Foetal circulation

The foetal circulation is unique in the sense that less than 10% of cardiac output passes through the lungs. This is suited to life in utero because the foetus does not breathe, instead gets all it’s gas exchange via the placenta, however after birth this situation has to rapidly change to ensure survivial outside the womb.

Schematic

foetal circulation

Blood leaves the placenta via the umbilical vein with an oxygen saturation of around 80%. The ductus venosus shunts half of the blood across the liver directly into the inferior vena cava. The mixed venous blood oxygen saturation is around 65%. Two thirds of blood is shunted directly into the left atrium via the foramen ovale.

There is intense hypoxic pulmonary vasoconstriction, therefore the majority of the blood in the pulmonary artery flows through the ductus arteriosus into the aorta. Less than 10% of the cardiac output passes through the pulmonary circulation.

The umbilical arteries arise from the internal iliac arteries and pass to the placenta.

Birth

With the first breath a negative intrathoracic pressure of around -50cmH20 is generated, expanding the FRC and encouraging blood flow through the lungs. Ventilation of the alveoli reduces hypoxic pulmonary vasoconstriction and therefore reduces pulmonary vascular resistance. This reduces the amount of blood flowing across the ductus arteriosus and when the umbilical cord is clamped this raises the systemic vascular resistance and can reverse the flow of blood through the ductus.

Exposure to oxygenated blood and a drop in prostaglandin E2 causes closure of the ductus arteriosus in less than 24 hours.

Oxygen saturation at different points

Umbilical vein: 80%
IVC pre-ductus venosus: 25%
Mixed IVC: 65%
Aorta pre-ductus arteriosus: 60%
Descending aorta: 50%

Foetal haemoglobin

Contain two alpha and two gamma chains, the gamma chains do not bind to 2,3-diphosphoglycerate shifting the oxyhaemoglobin dissociation curve to the left. The P50 of foetal haemoglobin is 2.5kPa, compared to adult haemoglobin which is 3.5kPa. The Hb concentration is around 160g/l at birth.

A baby starts to synthesis haemoglobin A (adult haemoglobin: two alpha, two beta chains) a few weeks before birth and by the age of 2 years haemoglobin F is no longer present.

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