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.