The Liver is one of two principal accessory organs of the digestive system. Its metabolic capabilities is so crucial for maintaining several of the body’s critical functions that a wide array of physiologic disturbances occur when there is damage to the liver.
The adult human liver normally weighs between 1.0- 2.5 kilograms and is a soft, pinkish-brown boomerang shaped organ. It is the second largest organ right after the skin. Its anatomical position in the body is immediately under the diaphragm on the right side of the upper abdomen. The liver lies on the right of the stomach and makes a kind of bed for the gallbladder.
The liver is supplied by two major blood vessels on its right lobe: the hepatic artery and the portal vein. The hepatic artery normally comes off the celiac trunk. The portal vein brings venous blood from the spleen, pancreas and small intestines, so that the liver can process the nutrients and byproducts of food digestion. The hepatic veins drain directly into the inferior vena cava.
The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. These eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic duct. The cystic duct (from the gallbladder) joins with the common hepatic duct to form the common bile duct. Bile can either drain directly into the duodenum via the common bile duct or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the duodenum together at the ampulla of Vater. The branching of the bile ducts resemble those of a tree, and indeed the term “biliary tree” is commonly use.
The liver is among the few internal organs capable of tissue regeneration. Even as little as 25% of a remaining liver can regenerate to a whole liver again. This is predominantly due to the hepatocytes acting as unipotential stem cells.
The various functions of the liver are carried out by liver cells or hepatocytes.
- The liver produces and excretes bile required for food digestion. Some of the bile drains directly into the duodenum and some are stored in the gallbladder.
- The liver performs several roles in carbohydrate metabolism:
- Gluconeogenesis– formation of glucose from certain amino acids, lactate or glycerol
- Glycogenolysis- formation of glucose from glycogen.
- Glycogenesis– formation of glycogen from glucose.
- Breaks down insulin and other hormones.
- The liver also performs several roles in lipid metabolism:
- Cholesterol synthesis.
- The production of triglycerides (fat).
- The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX and XII as well as protein C, protein S and antithrombin.
- The liver breaks down hemoglobin (bile pigments are its metabolites), toxic substances and most medicinal products.
- The liver converts ammonia into urea.
- The liver stores a multitude of substances including glucose in the form of glycogen, vitamin B12, iron and copper.
- In the first trimester fetus, the liver is the main site of red blood cell production. By the 42nd week of gestation, the bone marrow has almost completely taken over that task.