Altered Cardiovascular Functions
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After the umbilical cord has been cut, the newborn
must quickly adapt to receive oxygen from the lungs.
The transition from foetal to pulmonary circulation
occurs in just a few hours. During foetal circulation,
the constricted pulmonary vessels limit blood flow
to the lungs (high pulmonary vascular resistance).
Blood, however, flows easily to the extremities because
systemic vascular resistance is low. The foramen ovule,
an opening between the atria in the foetal heart, allows
blood to flow from the right to the left atrium. Systemic
vascular resistance increases after the umbilical cord is
cut, causing a backup of blood flow. The pressure in the
left side of the heart increases stimulating closure of
the foramen ovule. Once breathing has been initiated,
the lungs expand and pulmonary vascular resistance
falls. Blood that was previously shunted through the
ductus arteriosus to the aorta flows to the lungs.