Hemodynamics of the ductus venosus.

Although the ductus venosus has a similar function in human as in animal pregnancies (to regulate the shunting of oxygenated blood from the umbilical vein towards the left atrium), the amount of blood shunted in the human fetus seems to be less (25-40%) than in the animal (50%). The degree of shunting depends both on the resistance of the portal vasculature in the liver as well as the resistance of the ductus venosus itself. Neural and endocrine regulation plays a role in this distribution, as do fluid mechanical forces; blood viscosity and umbilical venous pressure are powerful determinants. There is a high degree of shunting at reduced umbilical venous pressure, and by increasing hematocrit, and viscosity, the distribution shifts from the liver to the ductus venosus. Additionally, the ductus venosus acts as transmission line in the opposite direction for the atrial pressure waves. Shape, viscosity, compliance, and particularly the diameter of the inlet are suggested to influence the pulsatility of the blood velocity at the ductus venosus inlet--and determine the degree of wave transmission into the umbilical vein. Occurrence of umbilical venous pulsation, an important diagnostic sign, is also dependent upon the size and compliance of the umbilical vein.