
postnatal changes in the portal circulation
during foetal development a large portion of the
oxygenated umbilical blood flows through the
umbilical recess directly into both main portal
branches.  accordingly, these are exposed to the
umbilical blood pressure and have a wide
by contrast, the trunk of the portal vein is narrow
and there is probably only a small blood flow through
it from the inactive digestive tract, though the
pressure in it is approximately the same as in the
umbilical vein or perhaps somewhat higher.
immediately after birth the liver is deprived of the
substantial umbilical blood flow.  the portal
venous pressure falls to about one-quarter of the
umbilical venous pressure at birth.  however the
pressure gradient between the portal veins and
inferior vena cava remains constant.
