The fetal circulation

Distribution of blood volume v s Systemic circuit 80%

l Arteries 10% si y Capillaries 5%

t Veins and venules 65% e

3 Pulmonary circuit 12% Heart 8%

* Deoxygenated blood from the superior vena cava flows into the right atrium, being primarily directed into the right ventricle and ejected into the pulmonary artery.

* Mixing of the venous returns occurs.

* The pulmonary circulation is a high-impedance and low-flow system.

* The placental circulation is a low-impedance and high-flow system At birth, a transitional circulation ensues, with the following changes:

A reduction in pulmonary vascular resistance secondary to expansion of the lungs and an increase in arterial pO2.

An increase in systemic vascular resistance caused by removal of the low resistance placental circulation.

The systemic vascular resistance exceeds the pulmonary vascular resistance. A left-to-right shunt through the ductus arteriosus, with progressive closure. Functional closure of the foramen ovale as a result of raised left atrial pressure and volume.

Closure of the ductus venosus as a result of removal of the placenta from the circulation.

The adult circulation consists of two chambers, the right and left ventricles, in series, with two interposed vascular beds (systemic and pulmonary).

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