Briefly describe fetal circulation

Fetal circulation: blood from the placenta returns to the fetus through the umbilical vein. This blood is about 80% saturated. On approaching the liver most of the blood is shunted into the inferior vena cava through the ductus venosus. The remaining portion enters the liver and mixes with blood from the portal circulation. In the inferior vena cava the blood mixes with deoxygenated blood returning from the lower limb and enters the right atrium where it is directed towards the foramen ovals by valve of the inferior vena cava. Most of the blood enters the left atrium. The small portion remaining in the right artrium mixes with desaturated blood from the head and arm through the superior vena cava.
From the left atrium blood mixes with desaturated blood from the lungs and enters the left ventricle and then to the ascending aorta. Also, blood in the right atrium flows to the right ventricle and then to the pulmonary trunk. From here, most of the blood passes through the ductus arteriosus to the descending aorta.
The blood from the umbilical vein during its course has a high oxygen content, however, this is gradually decreased by mixing with desaturated blood. This may occur:
1. In the liver by mixing with a Small amount of blood returning from the portal system.
2. In the inferior vena cava by mixing with deoxygenated blood from the lower limbs, pelvis and kidneys.
3. In the right atrium by mixture with blood from the head and upper limb.
4. In the left atrium by mixing with blood from the lungs.
5. At the entrance if the ductus arteriosus into the descending aorta.

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