The heart of a fetus is formed early. It is not yet fully functional because the fetus’s lungs are not used until after birth. The blood must be directed around the lungs.
Soon after conception, the heart and circulatory system of the fetus to shape. At the end of the fifth week of pregnancy, the fetus’s heart is able to pump blood throughout its own body. Because a fetus’s lungs don’t function until after birth, when the baby begins to breathe, the mother supplies the unborn child with oxygen-rich blood.
The heart of a fetus develops and four cavities and four valves develop, just like in an adult. But because the fetus’s lungs are not used until after birth, the blood has to be routed around the lungs. Two structures develop in the fetal heart that allow blood to be diverted: the foramen ovale and the ductus arteriosus. The foramen ovale is an opening between the left and right atria. The ductus arteriosus is a blood vessel that aorta connects to the pulmonary artery.
With a normal prenatal circulatory system the mother’s body supplies oxygenated blood to the inferior vena cava of the fetus via the placenta and umbilical cord. The vena cava also receives deoxygenated blood from the fetus’s body. Both oxygenated and deoxygenated blood flows through the vena cava into the right atrium.
Most of the mixed blood in the right atrium is pushed into the left atrium through the foramen ovale. From there, it flows into the left ventricle, which pumps blood into the aorta. The aorta carries blood to the fetus’s body.
The leftover blood in the right atrium flows into the right ventricle, and from there it is pumped to the lungs via the pulmonary artery. But because the lungs do not yet function as a respiratory organ, the blood from the pulmonary artery is diverted to the aorta via the ductus arteriosus. And even now the aorta carries the blood to the fetus.
After birth, the foramen ovale and ductus arteriosus close as soon as the baby begins to breathe. Deoxygenated blood in the right side of the heart is now sent via the pulmonary artery to the lungs pumped. Oxygenated blood flows to the left side of the heart and is pumped through the aorta to the rest of the newborn’s body.