MD Nexus

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

Henderson-Hasselbach Equation

Kassirer-Bleich Equation (a Modification of the Henderson-Hasselbach Equation)

  • Utility of Kassirer-Bleich Equation
    • Use the Kassirer-Bleich 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

Metabolic Acidosis (see Metabolic Acidosis-General, 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 Arterial pH Toward Normal
    • 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)
    • Degree of Respiratory Compensation in Response to Metabolic Acidosis is Limited
      • In Patients with Normal Neural/Respiratory Function, the Arterial 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 Arterial pH Toward Normal
    • Degree of Respiratory Compensation in Response to Metabolic Alkalosis is Limited
      • Even in Severe Metabolic Alkalosis, the pCO2 Usually Does Not Increase >55 mm Hg (Due to the Presence of Competing Hypoxic Respiratory Drive, Which Continues to Stimulate Ventilation to Maintain Adequate Oxygenation)
  • Equations to Determine Expected pCO2
    • Expected pCO2: should increase 0.7 mmHg for each 1 mEq/L increase in HCO3

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 Arterial 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 Arterial 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 Arterial 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 Arterial 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