A fetus (alternatively foetus) is an embryo in later stages of development, from the third month of pregnancy until birth in humans.
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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.
Blood from the placenta is carried by the umbilical vein. About half of this enters the ductus venosum and is carried to the inferior vena cava, while the other half enters the liver proper. 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 from the right into the left atrium, then into the left ventricle from where it is pumped through the aorta into the body. Some of the blood moves from the aorta through the internal iliac arteries to the placental arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the mother's circulation.
Some of the blood from the right atrium does not enter the left atrium, 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 anyway).
With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced. More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the foramen ovale to the left atrium. The blood from the lungs travels through the pulmonary veins to the left atrium, increasing the pressure there. The decreased right atrial pressure and the increased left atrial pressure pushes the septum primum against the septum secundum, closing the foramen ovale, which now becomes the fosse ovalis. This completes the separation of the circulatory system into two halves, the left and the right.
The ductus arteriosus normally closes off within one or two days of birth.
Infants with certain congenital anomalies of the heart can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of prostaglandins to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of patent ductus arteriosus, where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided.
Remnants of the fetal circulation can be found in adults:
The word fetus comes directly from the Latin fetus meaning "offspring". The pseudo-Greek foetus is thus etymologically incorrect. In general, the medical community only permits the spelling fetus, but the spelling foetus persists in the wider community, most notably in Britain, and is accepted in some dictionaries.
See also: Child, SuperfetationDevelopment
Circulatory system
Etymology and spelling variations
