Physiology - Renal Topic #6

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9spr
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70618
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Physiology - Renal Topic #6
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2011-03-05 00:58:36
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Topic #6 - Acid / Base BalancePlasma pH
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  1. Acid-Base Balance
    • in water, acids dissociate in solution to liberate free H+ions
    • STRONG acids completely dissociate (HCl--> H+ Cl-)
    • WEAK acids have more limited dissociation (H2CO3<---> H+ + HCO3)
    • bases bind free H+and remove it from solution
  2. Acid Production in the body
    • H+is continually produced by metabolic activity:
    • volatile acids: carbonic anhydrase
    • CO2 + H2O <--> H2CO3 <--> H++ HCO3
    • non-volatile acids:
    • ingested acids
    • products of fat, amino acid, and sugar metabolism: phosphate, sulphate, lactate
  3. H+concentration
    • pH is the log of the reciprocal of the free H+ion concentration (negative log)
    • pH =
  4. Plasma pH
    • average plasma pH = 7.35-7.45
    • most enzymes do not function outside this range
    • represents very small H+concentrations: 40 nM
    • much higher bicarbonate levels in plasma: 24 mM
  5. body regulates changes in pH by using:
    • 1: buffers
    • 2: H secretion
    • 3: HCO3 reabsorption
  6. Buffer Systems
    • a buffer system is a combination of two compounds that: minimize pH changes in acids or bases to solution
    • BODY BUFFER SYSTEMS:
    • in plasma: bicarbinate/carbonic acid (pKa=6.1)
    • hemoglobin (unoxygenated/oxygenated) helps buffer plasma
    • in urine: phosphate (h2po4 -/h2po4 2-) (pKa=6.8)
    • ammonium (NH3 / NH4) (pKa=9.0)
  7. buffers contain BOTH dissociated and undissociated forms of an acid
    • eg. bicarbonate buffer is actually H2CO3 and (HCO3)
    • the ratio of these forms is CONSTANT
    • K = [H+][HCO3]/[H2CO3]
  8. Henderson-Hasselbalch equation:
    • eq PH=pK+log[HCO3]/[H2CO3] = pK+log [HCO3]/a [HCO2] - a=solubility constant (0.03)
    • only the amount of FREE H+determines pH
    • more HCO3 than H2CO3 (i.e. CO2) is needed to maintain physiological pH
  9. in addition to buffering
    • bicarbinate buffersystem can alter pH by altering conc of its components
    • bicarbonate conc. controlled by kidney ---> HCO3
    • CO2 conc controlled by lungs ---> CO2
    • alkalosis can be caused by: rise in HCO3 or fall in CO2
    • acidosis can be caused by: fall in HCO3 or rise in CO2
  10. Renal Hydrogen Ion Secretion - Proximal Tubule
    • very low (nM) plasma H+conc. , so little is filtered
    • instead, H+is secreted by tubular cells and HCO3-is reabsorbed
    • large acid load excreted means urine pH is normally about (pH6 slightly acidic)
    • Na+concentration gradient created by
    • H+moves out of cell using
    • H+in filtrate combines with HCO3-to form CO2
    • enzyme required
    • CO2 diffuses into cell, converted back to HCO3
    • HCO3-transported OUT of cell into capillary
    • net effect: H+is secreted in order to REABSORB HCO3
  11. Other Mechanisms of Proximal H+secretion - Phosphate buffering
    • H+buffered by HPO42-after filtered HCO3-has been reabsorbed
    • H+and HCO3-made in cell using:
    • source of HCO3
    • H+transported into filtrate using
    • net effect: new HCO3-enters blood (i.e. is produced, not just reabsorbed)
    • high pCO2 incr H+secretion
  12. Ammonium secretion - controlling urine pH
    • bicarbonate is normally reabsorbed, so it cannot buffer urinary H+
    • the kidney can make more HCO3-by: daminating glutamine
    • (yeailds ammonia which is lipid soluble, crosses cell membrane easily)
    • in the urine ammonium is formed:
    • NH3 + H+ NH4
    • ammonium is not very permeable
    • stays in the urine to act as a buffer
  13. H+Handling by the Distal Nephron
    • intercalated cells secrete H+using H/K ATPase
    • (H out - K in)
  14. Acid Base Disorders
  15. Respiratory acidosis =
    • reduced pH due to high pCO2
    • caused by: inadequeate ventilation
    • COMPENSATION: increased renal H secretion and HCO3 reabsorption (causes increased plasma HCO3)
  16. Metabolic acidosis =
    • lower pH with low plasma HCO3
    • caused by: diabetic ketoacidosis, lactic acidosis, ethylene glycol or salicylate poisoning, diarhea
    • COMPENSATION: hyperventilation (decreased CO2)
    • increased renal H secretion and HCO3 absorption
  17. Metabolic alkalosis =
    • Elevated pH due to high plasma HCO3
    • caused by: excessive alkali ingestion (antacids)
    • COMPENSATION: hypoventilation (increased CO2)
    • less renal HCO3 reabsorption
  18. Respiratory alkalosis =
    • elevated Ph due to low pCO2
    • caused by: hyperventilation
    • COMPENSATION: less renal H secretion, less HCO3 reabsorption
  19. Analyzing Acid-Base Disorders
    • 1. is condition: Acidosis (pH < 7.4) Alkalosis (pH > 7.4)
    • 2. what can causethis acidosis/alkalosis? low HCO3 or high pCO2 high HCO3 or low pCO2
    • 3. is there a COMPENSATION?low pCO2 high HCO3 high pCO2 low HCO3

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