The ductus venosus and the mechanism of its closure.

Although recognized since Vesalius as one of the distinguishing features of the foetal circulation, the precise function of the ductus venosus is uncertain. Anatomically, it provides a direct connexion between the umbilical vein and the inferior vena cava. Functionally, it represents a bypass through which part of the oxygenated umbilical blood flows directly into the inferior vena cava. However, what portion of the umbilical venous blood flow passes through the ductus venosus in the human foetus is not yet known and has so far only been estimated approximately. Not all species of mammals possess a ductus venosus at maturity; it disappears at an early stage of gestation in the horse and the pig. Experiments have shown that occlusion of the ductus venosus in mature foetal lambs causes no significant change in blood pressure, heart rate, or carotid oxygen saturation (Amoroso, Dawes, Mott, and Rennick, 1955). Its continued patency seems to be unnecessary for survival (Blanc, 1960). On the other hand, its absence is occasionally associated with some other malformations, and can cause portal hypertension in the neonatal period (MacMahon, 1960). The findings concerning the structure of the ductus venosus are contradictory and the mechanism of its postnatal functional clQsure is not yet satisfactorily explained; nor is there any agreement concerning the time of closure after birth and the significance of its patency in the neonatal period of life. We have therefore reviewed the published papers in the light of our own studies.