Control of blood water potential

• Nephron


  The functional unit of the kidney, found in the medulla
• Role of the kidneys

  • Osmoregulation - control blood glucose level
  • Excretory organ - excrete toxic waste products of metabolic processes (urea) and substances in excess (salts)
  • Around 1 million nephrons in each kidney
  • Long tubule surrounded by capillaries
• Glomerulus


  • Network of blood capillaries
  • At the beginning of the nephron
  • Capillaries branched off from afferent arteriole
• Renal capsule


  Also bowman's capsule, surrounds the glomerulus
• Proximal convoluted tubule


  Branches from bowman's capsule
• Loop of Henle


  Long U shaped portion of tubule, leads on from proximal convoluted tubule
• Collecting duct


  Series of tubules which connects nephrons
• Interstitial space


  Space between the nephron and the capillaries
• Components of urine

  • Water
  • Dissolved salts
  • Urea
  • Other small substances e.g. hormones and excess vitamins
• Proteins and blood cells are not found in urine because they are too large to be filtered out of the blood
• Glucose is not found in urine because all glucose is absorbed at the selective reabsorption stage in the proximal convoluted tubule
• How blood first enters the nephron


  From the afferent arteriole
• How high hydrostatic pressure is established in the glomerulus


  1. The afferent arteriole splits into smaller capillaries

  2. Blood moves from a wider lumen to a smaller lumen

  3. Creating a higher hydrostatic pressure
• The high pressure in the glomerulus forces out small molecules, water, glucose, and mineral ions, forming the glomerular filtrate
• Large proteins and blood cells stay in the blood and leave via the efferent arteriole during the formation of the glomerular filtrate
• Glomerular filtrate formation


  In the glomerulus, through ultrafiltration
• Where the glomerular filtrate enters


  Enters the renal/bowman's capsule, flows into the proximal convoluted tubule
• What occurs in the proximal convoluted tubule


  Reabsorption of glucose and water, leaving urea and excess mineral ions behind
• Adaptations of the proximal convoluted tubule

  • Microvilli - provide a large surface area for reabsorption
  • Lots of mitochondria - provide energy for active transport
• The filtrate is dilute when it reaches the top of the ascending limb/in the distal convoluted tubule due to the active transport of sodium ions out and into the medulla
• What happens as the filtrate moves through the distal convoluted tubule and the collecting duct

  Due to the lower water potential in the medulla, even more water moves out of the DCT and collecting duct by osmosis into the surrounding capillaries
• The rest of the filtrate in the collecting duct goes on to form the urine
• Desert animals have a longer loop of Henle


  More sodium ions would be actively transported out in the ascending limb, creating an even lower water potential in the medulla, allowing for more water to be reabsorbed
• Low blood water potential

  Loss of water through sweating, not drinking enough water, lots of ions in drinks and food
• Hypertonic


  Low water potential, high concentration of ions
• Corrective mechanism for low blood water potential


  More water is reabsorbed by osmosis into the blood from the tubules in the nephron, urine is more concentrated in ions and less water is lost
• Low blood water potential needs to be regulated because too much water will leave cells and move into blood via osmosis, causing cells to shrivel (crenation)
• High blood water potential


  Drinking too much water, lack of ions in diet
• Hypotonic


  High water potential, low concentration of ions
• Corrective mechanism for high blood water potential


  Less water is reabsorbed by osmosis into the blood from the tubules in the nephron, urine is more dilute and more water is lost
• High blood water potential needs to be regulated because too much water will move from blood into cells by osmosis, causing cells to burst (lysis)
• Role of the hypothalamus in osmoregulation


  To detect changes to the blood water concentration through osmoreceptors, to produce ADH
• How osmoreceptors respond to low blood water potential


  Water leaves the osmoreceptors by osmosis and they shrivel, stimulating the hypothalamus to produce more ADH
• How osmoreceptors respond to high blood water potential


  Water enters the osmoreceptors by osmosis, stimulating the hypothalamus to produce less ADH
• ADH


  Produced in the hypothalamus, released by the posterior pituitary gland
• Role of the posterior pituitary gland in osmoregulation


  Releases/secretes ADH into blood
• Role of ADH in osmoregulation


  When it reaches the kidneys, it causes the walls of the collecting duct and distal convoluted tubule to become more permeable to water, more water leaves the nephron and is reabsorbed into the blood, urine is more concentrated
• How ADH works

  1.ADH binds to complementary receptors on the distal convoluted tubule and collecting duct

  2. Activates an enzyme called phosphorylase

  3. The enzyme causes vesicles containing aquaporins to fuse with the cell surface membrane

  4. This results in more aquaporins embedded in the cell surface membrane
• Aquaporins


  Protein channels that allow the passage of water, more aquaporins means more water passes out of the distal convoluted tubule and collecting duct
• How blood water potential is returned to normal when too low


  • Detected by osmoreceptors in the hypothalamus
  • stimulates the hypothalamus to produce more ADH
  • more ADH released by the posterior pituitary gland
  • more water reabsorbed into the blood
  • urine is more concentrated