Fetal circulation

In the adult, oxygenated blood is sent from the left atrium to the left ventricle and then out the aorta to arteries in the rest of the body. Blood then returns through veins to the right atrium and goes into the right ventricle, which pumps it to the lungs in order to drop off carbon dioxide and pick up oxygen.

In the fetus, the lungs are not mature enough to do that, so oxygenation happens in the placenta, and four key adaptations or structures make this possible.

These are the umbilical veins and arteries in the umbilical cord, the ductus venosus, the foramen ovale, and the ductus arteriosus.

So imagine you’re an oxygen rich red blood cell that has to get from the placenta to the fetal tissues. Blood from the placenta is highly oxygenated blood, so let’s color that red.

From the placenta, blood heads through the umbilical vein, the first adaptation of fetal circulation, that carries oxygenated blood toward the liver.

When the umbilical vein reaches the liver, it dumps blood into the portal vein. The blood in the portal vein goes out to every lobule of the liver, and becomes deoxygenated so we’ll color it blue, although in reality it’s more of a dark, dark red color.

This deoxygenated blood enters the hepatic vein, which then drains into the inferior vena cava, which is one of two enormous veins that carries deoxygenated blood from the lower half of the body to the right atrium.

Now, from the umbilical vein, a vessel called the ductus venosus forms and connects to the inferior vena cava. This bypasses the liver circulation, and represents the second adaptation of fetal circulation..

From there, the red oxygenated blood from the placenta mixes with the blue deoxygenated blood from the lower body, so red and blue make purple, and that purple blood is joined by the blood from the hepatic vein before it all flows into the right atrium.