Renal Regulation of Urea, Phosphate, Calcium, and Magnesium

Question 1

Urea Regulation

Answer 1

• 50% of filtered urea is reabsorbed passively in proximal tubule
• Distal tubule, cortical collecting ducts, and medullary collecting ducts are impermeable to urea
• ADH increases urea permeability of inner medullary collecting ducts
  
  • Contributes to urea recycling in inner medulla
  • Contributes to developing corticopapillary osmotic gradient
• Urea excretion varies w/ urine flow rate
  
  • Low urine flow: greater urea reabsorption and less excretion
  • High urine flow: less urea reabsorption & more excretion

Question 2

Phosphate Regulation

Answer 2

• 85% filtered phosphate reabsorbed in the proximal tubule
  
  • Via Na+/Phosphate cotransport
  • distal segments don’t reabsorb: 15% excreted
  • PTH inhibits phosphate reabsorption in proximal tubule
    
    • activates AC to form cAMP & inhibit cotranporter
    • Phosphaureia and increased urine cAMP
  • Urinary buffer for H+

Question 3

Calcium Regularion

Answer 3

• 60% of plasma Ca2+ is filtered across glomerular capillaries
• Proximal tubule and TAL reabsorb >90% filtered Ca2+
  
  • Coupled to Na+ reabsorption
• Loop diuretics (Furosemide) increase urinary Ca2+ excretion
  
  • b/c Ca2+ linked to Na+ reabsorption in loop
  • If volume replaced, loop diuretics can tx hypercalcemia
• Distal tubule and collecting ducts reabsorb 8% of filtered Ca2+
  
  • PTH increases Ca2+ reabsorption: activates adenyl cyclase
  • Thiazide diuretics increase Ca2+ reabsorption

Question 4

Magnesium Regulation

Answer 4

• Reabsorbed in proximal tubule, TAL, and distal tubule

- In TAL Mg2+ and Ca2+ compete for reabsorption