Control of blood water potential

• What is the purpose of the kidney?

  To filter the blood to remove urea and other waste products by forming urine
• Name the components of the structure of the nephron

  • Afferent arteriole
  • Glomerulus
  • Efferent arteriole
  • Capillaries
  • Bowman's capsule
  • Podocytes
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting duct
• Describe ultrafiltration between the glomerulus and Bowman's capsule

  1. Blood in the glomerulus has high hydrostatic pressyre because the efferent arteriole is narrower than the afferent
  2. This forces water, glucose, amino acids, urea and ions into Bowman's capsule, forming glomerular filtrate
  3. Pores between the glomerulus endothelial cells prevent red blood cells from entering the filtrate
  4. A basement membrane prevents proteins from entering the filtrate
  5. Podocytes lining Bowman's capsule allow the rest of the small molecules to pass through
• Describe selective reabsorption of glucose in the proximal convoluted tubule

  1. Na+ is actively transported from epithelial cells into the blood, creating a concentration gradient for Na+
  2. Na+ enters cells from filtrate by facilitated diffusion along with glucose through co-transport, creating a concentration gradient for glucose
  3. Glucose enters the blood by facilitated diffusion
  4. This increases the water potential of the filtrate, so water leaves by osmosis and enters the capillaries
• Describe the adaptations of proximal convoluted tubule cells

  • Lots of microvilli for a large surface area
  • Lots of mitochondria to produce ATP for active transport
  • Lots of ribosomes for protein synthesis to produce channel/carrier proteins
• Describe the effect of high blood glucose concentration on glucose reabsorption

  • Higher blood glucose concentration
  • Higher concentration of glucose in filtrate
  • Not all the glucose can be absorbed at the proximal convoluted tubule
  • Carrier proteins in the cell membrane become saturated
• Describe water reabsorption by the loop of Henle

  1. Na+ is actively transported out of the ascending limb which lowers the water potential of interstitial fluid
  2. Water cannot leave the ascending limb as its thick walls are impermeable to it
  3. Instead water in the descending limb leaves the filtrate by osmosis, entering interstitial fluid and is absorbed by capillaries
  4. Some Na+ diffuses into the descending limb which, along with the loss of water, concentrates the filtrate
  5. This creates a water potential gradient for the medulla, which is used by the DCT and the collecting duct to reabsorb water
• Describe properties of ascending and descending limb of loop of Henle


  Descending limb - permeable to water

  Ascending limb - impermeable to water
• Describe the role of the distal convoluted tubule

  Water reabsorption occurs in DCT along with reabsorption of some ions
• Describe the role of the collecting duct

  1. Filtrate in the ascending limb of the loop of Henle flows in the opposite direction to the collecting duct
  2. This maintains a water potential gradient, allowing water to be reabsorbed from the whole length of the collecting duct
  3. This is called the counter-current multiplier
• Describe osmoregulation

  1. Osmoreceptors in hypothalamus detect lower blood water potential
  2. They send APs to the posterior lobe of the pituitary gland which secretes more antidiuretic hormone (ADH) to blood
  3. ADH binds to complementary receptors on collecting duct cell membranes, and this causes vesicles with aquaporins to fuse w/ cell membrane
  4. This increases permeability of cells to water- more water is absorbed from filtrate to capillaries
  5. This produces urine with a higher concentration and a smaller volume
  6. When BWP increases to normal, less ADH is secreted - controlled by negative feedback