Excretion, homeostasis and the liver

• Excretion = the removal of waste products of metabolism from the body
• The main metabolic waste products in mammals are:

  • carbon dioxide
  • bile pigments
  • nitrogenous waste products (urea)
• Oxygenated blood is supplied to the liver by the hepatic artery and removed from the liver and returned to the heart in the hepatic vein.
• The liver is also supplied with blood by a second vessel, the hepatic portal vein. This carries blood loaded with the products of digestion straight from the intestines to the liver and this is the starting point for many metabolic activities of the liver.
• Liver cells or hepatocytes have large nuclei, prominent Golgi apparatus and lots of mitochondria, indicating they are metabolically active cells.
• The blood from the hepatic artery and the hepatic portal vein is mixed in spaces called sinusoids which are surrounded by hepatocytes. This mixing increases the oxygen content of the blood from the hepatic portal vein, supplying the hepatocytes with enough oxygen for their needs.
• The sinusoids contain Kupffer cells, which act as the resident macrophages of the liver, ingesting foreign particles and helping to protect against disease.
• The hepatocytes secrete bile from the breakdown of the blood into spaces called canaliculi, and from these the bile drains into the bile ductules which take it to the gall bladder.
• The liver plays a vital role in protein metabolism where hepatocytes synthesise most of the plasma proteins.
• Hepatocytes carry out transamination - the conversion of one amino acid into another.
• Transamination is important because the diet does not always contain the required balance of amino acids but transamination can overcome the problems this might cause.
• The most important role of the liver in protein metabolism is deamination - the removal of an amine group from a molecule.
• The body cannot store either proteins or amino acids. Any excess ingested protein would be excreted and therefore wasted if it were not for the hepatocytes.
• Hepatocytes deaminate the amino acids, removing the amino group, and converting it first into ammonia which is very toxic and then to urea which is only toxic in high concentrations. Urea is excreted by the kidneys. The remainder of the amino acids can then be fed into cellular respiration or converted into lipids for storage.
• The ornithine cycle:

  • ammonia reacts with ornithine and carbon dioxide
  • this forms citrulline - this is a condensation reaction so water is released
  • citrulline combines with more ammonia to form arginine -this is also a condensation reaction so water is released
  • arginine combines with water in a hydrolysis reaction to form urea and ornithine
  • the cycle repeats