Acid-Base Balance and Blood Gases

Extracellular hydrogen ion concentration sits between 36 and 44 nmol/L, which corresponds to the pH interval 7.35 to 7.45. Metabolism produces acid continuously, so this narrow band is maintained by buffers, ventilation, and renal excretion working together. Blood-gas results therefore couple measurement quality to rapid physiological reasoning: specimen handling errors can manufacture a disorder, and a correctly drawn specimen can hide a second disorder behind an apparently normal pH.¹

Hydrogen ion physiology and buffering

pH is the negative logarithm of hydrogen ion activity. Each one-unit fall in pH represents a tenfold rise in hydrogen ion concentration, which is why small pH changes carry large physiological weight. An acid donates a proton in water and a base accepts one. Acid strength is expressed by the dissociation constant, where pKa equals the negative logarithm of Ka. At a pH equal to the pKa, the protonated and unprotonated forms are present in equal amounts; below the pKa, the protonated form dominates. A buffer pairs a weak acid with its conjugate base and resists pH change because added hydrogen ion is absorbed by the conjugate base while added hydroxide is absorbed by the weak acid. A buffer works most effectively near its own pKa.¹

The Henderson-Hasselbalch equation describes buffer behavior:

pH = pKa′ + log([A^-]/[HA])

For the bicarbonate system, A^- is HCO₃^- and HA is H₂CO₃, which is measured as dissolved CO₂ because carbonic acid itself is unmeasurable and collapses instantly to CO₂ and water. Converting mm Hg of pCO₂ to mmol/L of dissolved CO₂ at 37 °C uses the solubility coefficient 0.0307 mmol/L per mm Hg, and pKa′ for plasma bicarbonate is fixed at 6.1:¹

pH = 6.1 + log₁₀( [HCO₃^-] / (0.0307 × pCO₂) )

A normal 20:1 ratio of bicarbonate to dissolved CO₂ gives 6.1 + log(20) = 6.1 + 1.3 = 7.4. The kidney sets the numerator and the lung sets the denominator, so pH is their ratio. A metabolic disturbance and a respiratory disturbance that move in opposite directions can therefore leave pH deceptively close to normal.

Carbon dioxide diffuses from metabolizing cells into capillary blood, and most of it enters the erythrocyte, where carbonic anhydrase hydrates it to carbonic acid, which dissociates to hydrogen ion and bicarbonate. Bicarbonate leaves the cell down its concentration gradient and chloride enters to preserve electroneutrality; this exchange is the chloride shift, also called the Hamburger shift. The liberated hydrogen ion binds newly deoxygenated hemoglobin. In the lung, every step reverses: oxygen loads hemoglobin, displacing hydrogen ion, which recombines with bicarbonate to regenerate carbonic acid, then CO₂ and water, and CO₂ is exhaled. Because retaining CO₂ behaves mathematically like adding acid and exhaling it like removing acid, ventilation changes are treated as acid-base disturbances even though CO₂ itself is neither acid nor base.¹

The kidneys require two to four days to fully engage but ultimately reclaim filtered bicarbonate and excrete the roughly 50 to 100 mmol/day net acid load. Because the filtrate carries the same bicarbonate concentration as plasma, reabsorption is indirect: proximal tubule cells secrete hydrogen ion in exchange for sodium; the secreted hydrogen ion combines with filtered bicarbonate to form carbonic acid, which carbonic anhydrase splits to CO₂ and water; the CO₂ diffuses into the cell, regenerates carbonic acid, and the resulting bicarbonate returns to blood. A luminal pH floor of 4.6 limits free hydrogen ion excretion. Phosphate carries titratable acid, and ammonia and ammonium provide the major adaptable route for net acid excretion, increasing markedly in acidosis.¹

Arterial blood gas reference intervals

Reference values at 37 °C, sea level, breathing room air:¹

These intervals are laboratory- and analyzer-specific. The ASCP BOC examination reference ranges topic carries the composite values used for examination answers, and the reporting laboratory’s verified interval governs patient results.

The four primary acid-base disorders

pH below 7.35 is acidemia, produced by a process called acidosis. pH above 7.45 is alkalemia, produced by alkalosis. The respiratory component of pH is pCO₂ and the metabolic component is bicarbonate. Changes in these two variables produce four primary disorders:¹

Compensation is the response of the organ that did not cause the disturbance. Lungs compensate metabolic disorders within minutes by resetting ventilation, while kidneys compensate respiratory disorders over days by resetting bicarbonate reabsorption and excretion. Compensation moves pH toward normal without overshoot. Compare the compensating variable with its expected range; a value outside that range indicates another primary process.¹

These expected-compensation rules are teaching approximations. A deviation from these values indicates a second disorder. Clinicians use the specific numeric rules with bedside clinical judgment:¹

Expected renal compensation differs between acute and chronic respiratory disorders because full renal adjustment takes days. An acute airway emergency therefore shows severe acidemia with little bicarbonate rise, while the same pCO₂ sustained for weeks shows a much larger compensatory bicarbonate.

