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Foetal circulation
Whilst the baby is in the
womb, foetal circulation is rather complicated:
Gaseous exchange takes place
at the placenta and oxygenated blood returns to the heart
via the umbilical vein and inferior vena cava. The oxygenated
blood enters the heart through the right atrium and then the
majority is directed into the left atrium through a flap-like
valve called the foramen ovale.
From the left atrium it
enters the left ventricle which pumps blood to the brain and
the rest of the body and will finally return to the placenta
for reoxygenation.
After birth the deoxygenated
blood returning to the right side of the heart is pumped to
the lungs via the right atrium and ventricle through the pulmonary
artery. However in the unborn baby the blood returning to
the right side of the heart is oxygenated, the majority therefore
bypasses the lungs by passing directly to the left side of
the heart through the foramen ovale. A small amount enters
the right ventricle and is pumped out into the pulmonary artery.
As the lungs are not in use most of this blood diverted away
from the lungs through the ductus arteriosus,
a blood vessel that connects the pulmonary artery to the aorta.
The blood can bypass the
lungs because the blood vessels of the lungs are constricted
and therefore the blood pressure in the pulmonary circulation
is higher than in the body circulation, the blood therefore
flows more easily through the ductus arteriosus.
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