QS Study

Foetal circulation is a special type of circulation occurred in the foetus which can be differentiated from the circulation in adult in several aspects and is essential for the survival of the foetus.

Mechanism –

Blood from the placenta – about 80% saturated with oxygen-passes to the foetus through the umbilical vein.

On approaching the liver, the main portion of this blood flows through the ductus venosus directly into the inferior vena cava. A smaller portion enters the liver sinusoids & mixes here with the blood from the portal circulation.

Blood from the inferior vena cava after reaching the right atrium divides into two portions – a larger portion of it passes through the foramen ovale to the left atrium & smaller portion mixed here with the deoxygenated blood from the superior vena cava, which collects blood from the upper half of the body.

Blood from the left atrium passes to the left ventricle & pumped out through the aorta, mainly to the upper half of the body. The mixed blood from the right atrium passes to the right ventricle. This blood passes through the pulmonary artery to the lung & through the ductus arteriosus to the descending thoracic aorta.

Blood from the descending thoracic aorta passes to the lower half of the body, but most of it passes to the placenta via the umbilical artery for oxygenation.

Fig: Foetal Circulation

Vascular changes that occur at birth

  • Closure of umbilical arteries.
  • Closure of the left umbilical vein.
  • Closure of the ductus venosus.
  • Closure of the ductus arteriosus.
  • Closure of foramen ovale.

Percentage of oxygen at the different level of foetal blood:

  • Maternal arterial blood: 95%
  • Umbilical venous blood: 80%

[95% – 80% = 15% oxygen reduced due to active metabolites of placenta]

In right atrium: 70% (Due to a mixture of blood into the inferior vena cava)

In descending aorta: 60% (Due to an admixture of blood which has come through the ductus arteriosus).

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