Acid-Base Balance

Normal blood pH - 7.3-7.4

Chemistry involved

Tests

• pH
• P_aCO₂ (normal range 4.7 - 6.0 kPa
• PaO₂ (normal > 10.6 kPa)
• HCO₃^- (normal 24-30 mmol/l)

1. Is the pH high or low? (acidosis vs alkalosis)
   • are there more or less hydrogen ions than there should be?
2. Is the bicarbonate high or low?
   • has the bicarbonate been used up absorbing hydrogen ions from the extra acid? (metabolic acidosis)
   • is the patient making bicarbonate from the CO₂ they are not breathing out? (respiratory acidosis)
   • is the patient unable to excrete unnecessary bicarbonate? (metabolic alkalosis)
   • has the bicarbonate been used up making carbon dioxide for the patient to breathe out? (respiratory alkalosis)

Compensatory mechanisms for change

Respiratory

•  rate
  • can increase amount of CO₂ removed from system
  • controlled by chemo-receptors in brain

Renal

• acid excretion
  • buffered by phosphate in urine
  • glutamine metabolism to ammonia
• bicarbonate filtration
• bicarbonate re-absorption
• renal bicarbonate formation

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