The placenta is an organ that connects the fetus to the uterine wall, allowing uptake of nutrients, as well as gas exchange. Blood leaving the placenta travels through the umbilical vein. It typically has a PO2 of 30mmhg, making it roughly 80% saturated with O2.
Though veins typically carry deoxygenated blood, in fetal circulation, this is not true. Via the placenta, the umbilical vein carries oxygenated blood to fetal tissues.
The ductus venosus is a shunt, allowing oxygenated blood in the umbilical vein to bypass hepatic circulation and be conducted into the inferior vena cava (IVC).
In the right atrium deoxygenated blood from the SVC and mixed O2 blood from the IVC (and umbilical vein via ductus venosus) are pumped. At this point, blood goes through either the foramen ovale or the right ventricle and ductus arteriosus.
After the right atrium, most oxygenated blood is pumped through the foramen ovale, a shunt between the right and left atria. This shunt allows right heart blood to bypass pulmonary circulation and flow into the left heart.
After passing through the foramen ovale, blood goes into the left atrium. The foramen ovale is a flap and allows blood flow only one way, thus, when the left atrium contracts, the foramen closes and blood can only flow through the mitral valve into the left ventricle.
Blood which has been shunted to the left heart goes through normal circulation. After passing through the foramen ovale, left atrium and ventricle, the blood goes through the aorta and perfuses various tissues and organs throughout the body.
Deoxygenated blood enters the right atrium through the superior vena cava (SVC) and is pumped into the right ventricle from the right atrium.
Coming from the right ventricle, blood would typically flow into the pulmonary vasculature and circulation. This, however, is not the case in the fetus and the deoxygenated blood in the right heart is conducted back into circulatory flow via the ductus arteriosus. This is a shunt connecting the pulmonary artery to the aortic arch.
The umbilical arteries are the anterior branches of the internal iliac arteries. These arteries carry blood from the descending aorta back to the placenta for gas exchange.
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