Foetal Circulation
Prior to birth the foetus is not capable of respiratory function and
thus relies on the maternal circulation to carry out gas, nutrient and
waste exchange. The foetal and maternal blood never mix, instead they
interface at the placenta.
Consequently the liver and the lungs are non-functional, and a series
of shunts exist in the foetal circulation so that these organs are
almost completely by-passed.
Shunt 1: The Ductus Venosus
Oxygenated blood travels from the placenta via the umbilical vein and most of it bypasses the liver
by way of the ductus venosus. The ductus venosus links the umbilical
vein to the caudal vena cava and the flow of blood is controlled by a
sphincter, enabling the proportion travelling to the heart via the liver to be altered.
Shunt 2: The Foramen Ovale
The foramen ovale is an opening between the two atria enabling blood
to be channelled directly into the systemic circulation thereby
bypassing the lungs.
The septum secundum directs the majority of the blood entering the
right atrium through the foramen ovale into the left atrium. Here it
mixes with a small volume of blood returning from the non-functional
lungs via the pulmonary veins.
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