Liver shunts are a congenital problem in some dogs. During gestation
the placenta delivers blood with food and oxygen from the mother through
the umbilical vein. This means that in the fetus, circulation is the reverse
of circulation after birth, because the fetus' veins have the oxygenated
blood and arteries return unoxygentated blood to the heart. In order to
make this work, there is a shunt from the liver venous circulation to the
arterial circulation. At birth, the pressure within the circulatory system
changes as respiration occurs and this shuts the shunt, which eventually
disappears. If this reverse in circulation doesn't happen for some reason,
the liver is deprived of a blood supply and doesn't develop properly after
birth. Many puppies can live with the small functioning portion of the
liver for some time but eventually have problems and usually die if the
situation is uncorrected. It is possible to surgically close the shunt
and the surgery works well. I can remember hearing of one sheltie that
was 6 years of age (or possibly older) before a congenital liver shunt
was recognized, so some dogs can live a long time with this problem.