5.1.2. Excretion as an example of homeostatic control

• The blood vessels in the liver are

  • hepatic vein; blood leaves the liver
  • hepatic artery; blood enters via a narrow vessel
  • hepatic portal vein; blood from the gut (nutrient dense) enters through branched vessel
• What is deamination?


  Ammonia‘s NH2 group removed from amino acid
• CO2 disassociates into hydrogen carbonate ions and ammonia is similarly acidic thus reducing the ph of the cytoplasm. the buildup can denature enzymes. CO2 also inhibits haemoglogin
•  CO2 is diffuses out of all cells in respiring tissues into the bloodstream, where it is transported, mostly as hydrogencarbonate and hydrogen ions, to the lungs, where carbon dioxide diffuses into the alveoli and is excreted in expiration.
• The liver has many metabolic roles and the waste products are passed into the bile for excretion in the faeces, for example bilirubin. It is involved in converting deaniminting excess amino acids, producing ammonia. This is a highly soluble and toxic molecule, so is converted to the less soluble and toxic compound, urea. The urea is passed into the bloodstream and carried in solution to the kidney and excreted in the urine.
•  is important for the liver to have a good blood supply so hepatocytes can carry of hundred of metabolic processes, that play a role in excretion and homeostasis. A good blood supply is maintained by the hepatic artery, and the hepatic portal vein.
• The arrangement of the liver into lobes, further divided into lobules each supplied by an intralobular-branch of the hepatic artery and hepatic portal veins, ensures that the greatest possible contact is maintained between the the blood and the liver cells. Oxygenated blood travels from the aorta into the liver in the former, supplying a high pO2 of oxygen to sustain a high rate of aerobic respiration, to sustain active processes in hepatocytes.
• The hepatic portal vein supplies deoxygenated blood from the digestive system. This provides hepatocytes with high concentrations of respiratory substrates from digestion, but mainly is needed so that the toxic compounds not absorbed by the intestine can have their concentrations adjusted in the liver before they circulate around the body.
• The blood is mixed in the sinusoid to provide both the products of digestion, and oxygen needed to carry out the metabolic functions, such as protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification, etc in the dense cytoplasm of the cuboidal hepatocytes. They have microvilli to increase the surface area in contact with blood in the sinusoid. Kupffer cells, specialised macrophages, are mobile within the sinusoid, breaking down and recycling old red blood cells, the by-product of which is the pigment bilirubin.
• Blood then flows out through the hepatic vein, via an intra-lobular vessels. Bile made in the hepatocytes is released into bile canaliculi, which join to form the bile duct, which carries the secretion from the liver’s metabolic processes from the liver to the gallbladder, where it is stored until required to aid the digestion of fats in the small intestines. Bile also contains excretory products which will be excreted in faeces.
• Role of the liver

  • The liver carries out a wide number of functions including control of blood glucose levels, amino acid, and lipid levels, synthesis of bile, plasma proteins, and cholesterol, storage of vitamins, and glycogen, breakdown of hormones, and erythrocytes.

  Glucose is stored in the form of glycogen, with live cells performing glycogenolysis to a combine approximate value of 100-120g of glycogen. The glycogen is either broken down, or synthesised in response to the hormones glucagon and insulin, maintaining blood glucose concentration.
• Another important role is to detoxify substances, either found in the body, such as hydrogen peroxide, or from consumption, such as ethanol. Through oxidation, reduction, methylation, or combination with another molecule, toxins can be rendered harmless. The liver contains enzymes which perform this. Catalase converts hydrogen peroxide to oxygen and water; whilst, cytochrome P450 is a group of enzymes used to breakdown many drugs, including cocaine, as well as being used in the electron transport chain.
• Another important role is to detoxify substances, either found in the body, such as hydrogen peroxide, or from consumption, such as ethanol. Through oxidation, reduction, methylation, or combination with another molecule, toxins can be rendered harmless. The liver contains enzymes which perform this. Catalase converts hydrogen peroxide to oxygen and water; whilst, cytochrome P450 is a group of enzymes used to breakdown many drugs, including cocaine, as well as being used in the electron transport chain.
• Ethanol is toxic both to cells and in depressing nerve activity, but also contains chemical potential energy as a respiratory substrate. Alcohol dehydrogenase oxidises ethanol to ethanal, and then to ethanoate/acetate by ethanal dehydrogenase. This can combine with Coenzyme A to form acetyl CoA, which enters the Krebs cycle. In this process two molecules of NAD are reduced.
• If stored, the amine group would make amino acids toxic, but they also contain chemical potential energy. Therefore, in the liver the amino acids are deaminated, removing hydrolysing the amine group in the presence of oxygen to leave a keto acid, which can be respired, and ammonia. As ammonia is toxic, it is combined with CO2 and ornithine in the ornithine cycle to produce urea. A condensation  reaction, this first step produces citrulline, another ammonia molecule is added to form arginine, which is converted back to ornithine by the removal of one molecule of urea.