Brent Cornell

    
The liver plays an important role in regulating the levels of nutrients in the bloodstream

• Nutrients absorbed by the small intestine are transported by the hepatic portal vein to the liver for metabolism
• The liver converts these nutrients into forms that can be stored or used and mediates their transport to various tissues
• Nutrients stored within the liver include glycogen, iron, vitamin A and vitamin D

Carbohydrate Metabolism

• Excess glucose in the bloodstream (e.g. after meals) is taken up by the liver and stored as glycogen 
• When blood glucose levels drop, the liver breaks down glycogen into glucose and exports it to body tissues
• When hepatic glycogen reserves become exhausted, the liver synthesises glucose from other sources (e.g. fats)
• These metabolic processes are coordinated by the pancreatic hormones – insulin and glucagon

Protein Metabolism

• The body can not store amino acids, meaning they must be broken down when in excess
• Amino acid breakdown releases an amine group (NH₂), which cannot be used by the body and is potentially toxic
• The liver is responsible for the removal of the amine group (deamination) and its conversion into a harmless product
• The amine group is converted into urea by the liver, which is excreted within urine by the kidneys
• The liver can also synthesise non-essential amino acids from surplus stock (via transamination)

Fat Metabolism

• The liver is the major site for converting excess carbohydrates and proteins into fatty acids and triglycerides
• It is also responsible for the synthesis of large quantities of phospholipids and cholesterol 
• These compounds are then stored by the liver or exported to cells by different types of lipoproteins
• Low density lipoprotein (LDL) transports cholesterol to cells, for use in the cell membrane and in steroid synthesis
• High density lipoprotein (HDL) transports excess cholesterol from cells back to the liver (for storage or conversion)
• LDL is considered ‘bad’ as it raises blood cholesterol levels, while HDL lowers cholesterol levels and is therefore ‘good’
• Surplus cholesterol is converted by the liver into bile salts, which can be eliminated from the body via the bowels

The Role of the Liver in the Transport of Lipids