[Cardiopulmonary adaptation to birth]

The cardio-pulmonary situation of the foetus is very special; certain changes occurring at the end of pregnancy make preparations for extra-uterine life. In the foetus, the two ventricles eject different outputs to the systemic and pulmonary circulation, these communicate between each other by the ductus arteriosus and the foramen ovale. The pulmonary output is very small on account of the high pulmonary resistance. The lungs do not participate in gas exchange (ensured by the placenta) they secrete a liquid maintaining airway distension; the synthesis of surfactant, whose role will be vital after birth, begins at the end of the second trimester of pregnancy. Dramatic changes occur at birth. The two ventricles now function in series, the shunts between the two circulations are shut. The lowering of the pulmonary vascular resistance, the aeration of the lungs, the formation of the functional residual capacity and the appearance of a regular and rhythmic respiration now enable the new born to ensure its own gas exchange.