Hemodynamics of the ductus venosus

Although the ductus venosus has a similar function in human as in animal pr egnancies (to regulate the shunting of oxygenated blood from the umbilical vein towards the left atrium), the amount of blood shunted in the human fet us seems to be less (25-40%) than in the animal (50%). The degree of shunti ng 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, an d by increasing hematocrit, and viscosity, the distribution shifts from the liver to the ductus venosus. Additionally, the ductus venosus acts as tran smission line in the opposite direction for the atrial pressure waves. Shap e, viscosity, compliance, and particularly the diameter of the inlet are su ggested to influence the pulsatility of the blood velocity at the ductus ve nosus inlet - and determine the degree of wave transmission into the umbili cal vein. Occurrence of umbilical venous pulsation, an important diagnostic sign, is also dependent upon the size and compliance of the umbilical vein . (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.