Before birth the placenta, not the lungs, supplies oxygen, so most blood bypasses the lungs. At birth the first breaths and cord clamping switch the baby to adult-style circulation.
The umbilical cord
The umbilical vein carries oxygen-rich blood from the placenta to the fetus. Two umbilical arteries carry low-oxygen blood back to the placenta.
Three shunts
The ductus venosus lets much of the umbilical blood bypass the liver. The foramen ovale lets blood cross from the right atrium to the left atrium, and the ductus arteriosus sends blood from the pulmonary artery to the aorta, so both avoid the lungs.
The switch at birth
As the baby breathes, the lungs expand and their vessel resistance falls, while cord clamping removes the low-resistance placenta. Pressure in the left atrium rises above the right and pushes the flap of the foramen ovale closed.
Closing the ductus
The ductus arteriosus constricts within hours to days as oxygen rises and later becomes a fibrous ligament. If it stays open (patent ductus arteriosus), especially in premature babies, it can cause a murmur and extra blood flow to the lungs.
| Fetal structure | Connects | After birth becomes |
|---|---|---|
| Umbilical vein | Placenta to the liver and ductus venosus | Round ligament of the liver |
| Ductus venosus | Umbilical vein to inferior vena cava | Ligamentum venosum |
| Foramen ovale | Right atrium to left atrium | Fossa ovalis |
| Ductus arteriosus | Pulmonary artery to aorta | Ligamentum arteriosum |
| Umbilical arteries | Internal iliac arteries to placenta | Medial umbilical ligaments |
At the bedside
A healthy newborn's oxygen saturation rises slowly and reaches about 85 to 95 percent only by 10 minutes, so early low readings are expected. Place the oximeter on the right hand, which measures pre-ductal blood on its way to the brain.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).