Shunts for the blood in the developing heart serve a very practical need. Initially all of the blood returns to the right atrium. However, because the lungs develop very late, the pulmonary vessels are limited in their capacity and the resulting resistance is very high. Thus, the pulmonary circulation system cannot deal with all the blood.
So the circulation system remains balanced nonetheless, there are two shunts that provide shortcuts for most of the pulmonary circulation system. First there is a direct connection between the right and left atria in that the blood from the right atrium flows directly into the left via the foramen ovale and thus goes around the pulmonary circulation system. This shunt allows a normal development of the left atrium and the left ventricle in that the cardiac musculature on this side is trained.
On the other hand, the right ventricle could also not develop correctly when no blood would flow via the right side. Thus in fetal hearts blood also flows in small amounts from the right atrium via the tricuspid valve into the right ventricle but, going through the truncus pulmonalis it takes a shortcut into the aorta via the ductus arteriosus.

The fetal circulation system

The two cardiac shunts:

  • Connection between the right and left atria via the foramen ovale
  • Connection between the truncus pulmonalis and the aorta via the ductus arteriosus

From the parallel blood flow pathways through the heart together with the two shunts the following circulation system results: The blood from the placenta that has been enriched with oxygen and nutrients gets via the umbilical vein to the liver, part flows through it and part bypasses it via the ductus venosus and gets via the v. cava inferior into the right atrium.

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There are indications that a functional sphincter regulates the flow of blood through the ductus venosus. The umbilical vein transports nutrient-rich blood from the placenta and a large part of it is channeled through the bed of capillaries in the liver. Due to periodic contractions of the uterus, the pressure in the umbilical veins increases and more nutrient-rich blood gets directly via the ductus venosus into the inferior vena cava. Relaxation of the uterine musculature leads to a decrease in umbilical vein pressure and blood deriving from the fetal circulation and from the hepatic vein flows into the inferior vena cava and thereby into the right atrium.

The largest part of the blood from the right atrium flows via the foramen ovale into the left atrium and via the mitral valve into the left ventricle.
From there it empties into the aorta, which winds around the truncus pulmonalis (ascending aorta), glides over the bifurcation in the right and left pulmonary artery (aortic arch), and – heading towards the back - goes over into the descending part of the aorta. In contrast, CO2-rich, nutrient-poor blood flows from the superior vena cava into the right atrium, is partially mixed with the O2-rich blood from the placenta and, via the tricuspidal valve, gets into the right ventricle. Via the pulmonary trunc and the pulmonary arteries a small portion of the blood passes through the pulmonary circulation and, via the pulmonary veins, reaches the left atrium and then the large circulation system. Due to the high pressure in the lungs, though, a significantly larger part flows through the ductus arteriosus and goes into the aorta descendens and thus directly into the large (systemic) circulation system.

Fig. 34 - The blood circulation system situation at the end of pregnancy

Fig. 34

In the placenta the blood becomes enriched with nutrients and oxygen and gets via the unpaired umbilical vein into the fetal blood circulation system.

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Due to the higher pressure of the blood in the inferior vena cava, more blood flows from it directly into the left atrium via the foramen ovale.
The foramen ovale opens like a valve and can direct the blood stream that comes from below directly into the left atrium. Taken together, the diameters of the inferior and superior vena cava are larger than that of the foramen ovale and therefore a small portion of the blood seeps into the right ventricle via the tricuspid valve. The heart is filled only with a mixed blood (O2 saturation).