Reproductive Endocrinology, Birth and Breastfeeding. Let's start off by comparing and
contrasting the fetal circulation with the adult circulation. This is going to be an important
process because during birth there are a lot of things that are changing especially for the
fetal circulation. So let's start with the items that are very different between the two, fetus
and adult. The big ones are the lungs. The fetus will receive about 6% of cardiac output while
the adult gets 100% of cardiac output goes through the lungs, big difference. In the kidneys,
about 2% goes to the fetus and a 20% to the adult, so about a tenfold change. Finally in the
gut, splanchnic system deliver about 7% of cardiac output goes to the fetus and 30% to the
adult. The final really big change that happens between the fetus and the adult is the placenta.
The fetus has 40% of cardiac output going to the placenta while in the adult if it's not the mom,
none and so this is a very big difference in terms of a vascular bed that the adult doesn't even
have to take into account but the fetus needs to take over a third of the blood flow going to
that locale. So let's talk about why these differences exist. So there are a few structural
differences in the fetus that preferentially shunt blood to different locales. The first is the
foramen ovale. This is a hole in the heart that allows flow to travel through it and bypasses the
lung to a great extent. Then we have the ductus arteriosus and a ductus venosus and
especially the venosus is going to change what flow goes to the splanchnic system or to the gut.
Finally we have the placenta in the circulation for the fetus that is not present for the adult.
I mentioned these four places because these are going to be the sites at which circulation has
to change right during birth. Within that first breathe, you need to have these particular areas
change their flow all of a sudden, otherwise we will not have a good circulation and we'd have
to do some specialized care for the fetus. Fetal oxygenation. So one other aspect of the
circulation is to deliver oxygen to peripheral tissues. For the fetus, about 80-90% of
hemoglobin is saturated with oxygen. This is a lot different than the adult which has about 98%
saturation of oxygen on hemoglobin. The lower amount of O2 is because you're obtaining this
O2 from the mother at the level of the placenta rather than the fetus breathing on its own.
So there are lower amounts of oxygen circulating around in the blood. There are further times
in which you're going to start mixing deoxygenated blood with oxygenated blood. This is
minimized because of the stream flow effect where you have areas of higher pressure that is
pushing through blood vessels. You have faster flow and then you have certain areas with
slower flow. This is very similar to a river in which a certain portion of river is traveling
fast and another portion of the river is traveling slower. Same thing happens in the circulation.
Luckily for us at this particular juncture, we have higher oxygen-rich blood that's traveling
faster as it's coming from the placenta versus other blood flow that's deoxygenated that's
coming from places like the splanchnic system. This will mix a little bit and you will have a
decrease in O2 to maybe something like 70%. So as the blood flow travels through the heart
out into the systemic circulation, you only have about 58% of hemoglobin has oxygen bound to
it. So this is a lot different than in the arterial side of the circulation in the adult human, which
is about 98%. So there's about a 40% difference in that blood traveling around to the various
tissues and you might think this may lead to having a fetus that doesn't have enough oxygen
who has a hypoxemia. Right? Well, there's one other factor that we need to put into play and
that is the fetus has a different type of hemoglobin. That different type of hemoglobin has
a different oxygen-hemoglobin dissociation curve. So at a lower partial pressure of O2, you
have a higher percent of oxygen bound to hemoglobin. That helps to offset this particular
problem of having low PO2s in these particular circumstances that the systemic flow goes
around that only has 60% O2 bound to hemoglobin.