After birth the circulation is divided into two separate sides that are not connected. The left side of the heart consists of the pulmonary veins, left atrium, left ventricle, and aorta. The right side of the heart consists of the superior and inferior vena cava, the right atrium, the right ventricle, and the pulmonary artery. These two circulations are independent of each other and do not connect.
However, the fetal circulation is different. The right and left sides of the heart connect at the level of the foramen ovale and the ductus arteriosus. Because of these connections fetuses can develop serious heart defects and live while in the uterus, only to be severely compromised or die because of the circulatory changes that occur following birth. To understand the effect of certain types of heart defects it is therefore important to review fetal circulation. The following movies illustrate the fetal circulatory system.
Left Side of the Heart
1. Blood from the placenta returns to the fetus by the umbilical vein. The umbilical vein enters the fetal abdomen where blood continues through the intra-abdominal portion of this vessel.
2. Once inside the abdomen blood flows through the ductus venosus and is directed into the right atrial chamber. Because this blood has been enriched with oxygen and nutrients that it has picked up from the placenta, it is directed through the foramen ovale into the left atrium. The foramen ovale is a small hole in the wall that separates the right and left atrial chambers
3. Once inside the left atrium the blood mixes with blood returning from the lungs and then enters the left ventricle.
4. Blood from the left ventricle is pumped into the aorta, which distributes blood to the brain and upper extremities.
Right Side of the Heart
1. Blood from the placenta returns to the fetus by the umbilical vein. The umbilical vein enters the fetal abdomen where blood continues through the intra-abdominal portion of this vessel.
2. Once inside the abdomen blood flows through the ductus venosus and is directed into the right atrial chamber. Because this blood has been enriched with oxygen and nutrients that it has picked up from the placenta, it is directed primarily through the foramen ovale into the left atrium. The foramen ovale is a small hole in the wall that separates the right and left atrial chambers. There is some mixing of blood within the right atrium.
3. The majority of blood enters the right atrium from the superior and inferior vena cava, as well as some blood from the ductus venosus (see above). The superior vena cava brings blood back from the head and upper extremities to the heart while blood from the inferior vena cava brings it back to the heart from the remainder of the body.
4. Blood is pumped from the right atrium into the right ventricle.
5. Blood from the right ventricle is pumped through the pulmonary valve into the main pulmonary artery. From here it is distributed to the lungs and to the ductus arteriosus.
6. The ductus arteriosus distributed blood to the entire body (excluding the head and upper extremities) as well as returns blood back to the placenta through the two umbilical arteries.
