Fetal circulation

The circulatory system of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.Whitaker, Kent. " [http://books.google.com/books?vid=ISBN0766813738&id=R3WK8XyAHYgC&pg=PA18&lpg=PA18&ots=wcft0RkRxg&dq=%22fetal+circulatory+system%22&sig=_DSt-D7pBSBtikfJ5oQKYVlNKmU Comprehensive Perinatal and Pediatric Respiratory Care] " (Delmar 2001). Retrieved 2007-03-04.]

Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal "ductus venosus" and is carried to the inferior vena cava, while the other half enters the liver proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. The blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the "foramen ovale"), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing pulmonary circulation. The continuation of this blood flow is into the left ventricle, and from there it is pumped through the aorta into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the woman's circulation.

Some of the blood entering the right atrium does not pass directly to the left atrium through the "foramen ovale", but enters the right ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the "ductus arteriosus", which directs most of this blood away from the lungs (which aren't being used for respiration at this point as the fetus is suspended in amniotic fluid).

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