Liver and portal circulation
Liver and portal vasculature
This figure shows the major veins that supply the portal vein, which proceeds toward the liver.
It should be noted that the majority of venous blood from the rectum does not go through the portal vein, but bypasses the portal system on the way to the inferior vena cava.
The liver serves many functions in the human body. The regulation of fat, protein, and carbohydrate metabolism is the most basic liver function. However, the liver also plays a crucial role by breaking down hormones and inactivating drugs and toxins. Via bile production, the liver excretes all kinds of metabolites, including cholesterol, heavy metals, and drugs. Bile also serves an important digestive function of the liver. Furthermore, the liver synthesizes proteins for blood clotting and stores iron and certain vitamins.
Extra info: By avoiding the portal vein, rectal administration of drugs avoids the first pass metabolism of the liver.