Home PHGY 210 Wednesday, April 4, 2012

Wednesday, April 4, 2012 CC-BY-NC
Renal: Sodium Balance

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1Review from last lecture

  • Secretion: Clearance > GFR
    • basically means that a msall amount is filtered at the glomeruluar capillary, and the rest/majority is dump into the tubule after.
    • ex: para-amino =bippurate
  • Absorption": Clearance < GFR

2Water and sodium balance

  • need to be in good balance to maintain blood pressure
  • water intake = water output (.4L ~ 25L/day)
  • sodium intake = sodium output (.05g ~ 25g/day)
  • both are freely filtered but 99% is reabsorbed.
    • 2/3 of reabsorption occurs in proximal tubule
    • the rest is subjected to hormonal regulation on DCT and CD.

3Sodium reabsorption

  • an active process
    • on basolaterla membrane (opposite of the side facing the tubule), Na+/K+ ATPase pumps sodium out of cell to maintain low intracellular level of Na.
    • this low concentration of Na allows sodium from the tubule lumen to diffuse into the epithelial cells down the gradient. depending on what tubular segment:
      • proximal tubule:
        • NA+/H+ antiporter
        • Na+/glucose cotransporter
      • cortical collecting duct : diffusion via Na+ channels
    • and then water follows (so more Na reabsorption, more sodium reabsorption)
  • many reabsorption and secretion are coupled with movement of Na

4Regulation

  • since sodium is the major extracellular solute, change in Na in the body would also change extracellular fluid volume (which includes plasma volume)
    • but please note that plasma concentration of Na is relative to how much sodium is in the body
      • it's cuz rate of excretion for water and Na are different. People also sweat out more water than salt.
      • so the concentration is just a relative relationship
  • baroreceptors in the cardiovascular system sense the change
  • exccretion is is regulated by :
    • minor: GFR : loss of Na and water --> drop in blood pressure --> reflexex mediate by baroreceptor -->renal sympathetic nerve activity increases --> constriction of afferent arterial --> glomerular pressure decreases --> less secretion.
    • major: sodium reabsorption
      • upregulated by aldosterone and renin
      • downregulated by arterial natriuretic peptide (ANP)

4.1Aldosterone

  • steroid hormone synthesized by adrenal cortex's zona glomerulosa
  • mechanism
    • upregulate basolateral Na+/K+ ATPase
    • upregulate apial potassium and sodium channels.
  • no aldosterone: ~2% of filtered load is secreted
  • high aldosterone: ~0% secreted.

4.2Renin

  • act via aldosterone
  • secreted by kidneys and justaglomerular cells
  • mechanism:
    • liver secrete angiotensinogen inot the blood
    • renin from kidneys conver antiotensinogen into angiotensin I
    • angiotensin converting enzyme converts angiotensin I to Angeiotensin II
    • Angiotensin II acts on adrenal cortex to secrete aldosterone
      • act synergistically with high K+ plasma concentration and ACTH
      • ANP (discussed next) acts negatively on synthesis of aldosterone
  • Signal pathway:
    • plasma volume low --> renal juxtaglomerular cells secrete renin (upon activity of sympathetic nerve and drop in pressure sensed by macula densa)
    • -->decrease in Na excretion, thus less water excreted.

4.3ANP

  • peptie hormone synthesized by cells in cardiac atria, which sense distension when blood pressure is high.
  • inhibit sodium reabsorption
    • by decreasing secretion of aldosterone
    • also by decreasing NA reabsorption at level of tubule
    • also by dilating afferent arteriole --> higher glomerular pressure --> greater GFR
  • increased total body sodium, increased extracellular fluid/plasma volume, both would stimulate ANP secretion
    • high blood pressure increases Na excretion.