Fetal Circulation

Not everyone knows this but it's important in understanding infant arrhythmias and congenital heart defects: Fetal circulation is very different!! As a matter of fact, it seems as if everything is backwards. For now, I will discuss the circulation and later I will discuss the defects that can arise if fetal circulation does not make all of the proper changes to the normal postnatal circulation.

Normal Circulation/Flow of Blood Through the Heart
In a normal post-natal heart, blood flows in this order starting with deoxygenated blood:
Vena Cava (from the body and carries carbon dioxide)-->Right Atrium-->Triscuspid Valve-->Right Ventricle-->Pulmonary Arteries (carrying deoxygenated blood)-->Lungs-->Pulmonary Veins (carrying oxygenated blood)-->Left Atrium-->Mitral/Bicuspid Valve-->Left Ventricle-->Aorta (pumped to the rest of the body)

Below is a diagram of blood circulation in a normal heart: blue means deoxygenated blood and red is oxygenated blood.



Pressure in a normal post-natal heart is higher in the left heart (left atria and ventricle) than the right heart. The left heart is also larger, stronger and thicker than the right heart. This is due to the fact that the left heart must pump blood to the entire body and against gravity! The right heart simply has to pump blood into the lungs directly below it.

Fetal Circulation Similarities
1. The fetus has all of the same parts (2 atria, 2 ventricles, valves, arteries and veins) but as mentioned above may be different sizes.

2. The fetus receives deoxygenated blood into the right atrium.

Fetal Circulation Differences
1. The first and foremost important thing to recognize is that a fetus does not have mature lungs until approximately 36-37 weeks gestation. But the blood still needs to receive oxygen from somewhere...

2. Umbilical Vein
This is the somewhere that the oxygenated blood comes from. While veins typically carry deoxygenated blood from the body back to the heart to be oxygenated by the lungs, pulmonary veins and umbilical veins are different!! Again, fetal circulation is different! The UMBILICAL VEIN carries OXYGENATED blood from the mother's heart TO the PLACENTA.

3. Umbilical Arteries
Yes, plural. There are two arteries; as a matter of fact, when a baby is born this is one of the first things the doctor checks after cutting the umbilical cord. The doctor makes sure there are two arteries and one vein. Normally arteries carry oxygenated blood away from the heart to the body and pick up carbon dioxide to bring back to the lungs. BUT again, pulmonary arteries and umbilical arteries are different! The UMBILICAL ARTERIES bring DEOXYGENATED blood FROM the PLACENTA to the mother.

4. Foramen Ovale
Because the fetus does not have mature lungs to be able to exchange gases itself, blood that normally goes to the lungs must be shunted away from them in the fetus. The first of these shunts is the foramen ovale: this shunts blood from the right atrium to the left atrium (at the top of the heart) bypassing the right ventricle and the lungs. As opposed to the post-natal heart, the fetus' right heart has higher pressure than the left heart (DIFFERENT in yet another way)! This pressure differential pushes blood from the higher pressure heart to the lower pressure heart to avoid the lungs. The foramen ovale normally closes within 3 months of birth.

5. Ductus Arteriosus
The second cardiac shunt in the fetus is the ductus arteriosus. Any blood that has gotten to the right ventricle and not gone through the foramen ovale is shunted from the pulmonary arteries (normally carries deoxygenated blood to the lungs to become oxygenated) to the aorta (pumped to the rest of the body). Again, this bypasses the lungs. The ductus arteriosus typically closes once breathing regulates in the newborn baby.

6. Ductus Venosus
The ductus venosus is another shunt that is present in the fetus that is not present after birth. This does not directly involve blood flow in the heart but sends approximately 80% of the blood from the umbilical vein to the inferior vena cava (vein that carries blood from lower extremities back to the heart). The ductus venosus therefore bypasses the fetal liver. Within minutes of birth, the ductus venosus does not work but it will take 3-7 days for it to completely close.

As a general overview, everything above is basically how the fetal heart differs from that of a healthy, normal person after birth. As mentioned above, I will discuss how congenital heart defects and arrhythmias can be present at birth if something in the regular developmental cycle goes wrong.