Nutrient Supply

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•  Some nutrients in excess can be stored in the liver

•  Surplus cholesterol is converted into bile salts

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

liver lipoproteins