During pregnancy the fetal circulatory system works differently than it does after birth. The fetus is connected by the umbilical cord to the placenta. The placenta is an organ that develops and implants in the mothers uterus to provide oxygen and nutrients to the fetus.
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 monoxide from the fetus is sent back through the umbilical cord and placenta to the mothers circulation to be eliminated.
Blood from the mother enters fetal circulation through the vein in the umbilical cord which goes to the liver and splits into three branches. It then travels to the inferior vena cava, their vein that is connected to the heart.
Once inside the fetal heart the blood enters the chamber on the upper right side of the heart known as the right atrium. Most of the blood flows to the left side through a special fetal opening between the left atrium and the right atrium called foramen ovale.
The blood then passes into the lower left chamber of the heart known as the left ventricle. It continues on to the large artery coming from the heart, the aorta. From the aorta the blood is sent to the head and upper extremities. After circulating there the blood returns to the right atrium through the superior vena cava. Approximately one third of the blood entering the right atrium does not flow through the foramen ovale. Instead it stops in the right side of the heart where it will eventually flow into the pulmonary artery.
The placenta does the work of exchanging oxygen and carbon monoxide through the mothers circulation, therefore the fetuses lungs are not used for breathing. Instead of blood flowing to the lungs to pick up oxygen and then to the rest of the body, fetal circulation bypasses most blood away from the lungs. The blood is shunted from the pulmonary artery to the aorta through connecting blood vessels called ductus arteriousus. After birth the ductus arteriousus is no longer needed so it withers and closes off.
A mother’s circulation is not directly connected to the fetus. The placenta functions as the respiratory center. The uterine arteries carry oxygenated blood to the placenta where it then diffuses from the placenta to the chorionic villus an alveolus like structure. It is then carried to the umbilical vein. The placenta also acts as a filtration system for plasma, nutrients, and wastes.
