Fetal Circulation

How does the fetal circulatory system work?
During pregnancy, the fetal circulatory system works differently than after birth:

• The fetus is connected by the umbilical cord to the placenta, the organ that develops and implants in the mother's uterus during pregnancy.
• Through the blood vessels in the umbilical cord, the fetus receives all the necessary nutrition, oxygen, and life support from the mother through the placenta.
• Waste products and carbon dioxide from the fetus are sent back through the umbilical cord and placenta to the mother's circulation to be eliminated.

Blood from the mother enters the placenta and comes in close proximity to the fetal blood that has returned from the fetus to the placenta through the umbilical arteries. Once the two circulations are in close proximity in the placenta, the oxygen (O²) and nutrients, like sugar, protein and fat molecules can move from maternal to fetal blood, and carbon dioxide (CO²) and waste products can move from fetal to maternal blood. The maternal blood returns from the placenta to the mother's veins for her systems to take care of the waste. The new well nourished fetal blood returns to the baby through the umbilical vein. The umbilical vein goes to the liver and splits in to three branches, one of which connects to the inferior vena cava, a major vein connected to the heart. In this way the well nourished blood reaches the fetal heart to be sent to the rest of the body.

• 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, called the foramen ovale.
• Blood then passes into the left ventricle (lower chamber of the heart) and then to the aorta, (the large artery coming from the heart).
• From the aorta, blood is sent to the head and upper extremities. After circulating there, the blood returns to the right atrium of the heart through the superior vena cava.
• About one-third of the blood entering the right atrium does not flow through the foramen ovale, but, instead, stays in the right side of the heart, eventually flowing into the pulmonary artery.

With clamping of the cord, the detachment from the placenta and the first breaths of air the baby takes at birth, the fetal circulation changes. By removing the placenta reservoir, the baby's venous resistance/blood pressure goes up. By filling the lungs with air, the blood pressure in the lung arteries goes down. This eventually leads to the closing of the two areas for shunting, the patent foramen ovale (PFO) and the patent ductus arteriosus (PDA).

• Because the ductus arteriosus (the normal connection between the aorta and the pulmonary artery) is no longer needed, it begins to constrict and close off.
• 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 and blood circulates normally.