The circulatory system of the human fetus has four features not present in those of an adult (adult here meaning anyone outside the womb). The need for these features stems from the fact that there's no air in the womb, so the fetus has to acquire oxygen from the host... er, mother. These features are:

a) The foramen ovale, or oval opening. This is an opening between the atria of the heart, and is often called a 'hole in the heart'. It is covered by a flap of tissue that acts as a valve. Normally, blood would never flow between the atria, which are divided by the septum in adults. However, because the fetus doesn't need it's lungs the entire pulmonary pathway can be circumvented. Hence the use of such a hole.

b) The ductus arteriosus, or arterial duct. This is a connection between the pulmonary artery and the aorta. This exists because, despite the foramen ovale, blood still goes to the now-useless lungs. However, with the addition of the arterial duct most of the blood reaches the aorta (ie. body) rather than the lungs.

c) The umbilical arteries and vein. These are simply vessels which travel to and from the placenta, dumping waste and acquiring nutrients and oxygen.

d) The ductus venosus, or venous duct. This is a connection between the umbilical vein and the inferior vena cava, making the circuit complete.

The most common of all cardiac defects in newborns is the persistence of the oval opening. Normally, the duct closes as endothelial cells divide and block it off. However, in nearly 1 out of 4 individuals this closure fails to occur completely. Even then there is usually little passage of blood between the atria, because the hole is either small or actually closes when the atria contract. However, in a small number of cases the flow of deoxygenated blood is sufficient enough to cause a "blue baby". Such a condition can now be corrected by open-heart surgery, and in fact this has advanced so far that the operation can now be performed with the fetus still in the womb.

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