I. Acid/Base Regulation
  1. Energy metabolism produces acid.

    Acid is a waste byproduct of energy production and metabolism, as CO2 (~ 15 mole/day) and as nonvolatile acids (~ 100 mEq/day).

  2. Acid Excretion -- Mechanisms and Regulation

    1. Respiration eliminates CO2 and normally maintains PCO2 at 40 mm Hg. Respiration rate is controlled by:
      1. baroreceptors in the carotid arteries responding to PO2
      2. chemoreceptors in the medulla responding to CSF pH

    2. The kidneys normally maintain HCO3- concentration at 24 mM. Renal acid elimination rate and rate of HCO3- reabsorption and formation is passively controlled by blood/tubule fluid [H+]. However, the amount of the enzyme, glutaminase, in renal tubule epithelial cells limits the maximum rate of NH3 formation and therefore the maximum rate of renal acid elimination.

  3. The Henderson Hasselbach equation illustrates the significance of the [HCO3-] / [CO2] ratio.
    1. HCO3- serves as a buffer for elimination of H+ as CO2 :
      HCO3- + H+ -------> CO2       (with elimination by lungs)

    2. The [HCO3- ] / [CO2] ratio controls blood pH:
      (H+ ) = Ka / [(HCO3- ) / (CO2)]
              = 800 nM / [(HCO3- ) / (CO2)]
         -log(H+ ) = pH = 6.1 + log[(HCO3- ) / (CO2)]

    3. The Blood Gas CO2 result is expressed in terms of mm Hg.
      normal PCO2 = 40 mm Hg
      PCO2 results can be transformed to mM concentration units by:
      (CO2) = 0.03 x PCO2
      normal (CO2) = 1.2 mM

    4. Since normal (HCO3- ) = 24 mM and normal (CO2) = 1.2 mM:
      normal [(HCO3- ) / (CO2)] = 20
      normal (H+) = 800 nM / 20 = 40 nM
      normal pH = 6.1 + log(20) = 6.1 + 1.3 = 7.4

II. Characteristics of Acid/Base Disturbances
Acid / Base disturbances
are reflected in the
[(HCO3- ) / (CO2)] ratio:
  1. Acidosis results from
    [(HCO3- )/(CO2)]
    1. respiratory acidosis
      from PCO2
    2. metabolic acidosis
      from (HCO3- )
  2. Alkalosis results
    from [(HCO3-) / (CO2)]
    1. respiratory alkalosis
      from PCO2
    2. metabolic alkalosis
      from (HCO3- )

III. Compensation

  1. The kidneys compensate for respiratory acid/base disturbances by appropriately adjusting circulating (HCO3-).
  2. The lungs compensate for metabolic acid/base disturbances by appropriately adjusting PCO2.
  3. Mechanisms of compensation
    1. Respiratory compensation depends on equilibration of altered blood (HCO3- ) with CSF for which the t½ is ~ 2 hr. Respiratory compensation is therefore quite rapid. Metabolic acid/base disturbances develop relatively slowly and therefor are always compensated in practice.
    2. Renal compensation:
      1. for respiratory alkalosis is determined by glomerular filtration rate for which the t½ is ~ 6 hours.
      2. for respiratory acidosis requires increased synthesis of the NH3 producing enzyme, glutaminase, for which the t½ is ~ 1 - 2 days.
    3. The extent of compensation is limited and is never complete.
      1. increased respiration compensates metabolic acidosis to the extent of ~ 80% - opposed by hyperoxemia
      2. decreased respiration compensates metabolic alkalosis to the extent of only ~ 25% - opposed by hypoxemia
      3. renal compensation for respiratory acidosis is ~ 60% complete
      4. renal compensation for respiratory alkalosis is ~ 80% complete

IV. Summary

V. Laboratory Determinations

A. Blood Gas Measurement
The Blood Gas Analyzer is a clinical laboratory laboratory instrument which reports values for PO2, PCO2, pH and HCO3-. Specific electrodes directly determine PO2, PCO2 and pH. Bicarbonate is calculated from the Henderson/Hasselbach equation and the measured pH and PCO2 values. The specimen is usually arterial, whole blood. Normal ranges are given in the table to the right.
Blood gas results completely describe an acid/base disturbance in regard to:
  • acidosis or alkalosis
  • respiratory or metabolic
  • acute or compensated

B. Pertinent Chemistry Tests

Metabolic acid/base disturbances are almost always associated with disturbances in fluid and electrolyte balance. Tests in the panel to the right are used to assess metabolic acid/base disturbances and associated disturbances in fluid and electrolyte balance. Normal ranges are shown by the gray areas for each test. The position of the horizontal red line represents a test result. Total CO2 represents bicarbonate concentration and reflects acid/base status; urea (BUN)and the urea/creatinine ratio reflect fluid balance status; electrolyte concentrations are altered characteristically in specific disturbances.