The Umbilical Vessels:
The umbilical arteries carry venous blood of low P02 and oxygen content to the placenta (Figure 2.12). The umbilical vein carries arterialized blood of a higher P02 and oxygen content from the placenta to the fetus.
The ductus venosus arises from the umbilical recess, and is a relatively low resistance bypass of the liver to the heart. It carries approximately 50% of the blood from the placenta. There is little smooth muscle in the wall of the ductus venosus.
As explained above, the crista dividens directs blood to the left atrium by way of the via sinistra, through the foramen ovale. Blood passing into the right ventricle, follows the via dextra. The flow rate through the foramen ovale is less than that which reaches the right ventricle. It has approximately one-half the cross-sectional area of the vena cava.
The ductus arteriosus is a short shunt between the pulmonary artery and aorta, a remnant of the 6th left aortic arch. Like the pulmonary vessels it is responsive to P02, contracting above a P02 of 50-60 mmHg.
Return to Index