Shown here, the liver segments. The segments are defined by the bile duct, portal vein and branches of the liver arteries. Humans have six to ten segments, depending on how you count. The liver is the most important organ for the degradation and metabolism of exogenous and endogenous substances. It also is vitally important for absorption and utilization of food components, provides essential proteins and regulates substances in the immune system and hormones. Although only about 1500 - 2000g heavy, the liver is very well perfused (28% of total blood flow) and consumes about 20% of the total oxygen from the blood. The blood it receives comes partially from the intestinal blood vessels that carry nutrient-rich blood so that the nutrients can be processed immediately. It also receives arterial oxygen-rich blood from systemic circulation. After passing through the liver the blood is returned back into the circulation. Produced by the liver gall flows through the bile duct system into the intestine. Thus degradation products and toxic substances are discharged into the instestine where these substances simultaneously aid digestion.