Fetal Circulation
Quick Facts
- Blood circulation in a fetus is more complex than after birth.
- During pregnancy, the placenta does the work of the lungs, digestive system, liver and kidneys.
- The foramen ovale and ductus arteriosus are special connections that help the process before birth.
Blood flow in a fetus is more complex than after birth because the baby’s lungs aren’t working yet. Instead, the placenta does that job. It also acts like the baby’s digestive system, liver and kidneys while the baby is growing.
Blood carrying oxygen travels from the placenta to the fetus through the umbilical vein. Some of this blood passes through the liver, while some bypasses the liver through a special vessel called the ductus venosus. The blood then enters the right side of the heart.
The fetal lungs are not yet functioning for oxygen exchange. Much of the blood travels through the heart and bypasses the lungs through two special pathways.
One pathway is the foramen ovale, an opening between the right and left top chambers of the heart. This allows blood to flow from the right side of the heart to the left side of the heart. Then, it goes to the aorta and to the brain and the rest of the body.
A second pathway for blood to bypass the lungs is through the ductus arteriosus. Blood that moves into the bottom right chamber of the heart is pumped into the pulmonary artery. It then bypasses the lungs through this connection called the ductus arteriosus. This connection directs blood into the aorta and then to the rest of the body.
After blood delivers oxygen to the fetus, it returns to the placenta through the umbilical arteries. In the placenta, carbon dioxide and waste products are removed, and the blood picks up oxygen and nutrients. Then the blood returns to the fetus through the umbilical vein.
The foramen ovale and ductus arteriosus are normal and essential before birth. After birth, these pathways usually close as the lungs take over oxygen exchange. The circulation shifts to the newborn pattern.
