Highly oxygenated and nutritious blood from the placenta is carried by the left umbilical vein through the liver into the vena cava caudalis. In the liver, most of the blood flow directly “by pass” into the vena cava caudalis via the ductus venosus. However small volume of blood flow through into liver sinusoids. The vena cava caudalis receive highly-oxygenated and nutritious blood from the placenta and also low-oxygenated and notritious blood from the posterior part of fetal body and visceral organs. From the vena cava caudalis, the blood flow into the right atrium. In the right atrium, due to the high pressure of the blood from the placenta, most of the blood directly flow into the left atrium through the foramen ovale. However, small volume of blood in the right atrium mixes with low-oxygenated and nutritious blood from the vena cava cranialis. The vena cava cranialis carry blood from the head and the forelimbs.
In the left atrium, the highly oxigenated and nutritious blood from the right atrium mixes with low oxigenated and nutritious blood from blood vessel of unfunctional lung, and flow into the left ventricle. In the left ventricle, the blood mostly from the placenta which highly-oxygenated and nutritious. From the aorta, blood flow into subclavian artery and brachiocephalical trunk to supply the head and the forelimbs, respectively. Meanwhile, in the right ventricle which receives blood from the right atrium, moderate-oxygenated and nutritious blood is pumped into the lungs, and mostly is brought into the aorta by the ductus arteriosus. Understandably, because the lungs were not functioning before birth, most of the blood from the right ventricle directly flow into the aorta.
Furthermore, blood from the aorta flow to the posterior part of the body, the organs of viscera such as intestine and kidney, and the hindlimbs. Then the blood flow back into the fetal placenta through a pair of umbilical artery.
Fetal circulation are maintained in a balance of the concentration of oxygen, carbondioxide, nutrient, and metabolite by the mixing of blood from the placenta with the blood from any parts of the fetal organ.
Several important aspects of the fetal circulation are:
1. Blood from the placenta is carried by the left umbilical vein
2. Most of the blood from the plasenta flow through the liver “by pass” via ductus venosus directly to the vena cava caudalis
3. Most of the placental blood from the right atrium flow into the left atrium via the foramen ovale
4. The blood pumped by the right ventricle to the lungs, most of them pass through into the aorta via the ductus arteriosus
5. Blood from the aorta flow back to the placenta through a pair of umbilical artery.
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