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.