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.