Acid–Base Disorders and Compensation.

    

     Arterial blood pH is normally 7.35–7.45.  
  
    Disorder that tends to reduce pH to a less than normal value is an acidosis, whereas one tending to increase pH is termed an alkalosis .
    If the disorder primarily affects [HCO3 − ], it is termed metabolic . If the disorder primarily affects PaCO2 , it is termed respiratory.

Three phases of compensatory mechanisms:

 I. Chemical buffering (immediate)

  • bicarbonate (H2CO 3 /HCO 3 − ) (BLOOD BUFFER)
  • hemoglobin (HbH/Hb − ) (BLOOD BUFFER)
  • intracellular proteins (PrH/Pr − ) (INTRACELLULAR BUFFER)
  • phosphates (H2PO 4 − /HPO 4 2− ) (URINARY BUFFER)
  • ammonia (NH 3 /NH 4 + ). (URINARY BUFFER)


 II. Respiratory compensation (fast)

  • mediated by chemoreceptors within the brainstem. These receptors respond to changes in cerebrospinal fluid pH. Minute ventilation increases 1–4 L/min for every (acute) 1 mm Hg increase in PaCO2 .

 
III. Renal compensatory response (slow,but more effective)

  • The ability of the kidneys to control the amount of HCO3− reabsorbed from filtered tubular fluid, form new HCO3 − , and eliminate H + in the form of titratable acids and ammonium ions allows them to exert a major influence on pH during both metabolic and respiratory acid–base disturbances.

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