How It Works
Blood from the mother enters the fetus through the vein in the umbilical cord. It goes to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein linked to the heart. Inside the fetal heart:
- Blood enters the right atrium (the chamber on the upper right side of the heart). Most of the blood flows to the left side through a special fetal opening between the left and right atria. This is called the foramen ovale.
- Blood then passes into the left ventricle (lower chamber of the heart). It then goes to the aorta (the large artery coming from the heart).
- From the aorta, oxygenated blood is sent to the body. After circulating there, the blood returns to the right atrium of the heart through the superior and inferior vena cava.
- About one-third of the blood that enters the right atrium does not flow through the foramen ovale, Instead, it stays in the right side of the heart. In time it flows into the pulmonary artery.
Because the placenta does the work of exchanging oxygen (O2) and carbon dioxide (CO2) through the mother's circulation, the fetal lungs are not used for breathing. Instead of blood flowing to the lungs to pick up oxygen and then flowing to the rest of the body, the fetal circulation shunts (bypasses) most of the blood away from the lungs. In the fetus, blood is shunted from the pulmonary artery to the aorta through a linked blood vessel called the ductus arteriosus.
With the first breaths of air the baby takes at birth, the fetal circulation changes. A larger amount of blood is sent to the lungs to pick up oxygen.
- Because the ductus arteriosus is no longer needed, it starts to wither and close off. The ductus closes over the first few days of life. It may stay open longer in premature babies.
- The circulation in the lungs increases and more blood flows into the left atrium of the heart. This increased pressure causes the foramen ovale to close. Blood then circulates in a normal way.