Interactive physiology

Acid–Base Balance

Blood pH stays between 7.35 and 7.45 because three systems work on one equation. Chemical buffers act in seconds, the lungs set PaCO₂ within hours, and the kidneys set HCO₃⁻ over days. Pick a disorder to watch the primary change, then the compensation.

pH=6.1 + log₁₀ HCO₃⁻ 24.00.03 × PaCO₂ 40 =7.40
acid base 7.40 pH CO₂ 40 mmHg HCO₃⁻ 24 mEq/L HCO₃⁻ : dissolved CO₂ = 20.0 : 1 acidaemia alkalaemia
Each pan shows its value against normal (PaCO₂ 40 mmHg, HCO₃⁻ 24 mEq/L). The beam follows pH, which depends on their ratio. 6.8–7.8 is roughly the range compatible with life.

pH as a landscape over PaCO₂ and HCO₃⁻, coloured like an indicator. Lines of equal pH are straight rays from the origin because pH depends only on the ratio; the dark ray is pH 7.40, the 20 : 1 line. Drag to turn it.

Steady state Normal

HCO₃⁻ 24 mEq/L against dissolved CO₂ of 1.2 mmol/L (0.03 × 40) is a 20 : 1 ratio, which gives pH 7.40. pH tracks that ratio, not either value on its own.

40 mmHg
24.0 mEq/L

Chemistry

The bicarbonate buffer

The buffer is open at both ends. The lungs remove CO₂ and the kidneys remove H⁺ and return HCO₃⁻, so it holds pH far better than its pKa of 6.1 would suggest.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

CO₂H⁺HCO₃⁻Arrows show the net direction of each reaction.

Timing

Three lines of defence

After an acid load each system switches on at its own speed. Bars thicken from onset to full effect; time runs on a log scale.

Chemical buffers

Plasma and interstitial bicarbonate act instantly; haemoglobin, proteins and phosphate share the load, and intracellular and bone buffers join over hours. Buffers blunt the change in pH but remove no acid.

Lungs

Central and peripheral chemoreceptors change ventilation within minutes, moving PaCO₂. The response peaks at 12–24 hours and only partly corrects a metabolic disorder.

Kidneys

The proximal tubule reclaims filtered HCO₃⁻; the distal nephron excretes H⁺ as titratable acid and NH₄⁺, generating new HCO₃⁻. Slow, at 3–5 days, but the only exit for non-volatile acid.

Bedside

ABG interpreter

Enter a gas and the electrolytes. It follows the usual order: check the numbers, read the pH, find the primary process, test the compensation, then the anion gap and delta ratio.

Examples are illustrative values, not real patients.

mEq/L
mEq/L
mEq/L

Acidaemia · pH 7.23

High anion gap metabolic acidosis with appropriate respiratory compensation

    Expected compensation

    DisorderPrimary changeExpected compensationFull effect
    Metabolic acidosisHCO₃⁻ ↓PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2)12–24 h
    Metabolic alkalosisHCO₃⁻ ↑PaCO₂ ↑ 0.7 per 1 HCO₃⁻ (± 2)24–36 h
    Respiratory acidosis, acutePaCO₂ ↑HCO₃⁻ ↑ 1 per 10 PaCO₂minutes
    Respiratory acidosis, chronicPaCO₂ ↑HCO₃⁻ ↑ 3.5–4 per 10 PaCO₂3–5 days
    Respiratory alkalosis, acutePaCO₂ ↓HCO₃⁻ ↓ 2 per 10 PaCO₂minutes
    Respiratory alkalosis, chronicPaCO₂ ↓HCO₃⁻ ↓ 4–5 per 10 PaCO₂2–3 days

    PaCO₂ in mmHg (1 kPa = 7.5 mmHg), HCO₃⁻ in mEq/L. Anion gap taken as 12 ± 2 without K⁺; many modern analysers run lower, so read against your laboratory's range. Albumin correction: AG + 2.5 × (4.0 − albumin g/dL).