Structure
Blood supply
The liver has a dual blood supply with the portal vein supplying 60‐70% of the blood flow to the liver, the remainder being supplied by the hepatic artery. The efferent blood flow is via the hepatic vein which enters the caudal vena cava.
Microscopic structure
The liver is composed of plates of hepatocytes, between which run the hepatic sinusoids where blood from the portal vein and hepatic artery mix and flow through to the central vein. The sinusoids are lined by phagocytic Kupffer cells and hepatic stellate cells (or Ito cells) inhabit the space of Disse and function as storage centers for vitamin A and have potential to differentiate into myofibroblasts in the fibrotic liver. Small membrane indentations in adjacent hepatocytes form the bile canaliculi, into which bile is secreted, eventually draining into the hepatic duct, gall bladder and finally into the duodenum via the common bile duct.
Hepatocytes are specialized polygonal cells with high metabolic activity whose features reflect the multitude of functions of the cell:
- many mitochondria
- network of rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER)
- lysosomes, peroxisomes, lipid droplets and glycogen stores
- microvilli adjacent to sinusoids – each hepatocyte has at least one surface bordering on a sinusoid
- small indentations in the plasma membranes of adjacent hepatocytes form the bile
- bile canaliculi
Function
The liver is a complex organ with a multitude of functions. Understanding liver function facilitates the interpretation of clinical pathology data. Listed below are some of the functions of the liver that are relevant to the clinical pathology changes observed in liver disease.
- Carbohydrate metabolism
- glucose: gluconeogenesis, glycogenolysis
- glycogen storage
- Lipid metabolism
- cholesterol synthesis, esterification and excretion
- triglyceride metabolism and storage
- lipoprotein synthesis
- phospholipid metabolism
- Protein synthesis
- albumin
- fibrinogen
- acute phase proteins
- coagulation proteins and inhibitors of coagulation: e.g. coagulation factors, protein C, antithrombin
- Vitamin metabolism
- intestinal bile acids facilitate absorption of fat soluble vitamins, e.g. vitamin K
- Immune function
- Kupffer cells clear intestinal pathogens, particulates and damaged cells
- Detoxification and excretion
- excretion of nitrogenous wastes via urea cycle
- bile secretion
- biotransformation and detoxification of drugs and toxins
