Skip to content
Explore atlas
PH-FD08

Acid-base disturbances

How buffers, the lungs and the kidneys keep blood pH steady, what goes wrong in the four primary acid-base disorders, how the body compensates and why an abnormal pH affects potassium, calcium, the heart and the brain.

By the end, you can

  • Explain how the bicarbonate buffer, breathing and the kidneys keep arterial pH within a narrow range.
  • Classify a disturbance as metabolic or respiratory acidosis or alkalosis from its primary change.
  • Use the anion gap to separate acid gained from bicarbonate lost in metabolic acidosis.
  • Explain why respiratory compensation takes minutes and renal compensation takes days.
  • Relate symptoms such as deep breathing, tingling and drowsiness to the underlying disturbance.
  • Recognise the reference values, classifications and tests examiners ask about, including the anion and osmolar gaps, and explain how bicarbonate, ventilation and other treatments act on acid-base balance.

What it is

An acid-base disturbance is a change in the body’s balance of acid and base that moves, or would move, arterial pH out of its normal range. Acidosis and alkalosis name the processes; acidaemia and alkalaemia name the resulting blood pH. Each disturbance is metabolic (the primary change is in bicarbonate) or respiratory (the primary change is in carbon dioxide), and more than one can coexist.

4 types to tell apart
Metabolic acidosis
Bicarbonate falls because acid is gained (lactate, ketones, retained acids in kidney failure, some poisons) or bicarbonate is lost (diarrhoea, some kidney tubule disorders).
Metabolic alkalosis
Bicarbonate rises, usually after acid is lost (vomiting, drainage of stomach contents) or with volume and potassium depletion that makes the kidneys keep bicarbonate.
Respiratory acidosis
Carbon dioxide rises because breathing removes too little of it: suppressed breathing, weak respiratory muscles or severe lung disease.
Respiratory alkalosis
Carbon dioxide falls because breathing removes too much: anxiety, pain, low oxygen, fever, pregnancy or early aspirin poisoning.

US spelling: acidaemia → acidemia, alkalaemia → alkalemia, sulphuric → sulfuric.

Normal function

How the body keeps blood pH steady

Arterial blood is kept slightly alkaline, at a pH of about 7.35 to 7.45, even though metabolism produces acid all the time: carbon dioxide from respiration, and fixed acids such as sulphuric acid from protein. Enzymes, ion channels and the binding of calcium to proteins all depend on that narrow range.

The main buffer outside cells is bicarbonate. Carbon dioxide and water form carbonic acid, which splits into hydrogen ions and bicarbonate (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻). The pH depends on the ratio of bicarbonate to carbon dioxide, not on either alone.

Two organs set that ratio. The lungs control carbon dioxide within minutes by changing breathing. The kidneys control bicarbonate over hours to days: the reclaims almost all filtered bicarbonate, and the secretes hydrogen ions and makes new bicarbonate, excreting acid as ammonium and phosphate.

Point to it on the model
Three systems share the workSchematic
NormalLungsRemove carbon dioxide; act in minutes
  • CO₂ breathed out (from Blood to Lungs)
BloodBicarbonate buffers acid at once: CO₂ + H₂O ⇌ H⁺ + HCO₃⁻
  • H⁺ excreted, HCO₃⁻ reclaimed (both ways)
NormalKidneysExcrete acid and make bicarbonate; act over days

Schematic. Proteins, haemoglobin and phosphate also buffer; bone buffers acid in chronic acidosis.