Control of Plasma Volume

Question 1

Name how sodium can exit the body

Answer 1

Urine, faeces, sweat

Question 2

Explain S1 sodium reabsorption in proximal tubule

Answer 2

• Apical membrane has Na-H exchanger, co-transport with glucose (sodium glucose co-transporter 2), co-transport with AA, co-transport with phosphate
• Basolateral membrane has Na/K ATPase which pumps 3 sodium ions out into interstitial space
• Aquaporins present to allow water to follow
• Concentration gradient of chlorine establishing which will aid passive reabsorption in S2-S3

Question 3

Explain S2-S3 reabsorption in proximal tubule

Answer 3

• Apical membrane absorbs sodium via Na-H exchanger
  
  • Paracellular chlorine absorbed from lumen due to concentration gradient created in S1 - more negative due to sodium reabsorption
  • Transcellular chloride movement as well
  • Aquaporins present
• Basolateral membrane has Na/K ATPase and chlorine channels

Question 4

Explain sodium and chloride uptake in late proximal tubule

Answer 4

• Passive diffusion through tight junctions (paracellular reabsorption)
• High chlorine reabsorption as HCO3- levels decrease and Na is reabsorbed

Question 5

Explain how PCT reabsorption into peritubular capillary occurs

Answer 5

• Proximal tubule highly water permeable
  
  • Bulk transport or obligatory water reabsorption
• Reabsorption is isosmotic with plasma
• Driving force
  
  • Osmotic gradient established by solute absorption - osmolarity in interstitial spaces higher
  • Hydrostatic force in interstitium higher
  • Higher oncotic force in peritubular capillary as proteins remain in efferent arteriole

Question 6

Describe glomerulotubular (GT) balance and the effect of ECF volume (pressure natriuresis and diuresis)

Answer 6

• When renal artery blood pressure increases, causes reduction in sodium and therefore water reabsorption in proximal tubule
  
  • Could be caused by reduced number of Na-H antiporter and reduced Na/K ATPase activity in proximal tubule
• Leads to increased sodium excretion (pressure natriuresis) and increased water excretion (pressure diuresis)
• ECF volume decreased and initial blood pressure rise diminished

Question 7

Explain the difference in structure of the ascending and descending limb

Answer 7

• Descending limb have thin, flat cells with no mitochondria
  
  • No brush border
  • Aquaporins present to allow water movement
  • Loose junctions
• Ascending limb have thicker cells with mitochondria present
  
  • No aquaporins
  • Solute movement occurs through active transport
    
    • Tight junctions

Question 8

Outline the action of water reabsorption in the descending limb

Answer 8

• Increasing concentrations of sodium further into medulla allow paracellular reabsorption of water through aquaporins in the loose junctions
• This concentrates the sodium and chloride ions in the lumen ready for active transport in the ascending limb

Question 9

Explain the action within thin ascending limb

Answer 9

• Sodium ion reabsorption passive in thin ascending limb
  
  • Due to concentration gradient created from descending limb
• Epithelium in thin ascending limb permits passive reabsorption by paracellular route

Question 10

Explain the processes occuring in the thick ascending limb

Answer 10

• Apical membrane has NKCC2 transporter (sodium-potassium-chloride transporter) - active transport
• Sodium ions move into interstitium due to actions of Na/K ATPase
  
  • Sodium moves into the cell via NKCC2 and then through Na/K ATPase (transcellular)
  • High concentrations of sodium in nephron also allow paracellular reabsorption
• Potassium ions diffuse via ROMK (renal outer medullar potassium channel) back into lumen
  
  • In the filtrate, there is less potassium ions so in order to maintain activity of NKCC2 transporter, potassium diffuses back into the filtrate
• Chloride ions move into interstitium through NKCC2 and paracellular diffusion
• This region uses more energy than any other in the nephron and is particular sensitive to hypoxia - very metabolic active

Question 11

Describe sodium ion uptake in the early distal tubule

Answer 11

• Hypo-osmotic fluid enters
• Active transport of sodium through Na/K ATPase allows NCC (sodium chloride symporter) to reabsorb sodium from the lumen
  
  • NCC sensitive to thiazide diuretics
    Water permeability is fairly low

Question 12

Describe sodium ion uptake in the late distal tubule

Answer 12

• Na enters by NCC and ENaC and leaves through Na/K ATPase
  
  • ENaC sensitive to amiloride diuretics
• Movement through ENaC not electroneutral and difference drives paracellular chloride ion reuptake

Question 13

Outline how calcium reabsorption occurs in DCT

Answer 13

• Apical calcium transport
• Cytosolic calcium immediately bound by calbindin, which shuttles calcium to basolateral aspect of DCT cell
• Transported out by NCX (sodium calcium exchanger)
• Tightly regulated by hormones such as parathyroid hormone and 1,25-dihydroxyvitamin D

Question 14

Outline how sodium reuptake in principal cells occurs

Answer 14

• Principal cells (70%) allows reabsorption of sodium ions via ENaC on apical membrane
• Na/K ATPase on basolateral membrane driving force
• Transport of only sodium without chlorine produces a driving force for chloride uptake via paracellular route

Question 15

Outline the role of intercalated cells

Answer 15

• Play an important role in acidosis and alkalosis
• Acid-secreting type A-IC
  
  • Have H-ATPase and H/K ATPase on apical membrane
  • Have Cl/HCO3 exchanger at basolateral membrane
• Bicarbonate-secreting type B-IC
• Allows active reabsorption of chloride ions