Physiology · Renal, fluids and acid–base

Reading an arterial blood gas step by step !

Measurement
Exam priority: high

A blood gas shows the blood's pH, CO₂, bicarbonate and oxygen. Read in order, it tells you what is failing: the lungs, the kidneys or metabolism.

Updated 2026-11-01

Also called: ABG, Blood gases

See it in 3D: Kidneys · Right upper lobe · Medulla oblongata

Key points

Sea level: pH 7.35–7.45; PaCO₂ 35–45 mmHg; HCO₃⁻ 22–26 mEq/L; PaO₂ ≈ 80–100 mmHg.
Order: pH → which value explains the pH? (primary) → is compensation as expected? → anion gap if metabolic acidosis → oxygenation.
Compensation never fully corrects or overshoots: a normal pH with abnormal PaCO₂ and HCO₃⁻ suggests a mixed disorder.
The lungs compensate in minutes to hours; the kidneys take 2 to 5 days, which separates acute from chronic respiratory disorders.
At ≈ 2600 m (Bogotá), normal PaO₂ is about 60–70 mmHg and PaCO₂ ≈ 30–34 mmHg, with slightly lower HCO₃⁻: a physiological compensated chronic respiratory alkalosis.
Air in the sample or delay without ice distorts the gases; a low HCO₃⁻ with low pH and low PaCO₂ is metabolic acidosis.

What it is

An arterial blood gas measures pH, PaCO₂ (the respiratory component) and PaO₂ and calculates HCO₃⁻ (the metabolic component). Reading it in order tells you whether there is acidemia or alkalemia, which system caused it, whether the other compensates as expected and how oxygenation is. In Bogotá and other high cities, normal values differ from sea level.

Key facts

Step 1, pH: < 7.35 acidemia; > 7.45 alkalemia.
Step 2, primary: if PaCO₂ moves opposite to pH (high CO₂ with low pH) it is respiratory; if HCO₃⁻ moves the same way (low HCO₃⁻ with low pH) it is metabolic.
Step 3, compensation: compare with the expected formula (Winter's in metabolic acidosis; acute/chronic rules in respiratory disorders).
Step 4, if metabolic acidosis: albumin-corrected anion gap, and the delta ratio to look for a second metabolic disorder.
Step 5, oxygenation: PaO₂, saturation and the alveolar–arterial gradient (normal if hypoxemia is from hypoventilation or altitude).

Equations

Henderson–Hasselbalch (clinical form)pH = 6.1 + log [HCO₃⁻ / (0.03 × PaCO₂)]

0.03: CO₂ solubility (mmol/L per mmHg)
6.1: pKa of the bicarbonate system

pH depends on the kidney/lung ratio, not on either value alone.

Alveolar gas (simplified)P_AO₂ = F_IO₂ × (P_B − 47) − PaCO₂ / 0.8

P_B: barometric pressure (≈ 760 mmHg at sea level; ≈ 560 in Bogotá)
47: water vapour pressure at 37 °C
0.8: respiratory quotient

A–a gradient = P_AO₂ − PaO₂; < 10–15 mmHg in a young adult.

Arterial reference values

Parameter	Value	Note
pH	7.35–7.45
PaCO₂ (sea level)	35–45 mmHg
HCO₃⁻	22–26 mEq/L	slightly lower at altitude
PaO₂ (sea level)	≈ 80–100 mmHg
PaO₂ at ≈ 2600 m	≈ 60–70 mmHg	approximate; varies with age and study
PaCO₂ at ≈ 2600 m	≈ 30–34 mmHg	approximate
SaO₂	≈ 95–100%	≈ 90–94% at altitude

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

The four primary disorders

Disorder	pH	Primary change	Compensation
Metabolic acidosis	↓	HCO₃⁻ ↓	PaCO₂ ↓ (hyperventilation)
Metabolic alkalosis	↑	HCO₃⁻ ↑	PaCO₂ ↑ (hypoventilation)
Respiratory acidosis	↓	PaCO₂ ↑	HCO₃⁻ ↑ (kidney)
Respiratory alkalosis	↑	PaCO₂ ↓	HCO₃⁻ ↓ (kidney)

Clinical notes

A patient in ketoacidosis: pH 7.15, HCO₃⁻ 8, PaCO₂ 20. Winter's predicts 1.5 × 8 + 8 = 20 ± 2: appropriate compensation, a simple disorder.
COPD with PaCO₂ 60 and pH 7.36: HCO₃⁻ ≈ 32 indicates chronic renal compensation; a pH of 7.25 would mean an acute exacerbation.
When reading gases from a patient in Bogotá, use altitude ranges: a PaCO₂ of 32 mmHg can be normal there.

Test yourself

Think of the answer, then tap the question to see it.

A patient has pH 7.40, PaCO₂ 25 mmHg and HCO₃⁻ 15 mEq/L. Why is this not simply normal?

Both components are badly abnormal in directions that cancel out; compensation alone does not normalise pH, so there is a mixed disorder (e.g. metabolic acidosis plus respiratory alkalosis, as in salicylate poisoning).

Why does HCO₃⁻ help distinguish acute from chronic respiratory acidosis?

Cell buffering raises HCO₃⁻ only slightly in the acute phase; over 2 to 5 days the kidney generates new HCO₃⁻ and raises it much more per mmHg of CO₂.

Why is normal PaO₂ in Bogotá lower than at sea level?

Barometric pressure is lower (≈ 560 mmHg), so inspired and alveolar O₂ pressures fall; compensatory hyperventilation lowers PaCO₂ and raises PAO₂ somewhat, but does not normalise it.

What is the alveolar–arterial gradient good for in a hypoxemic patient?

If normal, hypoxemia is explained by hypoventilation or low inspired pressure; if raised, the lung itself is at fault (V/Q mismatch, shunt or diffusion).

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The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 26.4 Acid-Base Balance (OpenStax (Rice University), 2022); 26.5 Disorders of Acid-Base Balance (OpenStax (Rice University), 2022); Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 31). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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