Control of blood water potential

• Describe the structure of a nephron

  • Nephron = basic structural and functional unit of the kidney (millions in the kidney)
  • Associated with each nephron are a network of blood vessels

  
• Summarise the role of different parts of the nephron


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• Describe the formation of glomerular filtrate

  1. High hydrostatic pressure in glomerulus - as diameter of afferent arteriole (in) is wider than efferent arteriole (exit)

  1. Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by:
  2. Pores / fenestrations between capillary endothelial cells
  3. Capillary basement membrane
  4. Podocytes
  5. Large proteins / blood cells remain in blood

  
• Describe the reabsorption of glucose by the proximal convoluted tubule (PCT)

  1. Na+ actively transported out of epithelial cells to capillary
  2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient
  3. Glucose moves into capillary by facilitated diffusion down its concentration gradient

  
• Describe the reabsorption of water by the proximal convoluted tubule (PCT)

  • Glucose etc. in capillaries lower water potential
  • Water moves by osmosis down a water potential gradient
• How features of the cells in the PCT allow rapid reabsorption of glucose into the blood

  • Microvilli (folded cell-surface membrane) → provides a large surface area
  • Many channel / carrier proteins → for facilitated diffusion / co-transport
  • Many carrier proteins → for active transport
  • Many mitochondria → produce ATP for active transport
  • Many ribosomes → produce carrier / channel proteins
• Suggest why glucose is found in the urine of an untreated diabetic person

  • Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT
  • As glucose carrier / cotransporter proteins are saturated & working at maximum rate
• Explain the importance of maintaining a gradient of Na+ ions in the medulla (concentration increases further down)

  • So water potential decreases down the medulla (compared to filtrate in collecting duct)
  • So a water potential gradient is maintained between the collecting duct and medulla
  • To maximise reabsorption of water by osmosis from filtrate
• Describe the role of the loop of Henle in maintaining a gradient of Na+ ions in the medulla

  • In the ascending limb:
  • Na+ actively transported out (so filtrate concentration decreases)
  • Water remains (as ascending limb is impermeable to water)
  • This increases concentration of Na+ in the medulla, lowering water potential
  • In the descending limb:
  • Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases)
  • Na+ ‘recycled’ → diffuses back in

  The loop of Henle acts as a countercurrent multiplier 

  
• Suggest why animals needing to conserve water have long loops of Henle (thick medulla)

  • More Na+ moved out → Na+ gradient is maintained for longer in medulla (higher Na+ concentration)
  • So water potential gradient is maintained for longer
  • So more water can be reabsorbed from collecting duct by osmosis
• Describe the reabsorption of water by the distal convoluted tubule (DCT) and collecting ducts

  • Water moves out of DCT & collecting duct by osmosis down a water potential gradient
  • Controlled by ADH which increases their permeability
• What is osmoregulation?

  Control of water potential of the blood (by negative feedback)
• Describe the role of the hypothalamus in osmoregulation

  1. Contains osmoreceptors which detect increase OR decrease in blood water potential
  2. Produces more ADH when water potential is low OR less ADH when water potential is high
• Describe the role of the posterior pituitary gland in osmoregulation

  Secretes (more / less) ADH into blood due to signals from the hypothalamus
• Describe the role of ADH in osmoregulation

  1. Attaches to receptors on collecting duct (and distal convoluted tubule)
  2. Stimulating addition of channel proteins (aquaporins) into cell-surface membranes
  3. So increases permeability of cells of collecting duct and DCT to water
  4. So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis
  5. So decreases volume and increases concentration of urine produced

  = a decrease in water potential of the blood (eg. increased sweating, reduced water intake, increased salt intake). How the body responds to an incr. in water potential = the opposite.

  
• Note: PCT and DCT abbreviations not recognised in the specification, so they must be written in full at least once.