Tubular Mechanisms

Question 1

describe the early proximal tubule

Answer 1

• Na+ symport with uncharged molecules (glucose, amino acids)
  
  • accounts for 10% of Na+ reabsorption; H2O follows
• HCO3 reabsorption
  
  • Na/H antiport allows H secretion for HCO3 reabsorption and accounts for 20-25% Na+ reabsorption
  • carbonic anhydrase enzyme in lumen and in cell
    
    • Acetzolamide = inhibitor (weak diuretic) that allows for diuresis of HCO3-

Question 2

describe the late proximal tubule

Answer 2

• Na/H antiporter coupled to Cl- reabsorption and formate secretion; accounts for ~35% Na+ reabsorption
• Passive reabsorption of Na+ and Cl- paracellularly

Question 3

describe the glomerulotubular balance

Answer 3

• major regulatory mechanism of prox. tubule
• ensures that constant fraction of filtered Na+ (67%) is reabsorbed via filtration/reabsorption coupling
• increase in GFR increases FF which increases concentration of protein and oncotic pressure of glomerular capillaries and in proximal tubular capillaries
• since π_cap is the most important driving force for reabsorption in proximal tubule, reabsorption increases so proportionality of filtration and proximal tubule reabsorption is maintained

Question 4

describe the descending limb and thin ascending limb of the LoH

Answer 4

• descending limb:
  
  • low permeability to solutes Na, K, Cl (move in)
  • high permeability for H2O (move out)
• thin ascending limb
  
  • impermeable to H2O
  • permeable to NaCl and other solutes (move out)

Question 5

describe the thick ascending limb

Answer 5

• aka diluting segment
• impermeable to H2O
• load-dependent
• absorbs Na/K/2Cl
  
  • Furosemide diuretic
• Na/H antiporter
  
  • helps to make new bicarbonate
• 25% Na reabsorption

Question 6

describe the early distal tubule

Answer 6

aka cortical diluting segment

• load-dependent
• impermeable to H2O, further diluting fluid
• Na and Cl co-transporter
  
  • Thiazide diuretics
• 5% Na reabsorption

Question 7

explain the tubuloglomerular feedback mechanism

Answer 7

• if arterial pressure decreases:
  
  • GFR decreases
  • macula densa will sense low NaCl
  • will send info to JG cells to secrete renin
  • renin will catalyze formation of AG II, efferent arteriolar constrictor
  • macula densa will reduce afferent arteriolr resistance with prostaglandins –> vasodilation
• if arterial pressure increases:
  
  • GFR increases
  • macula densa will sense high NaCl
  • will send info JG cells to secrete adenosine
  • adenosine constricts afferent arterioles, reducing blood flow, reducing GFR back to normal

Question 8

describe the late distal tubule and collecting duct

Answer 8

• 2 cell types:
  
  • 1. principal: K+ secretion (coupled with Na reabsorption)
       * aldosterone effects principal cells
       * increased Na reabsorption leads to increased K secretion (K sparing diuretics inhibit Na and save K)
  • 2.
    
    • alpha-intercalated: K+ reabsorption; H+ secretion
    • B-intercalated: coupled with HCO3- secretion
• water permeability dependent on ADH
• 3% Na reabsorption (makes fine adjustments since last opp.)