she felt the baby move around a lot, bunch up to one side of her belly and then stop moving. He died with the umbilical cord around his neck and she referred to him as having been strangled. So it was just a coincidence??
Sorry, six months pregnant. I thought that looked wrong!
I stand corrected on “blue babies.”
Nice post by gabriela.
I’m imagining the strangulation refers to an impingement of the umbilical cord. I knew of a similar case that happened at full term. It’s very sad.
Although the pulmonary vein is the only vein that carries oxygenated blood, and the pulmonary artery is the only artery that carries oxygen-poor blood, yes?
^Outside of the uterus, and assuming no (abnormal) connections between arteries and veins (Arteriovenous Malformations, fistula, or anastomosis).
There are some other places where circulation undergoes some major perinatal change. The fetus does not require the liver to filter its blood. There is a shunt that allows some blood to bypass the liver. If this shunt remains open after birth, bad things happen. It is fairly common in toy breed dogs, and is called a hepatoportal shunt or a portosystemic shunt.
It’s ok, Most of the time, I’m bold enough!