Acid-Base Balance

The pH scale ranges between 0 (absolute acid) and 14 (absolute base, or alkali). A pH of 7.0 is neutral. Blood is normally slightly alkali, with a normal pH range of 7.35 to 7.45. Blood contains acids and alkalis, and the body’s careful balance between acidity and alkalinity is referred to as the acid-base balance. The acid-base balance is carefully controlled because even a minor deviation can severely affect many organs.

Blood pH is affected by both respiratory and metabolic functions and can be life threatening.

Carbon dioxide, when dissolved in water, is mildly acidic. As carbon dioxide accumulates in the blood, the pH becomes more acidic. The brain regulates the amount of carbon dioxide in the blood, and therefore the pH, by controlling ventilation. This ventilatory response to changing pH happens almost immediately.

Bicarbonate (HCO3) on the other hand is alkaline. The kidneys actively regulate HCO3 to maintain blood pH. Low levels of bicarbonate causes the blood to become less alkali, or more acidic. High levels of bicarbonate causes the blood to become more alkali, or less acidic. The kidneys make adjustments to HCO3 more slowly than the lungs do; this compensation generally takes several days.

When the blood is more acidic for example, the respiratory system will attempt to increase the blood pH by increasing ventilation, excreting more hydrogen ions in the form of CO2. The opposite will occur when the blood is alkalotic. Similarly, the kidneys will retain more bicarbonate if the pH low or excrete more if the pH is high. Whilst the changes induced by changing the breathing frequency respond much quicker than the kidneys, these processes are termed compensation.

The primary causes of respiratory and metabolic disturbances are identified below.

Respiratory Acidosis

  • Central alveolar hypoventilation
  • Drug induced respiratory depression
  • Lesions of the spinal cord
  • Musculoskeletal disorders (e.g. kyphoscoliosis and muscular dystrophy)

Respiratory Alkalosis

  • Increased central drive to respiration (hyperthermia)
  • Voluntary/ Psychogenic hyperventilation
  • Hyperventilation due to hypoxaemia
  • Excessive ventilation during assisted breathing

Metabolic Acidosis

  • Diabetic ketoacidosis
  • Starvation
  • Renal failure
  • Production of excessive lactic acid

Metabolic Alkalosis

  • Vomiting
  • Ingestion of basic substance (e.g. sodium bicarbonate)
  • Potassium deficiency
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