Liver is the largest organ in the abdominal cavity and occupies out 40% space of abdominal cavity. It is a solid organ and the cells of liver are called hepatocytes. Liver gets dual blood supply: (i) venous blood from stomach & intestine through portal vein and (ii) directly from arterial system through hepatic artery. Liver receives about 15-20% of blood supply of body therefore; it is highly vascularized organ. Metabolically liver is most active organ and it is associated with efficient utilization of digested nutrients. Associated with liver is a pouch like organ called gall bladder as explained earlier which stores and secrete bile juice. Bile juice is involved in the digestion of lipids
Main functions of liver are listed below:
Utilization/metabolism of digested nutrients.
Synthesis of blood proteins and lipids.
Storage of glycogen, minerals, vitamins and other nutrients.
Synthesis of bile and elimination of fat soluble metabolites.
Detoxification of toxic compounds, drugs and hormones.
Because of above mentioned important functions, liver is considered as a vital organ and without which life is impossible. After absorption in small and large intestine, the nutrients are first taken to liver through portal vein. In liver these nutrients are undergo following transformations:
Surplus nutrients are storage as glycogen, lipids and proteins etc.
Synthesis of blood proteins, lipids taken place in liver to replace metabolized products.
Toxic substances entering through GI tract are detoxified in the liver (e.g. conversion of NH3 into urea) and only clear and safe blood is allowed to enter general circulation.
Synthesis of bile salts occurs in the hepatocytes from cholesterol. Sodium glycocholate, sodium taurocholate and other derivatives of these bile salts are involved in emulsification of dietary lipids in small intestine. These bile salts have a detergent property, owing to their amphoteric nature, that helps in splitting fat globules in smaller droplets. Thus the surface area of lipid droplet is increased several thousands times for action of digestion enzyme. This action of bile salts on lipids is also known as 'Surfactant action'.
Gall bladder receives the bile from the liver and stores it temporarily. During storage bile is concentrated by absorption of water. The gall bladder opens into duodenum through cystic duct. When food passes through duodenum, the gall bladder undergo contraction and bile is discharged into the duodenum. Quantity of bile discharged into intestines is proportional to the amount of lipids entering duodenum and this is controlled by nervous system and cholecystokinin hormone.