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CVPR: Renal > Kidneys & Acid-Base Balance > Flashcards

Flashcards in Kidneys & Acid-Base Balance Deck (11)
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1

Production of metabolic nonvolatile acid

  • e.g. H2SO4 and H3PO4
  • 70 kg person ==> 60 mmol of H+ added to ECF/day

2

ECF buffers: bicarbonate buffer system

  • bicarbonate = HCO3-
  • H+ + HCO3- <=> H2CO3 <=> CO2 + H2O
    • converts to CO2 = volatile (gaseous) form than can be eliminated 

3

Challenges of bicarbonate buffer system

  • must eliminate acid anions that are the source of H+ ions (HSO4-, H2PO4-)
    • filtered and excreted in urine
  • elimination of H+ ==> sacrifice of HCO3- ==> bicarbonate must be replaced
    • ROLE OF KIDNEY = maintenance of ECF bicarbonate via reabsorption and synthesis

4

Kidney reabsorption of bicarbonate 

  • bicarb = small mlq = easily filtered = must be reabsorbed
  • primarily occurs @ proximal tubule: 85% of filtered load
  • Na/H exchanger: secretes H+ into lumen
  • Hcombines w/bicarb ==> carbonic acid (H2CO3) ==> carbonic anhydrase converts to CO2 + H2O
  • CO2 diffuses into cell + H2O ==> H2CO==> H+ + HCO3-
  • bicarb ion (HCO3-) transported across basolateral membrane via Na/HCO3- co-transporter
    • movement of Na helps acheive electroneutrality

5

Mechanism of bicarbonate synthesis by kidney

  • occurs @ intercalated cells @ distal segments
  • CO2 enters cells from serosal side
  • CO2 + H2O => H2CO2 => H+ + HCO3-
  • H+ and bicarb are physically seperated by transporters
    • H ATPase: H+ into lumen
    • HCO3-/Cl- exchanger: HCO3- into serosal ECF/blood

6

Mechanism of urinary buffering

  • must buffer H+ ions secreted as result of HCO3- synthesis
  • titratable acid: complexing of hydrogen ion to a filter acid anion (e.g. HPO42-
    • incidental buffering capacity
  • ammonia trapping: tubular cells break down glutamine ==> ammonia (NH3) diffuses into tubule + H+ ==> NH4+ = impermeable and "trapped" ==> excreted

7

Regulation of ECF/serum bicarbonate

  • any cell w/apical H+ secretion and basolateral HCO3- extrusion mechanisms can reabsorb or sythesize bicarb
  • no bicarbonate sythesis takes places until bicarbonate reabsorption is complete
    • acts as a buffer that competitively inhibits synthesis
  • metabolic acidosis ==> excess bicarb synthesis
  • metabolic alkalosis ==> bicarb secretion + hydrogen ion retention

8

Important factors in rate of bicarb reabsorp/synthesis

  • apical secretion + basolateral extrusion of bicarb = rate limiting step of bicarb homeostasis
  • rates of apical hydrogen ion secretion and basolateral bicarbonate extrusion depend on ECF pH and CO2 levels
    • membrane transporters regulated by these factors

9

Renal response to metabolic acidosis

  • metabolic acidosis => increased H+ produced => buffered by bicarb => increased CO2 production => eliminated by lungs
  • buffering => decreased bicarb/decreased filtered load of bicarb => increased H+ion secretion capacity = synthesis of bicarb

10

Regulatory range of bicarb

  • @ normal: all filtered bicarb (3800 mmol) must be reabsorped + 60 mmol/day synthesized to replaced bicarb lost in respiration

11

Impacts of potassium imbalance on acid/base status

  • primary hypokalemia ==> alkalosis
    • lowering ECF K+ = shift H+ into all cells including tubular cells
    • increased H+ secretion + excretion
  • hyperkalemia tends to induce acidosis
    • reduce rate of H+ secretion/excretion
    • H+ retained in ECF => acidosis