6.4.3 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
• nephron structure

  A) proximal convoluted tubule

  B) distal convoluted tubule

  C) bowman capsule

  D) loop of henle

  E) descending limb

  F) ascending limb

  G) collecting duct

  H) cortex

  I) medulla

  
• Summarise the role of different parts of the nephron

  1. Bowman’s / renal capsule - Formation of glomerular filtrate (ultrafiltration)

  2. Proximal convoluted tubule - Reabsorption of water and glucose (selective reabsorption)

  3. Loop of Henle - Maintenance of a gradient of sodium ions in the medulla

  4. Distal convoluted tubule + collecting duct - Reabsorption of water (permeability controlled by ADH)
• Describe the formation of glomerular filtrate

  1. High hydrostatic pressure in glomerulus

  ○ As diameter of afferent arteriole (in) is wider than efferent arteriole (out)

  2. Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by:

  1. Pores / fenestrations between capillary endothelial cells

  b. Capillary basement membrane

  c. Podocytes

  3. Large proteins / blood cells remain in blood
• bowmans capsule structure

  A) efferent arteriole

  B) bowmans capsule

  C) podocyte

  D) afferent arteriole

  E) glomerulus

  F) capillary endothelium

  G) podocyte

  H) basement membrane

  I) glomerular filtrate

  
• Describe the reabsorption of glucose

  by the proximal convoluted tubule

  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

  ● Glucose etc. in capillaries lower water potential

  ● Water moves by osmosis down a water potential

  gradient
• Describe and explain how features of the cells in the PCT allow the 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 sodium 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 sodium

  ions in the medulla

  1. 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

  2. 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 and

  collecting ducts

  ● Water moves out of distal convoluted tubule & 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 antidiuretic

  hormone (ADH) in increasing water potential

  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
• Describe the role of antidiuretic

  hormone (ADH) in decreasing water potential

  1. less attaches to receptors on collecting duct (and

  distal convoluted tubule)

  2. less addition of channel proteins

  (aquaporins) into cell-surface membranes

  3. So permeability of cells of collecting

  duct and DCT to water remains low

  4. So less water reabsorption from collecting

  duct / DCT (back into blood) by osmosis

  5. So increases volume and decreases

  concentration of urine produced
• describe how ultrafiltration occurs in a glomerulus

  • high hydrostatic pressure
  • small substances eg. water, urea, glucose, ions forced into glomerular filtrate
  • filtered by pores/fenestrations in capillary endothelium
  • through basement membrane
• how does a thicker kidney medulla affect concentration of their urine

  • thicker medulla means longer loop of henle
  • increase in sodium ion concentration
  • water potential gradient maintained
  • so more water reabsorbed
• explain how inhibiting absorption of sodium and chloride ions from filtrate, causes an increase in volume of urine produced

  • lower water potential in filtrate
  • less water reabsorbed by osmosis in collecting duct