The ductus venosus and the mechanism of its closure.

Abstract

During fetal life the ductus venosus provides a direct connection between the umbilical vein and the inferior vena cava. A significant amount of the oxygenated umbilical venous blood from the placenta bypasses the liver through this channel. The wall of the ductus venosus contains only small amounts of smooth muscle bundles. No muscular sphincter is found at its origin. There is no evidence of other preformed structures, like the so-called lips sometimes mentioned in the literature, and which are said to obstruct the duct immediately after birth. The postnatal functional closure of the ductus venosus appears to be a consequence of a decrease in pressure in the portal sinus, resulting in retraction and narrowing of its origin which thus assumes a slit-like appearance. This closure is, probably, reinforced by the muscular elements in the wall of the portal sinus near the origin of the duct. The functional closure of the ductus venosus occurs during the 1st minutes after birth and is not complete. An appreciable amount of the portal venous blood is shunted through this channel during the 1st hr. and perhaps days of life. Organic closure of the ductus venosus begins at its narrow origin, with proliferation of the obliterating tissue in specially structured border strips. Closure is completed between the 15th and 20th days.