S2: Excretion

• All chemical reactions that happen in your cells make up your metabolism
• Metabolism produces waste products - such as carbon dioxide and nitrogenous (nitrogen containing) waste
• Excretion is the removal of the waste products of metabolism from the body
• Excreting waste products from the body maintains normal metabolism and homeostasis by helping to keep the levels of certain substances in the blood roughly constant
• One of the functions of liver is to break down metabolic waste products and other substances that can be harmful like drugs and alcohol
• Hepatic artery supplies the liver with oxygenated blood from the heart so the liver has a good supply of oxygen for respiration, providing plenty of energy
• Hepatic vein takes deoxygenated blood away from the liver
• Hepatic portal vein brings blood from the duodenum and ileum so it's rich in the products of digestion. This means any ingested harmful substances are filtered out and broken down straight away
• Bile duct takes bile to the gall bladder to be stored
• Liver is made up of liver lobules - cylindrical structures made of cells called hepatocytes that are arranged in rows radiating out from the centre
• Each liver lobule has a central vein in the middle that connects to the hepatic vein
• Many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule
• The hepatic artery and hepatic portal vein are connected to the central vein by capillaries called sinusoids
• Blood runs through the sinusoids past the hepatocytes that remove harmful substances and oxygen from the blood
• hepatocytes break down harmful substances into less harmful substances that can re-enter the blood
• Blood runs to the central vein and the central vein from all lobules connect up to form the hepatic vein
• Kupffer cells are also attached to the walls of the sinusoids
• Kupffer cells remove bacteria and break down old red blood cells
• Hepatocytes produce bile and secrete it into tubes called canaliculi
• Canaliculi tubes drain into the bile duct
• The bile ducts from all the lobules eventually connect up and leave the liver
• One of the liver's most important roles is getting rid of excess amino acids
• amino acids contain nitrogen in their amino groups
• Nitrogenous substances can't be stored b the body
• Excess amino acids are broken down in the liver
• 1. Nitrogen containing amino groups are removed from any excess amino acids forming ammonia and organic acids - deamination
  2. organic acids can be respired to give ATP or converted to carbohydrates and stored as glycogen
  3. Ammonia is combined with CO2 in the ornithine cycle to create urea and water
  4. Urea released from liver into blood. Kidneys then filter the blood and remove the urea as urine
• Liver also breaks down other harmful substances like alcohol drugs and unwanted hormones
• Breaking down alcohol, drugs and unwanted hormones into less harmful compounds that can be excreted from the body is detoxification
• Alcohol is broken down in the liver into ethanal, which is then broken down into acetic acid.
• Excess alcohol over a long period can lead to cirrhosis and of the liver - when the cells of the liver die and scar tissue blocks blood flow
• Paracetamol in the blood can lead to liver and kidney failure if not broken down by detoxification
• Insulin is a hormone that controls blood glucose concentration
• Insulin is broken down by the liver as excess insulin can cause problems with blood sugar levels
• Liver converts excess glucose in the blood to glycogen in a process called glycogenesis
• Glycogen is then stored as granules in the liver cells until the glucose is needed for energy
• The kidney excretes urea produced by the liver
• KIDNEY EXCRETION

  Blood enters the kidneys through the renal artery and then passes through capillaries in the cortex of the kidneys
• KIDNEY EXCRETION

  ultrafiltration - blood passes through the capillaries, substances are filtered out of the blood and into long tubules that surround the capillaries
• KIDNEY EXCRETION

  Useful substances are reabsorbed back into the blood from the tubules in the medulla and cortex - selective reabsorption
• KIDNEY EXCRETION

  Remaining unwanted substances (e.g. urea) pass along tubules then along the ureter to the bladder where they're expelled as urine.

  The filtered blood passes out of the kidneys through the renal vein