Acid-Base Physiology

Biologic Acids Which Require Excretion From the Body

Volatile Acids

  • Carbon Dioxide (CO2)
    • Approximate Amount Produced Per Day: 15k mmol/day (on typical Western diet)
    • Derived From Metabolism of Sugars/Fats/Amino Acids with Oxygen
    • Generates Carbonic Acid (H2CO3) When it Combines with Water
      • Bicarbonate-Carbon Dioxide Buffer System: Dissolved CO2 + H2O <-> H2CO3 <-> HCO3 + H+
    • Excreted by Lungs

Non-Volatile Acids

  • General Comments: approximate amount produced per day is 50-100 mEq/day (on typical Western diet)
  • Sulfuric Acid
    • Derived From Metabolism of Sulfur-Containing Amino Acids
    • Anions are Excreted by Kidneys as Sodium Sulfate and Hydrogen Ions are Excreted by Distal Tubule Via the Acid Secretory Mechanism
  • Phosphoric Acid
    • Anions are Excreted by Kidneys as Sodium Phosphate and Hydrogen Ions are Excreted by Distal Tubule Via the Acid Secretory Mechanism
  • Other Non-Volatile Acids

Renal Excretion of Acids

  • Kidney Combines H+ with Ammonia (NH3) to Form Ammonium Ion: NH3 + H+ -> HN4+
    • This is the primary adaptive mechanism to increase renal acid excretion
    • Ammonia results from metabolism of glutamate (and can be induced in response to an acid load)
  • Kidney Combines H+ with Phosphate (HPO42-) to Form Dihydrogen Phosphate: HPO42- + H+ -> H2PO4-

Henderson-Hasselbach Equation

  • Utility of Kassirer-Bleich Equation (a Modification of the Henderson-Hasselbach Equation): use the equation to check arterial blood gas and bicarbonate values for validity -> inequality between the sides of the equation indicates that laboratory error has occurred with one of the measurements
  • [H+] = 24 x pCO2/HCO3
    • pH 7.0 -> [H+] = 100
    • pH 7.1 -> [H+] = 80
    • pH 7.2 -> [H+] = 63
    • pH 7.3 -> [H+] = 50
    • pH 7.4 -> [H+] = 40
    • pH 7.5 -> [H+] = 32
    • pH 7.6 -> [H+] = 25
    • pH 7.7 -> [H+] = 20

Rules of Expected Acid-Base Compensation

ACID-BASE

Metabolic Acidosis (see Metabolic Acidosis-Normal Anion Gap, and Metabolic Acidosis-Elevated Anion Gap)

  • General Physiologic Principle: as the bicarbonate decreases in metabolic acidosis, the patient would be expected to hyperventilate and the pCO2 should decrease in an attempt to increase the pH toward normal
    • The respiratory compensation begins within 30 min (and is complete within 12-24 hrs)
    • Respiratory compensation is similar, regardless of the type of metabolic acidosis (ketoacidosis, lactic acidosis, hyperchloremic acidosis, etc)
    • The degree of respiratory compensation to metabolic acidosis is limited: in patients with normal neural/respiratory function, pCO2 would not be expected to decrease lower than approximately 8-12 mm Hg
  • Equations to Determine Expected pCO2
    • Winter’s Equation: expected pCO2 = (HCO3 x 1.5) + 8 +/- 2
    • Expected pCO2: should decrease 1.2 mmHg for each 1 mEq/L decrease in HCO3
    • Expected pCO2: HCO3 + 15
    • Expected pCO2: should be approximately equal to the last two decimal digits of the arterial pH

Metabolic Alkalosis (see Metabolic Alkalosis)

  • General Physiologic Principle: as the bicarbonate increases in metabolic alkalosis, the patient would be expected to hypoventilate and the pCO2 should increase in an attempt to decrease the pH toward normal
  • Equations to Determine Expected pCO2
    • Expected pCO2: should increase 0.7 mmHg for each 1 mEq/L increase in HCO3
      • Even in severe metabolic alkalosis, the pCO2 usually does not increase >55 mm Hg

Acute Respiratory Acidosis (see Respiratory Failure)

  • General Physiologic Principle: as the pCO2 increases in respiratory acidosis, the kidneys would be expected to retain bicarbonate in an attempt to increase the pH toward normal
  • Equations to Determine Expected Serum Bicarbonate
    • Expected HCO3: should increase 1 mEq/L for each 10 mm Hg increase in pCO2

Chronic Respiratory Acidosis (>3-5 days) (see Respiratory Failure)

  • General Physiologic Principle: as the pCO2 increases in respiratory acidosis, the kidneys would be expected to retain bicarbonate in an attempt to increase the pH toward normal
  • Equations to Determine Expected Serum Bicarbonate
    • Expect HCO3: should increase 3.5-5 mEq/L for each 10 mm Hg increase in pCO2

Acute Respiratory Alkalosis (see Respiratory Alkalosis)

  • General Physiologic Principle: as the pCO2 decreases in respiratory alkalosis, the kidneys would Be expected to excrete bicarbonate in an attempt to decrease the pH toward normal
  • Equations to Determine Expected Serum Bicarbonate
    • Expect HCO3: should decrease 2 mEq/L for each 10 mm Hg decrease in pCO2

Chronic Respiratory Alkalosis (>3-5 days) (see Respiratory Alkalosis)

  • General Physiologic Principle: as the pCO2 decreases in respiratory alkalosis, the kidneys would Be expected to excrete bicarbonate in an attempt to decrease the pH toward normal
  • Equations to Determine Expected Serum Bicarbonate
    • Expect HCO3: should decrease 4-5 mEq/L for each 10 mm Hg decrease in pCO2

References

  • A new acid-base nomogram featuring hydrogen ion concentration: Henderson revisited. Ann Intern Med 1967; 66:159-164
  • The anion gap. N Engl J Med 1977; 297:814-817
  • Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore) 1980; 59:161-187
  • An analytic approach to diagnosis acid-base disorders. J Crit Ill 1990; 5(2):138-150
  • The delta gap: an approach to mixed acid-base disorders. Ann Emerg Med. 1990;19:1310–1313 [MEDLINE]
  • Management of life-threatening acid-base disorders. First of two parts. N Engl J Med 1998; 338:26-34 [MEDLINE]
  • Management of life-threatening acid-base disorders. Second of two parts. N Engl J Med. 1998 Jan 8;338(2):107-11 [MEDLINE]