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The fetal circulatory system works differently than it does after birth. The placenta is the organ that develops during the pregnancy and is implanted into the endometrium of the mother’s uterus. The fetus gets all of its nutrients, oxygen, and life support because the umbilical vein transports it through the placenta to the fetal body. Any kind of waste and carbon dioxide that is from the fetus is sent back to the mother’s circulation to be discarded through the umbilical cord and placenta. Blood from the mother enters the placenta and is very close to the fetal blood; once that happens exchanges can be made.
Things like sugar, protein and fat molecules can move from the mother’s blood to the fetus’s blood and the waste and carbon dioxide moves from the fetus’s blood to the mothers to be eliminated. All of this happened through the umbilical vein. The umbilical vein goes to the liver and there it is split into three branches. One going to the inferior vena cava, so all the nourished blood will go to the fetal heart and then to the rest of the body.
In a fetus the lungs are nonfunctional, so when the blood from the inferior vena cava enters the right atrium, most of it is sent straight to the left atrium. The blood moves from the right to left atrium through an opening called the foramen ovale. Enough blood is brought to the lung tissue to sustain them though. The blood that does not move through the foramen ovale stays in the right side of the heart and then flows into the pulmonary artery and out through the pulmonary trunk. The blood in the pulmonary trunk moves past the lungs by a fetal vessel called the ductus arteriosus. This vessel connects the pulmonary trunk to the descending aortic arch. Coronary and carotid arteries take oxygenated blood to the heart and brain. Some of the blood from the descending aorta is carried to different parts of the lower body. The rest of the blood from the descending aorta goes through the umbilical arteries, which then go to internal iliac arteries and from there to the placenta.