Fetal circulation
During embryological life gas exchange is not support by lungs but by placenta (lungs starting to work at birth), then it is not necessary to have high blood flow to the lungs. This and other fetal circulatory requirements are obtained through some communication between the right and the left heart side, typical of the fetal heart. These are foramen ovale the communication between the atria, Aransius ductus venosus the communication between umbilical artery and the inferior vena cava, ductus arteriosus the communication between aorta and pulmonary artery.

Fig. 2.2
schematic representation of fetal circulation: the oxygenated blood coming from the placenta flow from the umbilical vena (UV) through the ductus venosus of Aransius, in the inferior vena cava and from this , principally through the foramen ovale, to the left atrium, to the left ventricle and ascending aorta. Blood with low content of oxygen flow preferentially through the tricuspid valve in right ventricle, in pulmonary artery and through ductus arteriosus (DA) in the descending aorta. Part of this blood flows back to the placenta through umbilical arteries to be reoxygenated. PO2 indicates the oxygen level of the blood in the different sectors of circulation (see the text for more details)-.

The oxygenated blood coming from the placenta through the umbilical vein arrives in the inferior vena cava and then in the right atrium; from here it drives essentially through the foramen ovale, in the left atrium and then left ventricle and aorta. This oxygenated blood goes principally to the brain. The non-oxygenated blood coming from the superior vena cava enters the right atrium and goes to the right ventricle and then in pulmonary artery. Only a little part of this blood goes to the lung whereas the most part, through the ductus arteriosus, goes down the descending aorta, in the umbilical arteries and then the placenta.

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