Topic 6C- Homeostasis

• • The hypothalamus is the part of the brain that controls homeostasis
• What is homeostasis? 



  the maintenance of a stable internal environment
• • Beta cells secrete insulin when blood glucose concentration is too high
• • Alpha cells secrete glucagon which increases blood glucose levels
• 1. Insulin binds to specific receptors on the cell membrane of liver cells and muscle cells.
  2. This increases the permeability of glucose for liver/muscle cells, so cells take up more glucose by increasing number of channel proteins in the cell membrane
  3. Insulin also activates the enzymes in the liver to convert glucose into glycogen to store in the cytoplasm- glycogenesis
  4. Insulin also increases the rate of respiration
• When glucose levels are low

  1. Glucagon binds to specific receptors on the liver cell
  2. It activates enzymes in the liver cell which break down glycogen into glucose- glycogenolysis
  3. Also activates an enzyme that forms glucose from non-carbohydrates eg. glycerol and amino acid this is known as gluconeogenesis
  4. Glucagon decreases the rate of respiration of in cells
• Adrenaline: A hormone secreted by the adrenal gland
• Adrenaline is secreted when there is a low level of glucose in the blood when you are stressed or when you are exercising.
• Adrenaline binds to receptors in the cell membrane of the liver cell

  1. Inhibits glycogenesis
  2. Activates glycogenolysis
  3. Inhibits insulin secretion
  4. Activates glucagon secretion
• Adrenaline and Glucagon act via secondary messengers.
• Adrenaline and glucagon bind to their receptors and activate an enzyme called adenylate cyclase. This enzyme converts ATP into a chemical signal called a 'secondary messenger'. This messenger is called cyclic AMP ( cAMP). This enzyme activates an enzyme called protein kinase A which leads to glycogenolysis.
• The islets of Langerhans (a cluster of cells in the pancreas) secrete two hormones, insulin, and glucagon. Insulin is released when blood sugar concentration is high and glucagon is released when blood sugar concentration is low.
• Hypoglycaemia is when the blood glucose level is too low. Hyperglycaemia is when the blood glucose level is too high.
• In Type I diabetes, the immune system attacks the beta cells, so they can not produce any insulin
• In Type II diabetes, the pancreas produces insulin but the body cells are resistant to its action due to receptors on cells not working properly or when the beta cells don't produce enough insulin. This type is usually linked to obesity.
• The long tubules along with the bundle of capillaries where the blood is filtered is called nephrons.
• Blood from the renal artery enters the kidneys


  Enters smaller arterioles in the cortex
• Glomerulus


  Structure that each arteriole splits into
• Ultrafiltration


  Takes place in the glomerulus
• Afferent arteriole


  Arteriole that takes blood to each glomerulus
• Efferent arteriole


  Arteriole that takes the filtered blood away from the glomerulus
• Efferent arteriole is smaller in diameter than the afferent arteriole


  Blood in the glomerulus is under high pressure
• High-pressure forces


  Liquid and small molecules in the blood out of the capillary and into the bowman's capsule
• Layers of the bowman's capsule

  • Capillary wall
  • Basement membrane
  • Epithelium
• Larger molecules and blood cells can't pass through the bowman's capsule
• Glomerular filtrate


  Molecules that do reach the nephron tubules
• Glomerular filtrate passes along the rest of the nephron


  Useful substances are reabsorbed along the way
• Filtrate flows through the collecting duct


  Passes out of the kidneys along the ureter
• Selective reabsorption takes place as the glomerular filtrate flows along the proximal convoluted tubule, through the loop of Henle, and along the distal convoluted tubule.
• The epithelium of the wall of the PCT has microvilli to provide a large surface area for the reabsorption of useful materials from the glomerular filtrate into the blood.
• Useful solutes are reabsorbed along the Proximal convulated tubules (PCT) by active transport and facilitated diffusion
• Water enters the blood by osmosis because the water potential is lower than that of the filtrate. Water is reabsorbed along the proximal convoluted tubules, loop of Henle, Distal convoluted tubules, and the collecting duct.
• Urine is usually made up of water and dissolved salts, urea, and other substances eg. hormones and excess vitamins