Library · Physiology
Fetal circulation
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Before birth, the baby does not breathe air. Its blood is set up differently: it uses three detours, called shunts, to skip the lungs and the liver. The trip to the placenta and back happens through the umbilical cord.
The first detour is the ductus venosus. The blood arriving from the placenta through the umbilical vein, well oxygenated, with a saturation of 70 to 80%, instead of all entering the liver, heads straight down that route into the inferior vena cava, the large tube that runs up to the heart.
The other two sit inside and right next to the heart. The foramen ovale is an opening between the two atria, the upper chambers, and it lets blood pass from the right side to the left. The ductus arteriosus is a bridge that sends the blood headed for the lungs into the aorta instead, because in lungs that still hold no air the resistance to flow is high.
All of that changes with the first breath. As air comes in, that resistance in the lungs collapses and the detours start closing. And if one of them does not, the baby can end up with congenital heart disease.
In brief
Before birth, blood bypasses the lungs and liver through three shunts and reaches the placenta through the cord.
Three detours that lift at birth.
The ductus venosus carries umbilical blood, 70 to 80% saturated, to the inferior vena cava, skipping the liver.
The foramen ovale passes blood from the right atrium to the left; the ductus arteriosus sends lung-bound blood to the aorta.
With the first breath, high lung resistance falls and the shunts close; if one persists, congenital heart disease can result.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Fetal Circulation, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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