Causes by disorder

Metabolic acidosis. Fixed-acid gain includes ketoacidosis (diabetic, alcoholic, starvation), lactic acidosis (shock, hypoxia), toxic-alcohol or salicylate ingestion, and uremic acid retention. Bicarbonate loss through diarrhea or renal tubular acidosis produces a normal-gap, hyperchloremic pattern. Kussmaul respiration, deep and rapid breathing, is the characteristic respiratory compensation.

Metabolic alkalosis. Acid loss through vomiting or nasogastric suction removes gastric hydrochloric acid. Base gain includes bicarbonate or antacid loading, and diuretic-driven hydrogen ion loss contributes. Hypokalemia drives hydrogen ion into cells and enhances renal bicarbonate reabsorption. Respiratory compensation is self-limited because rising pCO₂ and falling pO₂ both stimulate breathing; correcting volume, chloride, and potassium restores the chemistry.¹

Respiratory acidosis. Ventilation fails relative to CO₂ production in obstructive lung disease, respiratory-center depression from opioids, barbiturates, or alcohol, and neuromuscular weakness.

Respiratory alkalosis. Excess ventilation accompanies anxiety or pain, hypoxemia from altitude or pulmonary embolism and fibrosis, and direct respiratory-center stimulation as in salicylate toxicity. Because CO₂ crosses the blood-brain barrier rapidly, acute respiratory alkalosis can lower ionized calcium enough to cause perioral tingling and carpopedal spasm with a normal total calcium.¹

Mixed disorders

Mixed disorders combine these mechanisms. Lactic high-anion-gap metabolic acidosis with respiratory alkalosis occurs in sepsis, and respiratory acidosis with diuretic-induced metabolic alkalosis occurs in heart failure. A measured value outside the expected compensation range identifies the superimposed disorder.

Systematic interpretation with worked examples

Interpret a blood gas in six steps:¹

1. Flag which values fall outside their reference interval.
2. Read pH and classify acidemia or alkalemia.
3. Identify the variable that explains the pH: low bicarbonate supports metabolic acidosis, high bicarbonate supports metabolic alkalosis, high pCO₂ supports respiratory acidosis, and low pCO₂ supports respiratory alkalosis.
4. Test the other, compensating variable against the expected-compensation table. A value outside the predicted range signals a second, superimposed disorder.
5. Compute the anion gap whenever metabolic acidosis is present.
6. Reconcile the pattern with the clinical picture.

Example A: high-gap metabolic acidosis with appropriate compensation. Results are pH 7.29, pCO₂ 30 mm Hg, HCO₃^- 15 mmol/L, Na⁺ 139 mmol/L, and Cl^- 102 mmol/L. The low pH and bicarbonate identify a primary metabolic acidosis. Winter’s formula gives 1.5(15) + 8 = 30.5 ± 2 mm Hg, so the measured pCO₂ of 30 mm Hg is appropriate compensation. The anion gap is 139 − (102 + 15) = 22 mmol/L, supporting a high-gap process rather than isolated bicarbonate loss.

Example B: chronic respiratory acidosis with appropriate renal compensation. A blood gas shows pH 7.33, pCO₂ 70 mm Hg, and HCO₃^- 36 mmol/L. The elevated pCO₂ with acidemia indicates primary respiratory acidosis. Because pCO₂ sits 30 mm Hg above 40, chronic compensation predicts a bicarbonate rise of (30/10) × 4 = 12 mmol/L, giving an expected HCO₃^- of 36 mmol/L. The measured value matches that expectation, supporting a chronic compensated process. The pH remains acidemic, so compensation has not fully normalized it.

Example C: acute respiratory alkalosis. Results are pH 7.50, pCO₂ 30 mm Hg, and HCO₃^- 22 mmol/L. The alkalemia with a low pCO₂ identifies primary respiratory alkalosis. A 10-mm Hg fall from 40 mm Hg should acutely lower bicarbonate by about 2 mmol/L, predicting 22 mmol/L. The measured value matches, so the pattern fits a simple acute hyperventilation process before substantial renal adaptation.

Example D: a mixed disorder hidden by a near-normal pH. A septic patient has pH 7.42, pCO₂ 22 mm Hg, HCO₃^- 14 mmol/L, Na⁺ 141 mmol/L, and Cl^- 103 mmol/L. Despite the nearly normal pH, the low bicarbonate indicates metabolic acidosis. The anion gap is 141 − (103 + 14) = 24 mmol/L. Winter’s formula predicts pCO₂ = 1.5(14) + 8 = 29 ± 2 mm Hg, but the measured value is only 22 mm Hg. That extra fall in pCO₂ indicates a second primary respiratory alkalosis. Two opposing disorders can leave the pH deceptively close to normal.

Oxygen transport and delivery

Tissue oxygenation depends on available atmospheric O₂, adequate ventilation, gas exchange at the alveolus, hemoglobin able to bind O₂, sufficient hemoglobin mass, adequate perfusion, and O₂ release to tissue. Failure of any component causes hypoxia, inadequate tissue oxygenation. Hypoxemia, abnormally low arterial oxygen tension, is one possible cause of hypoxia.²

Partial pressures. At sea level, where total pressure is 760 mm Hg and oxygen is 20.93% of dry air, humidified inspired gas displaces 47 mm Hg as water vapor at 37 °C. Inspired pO₂ therefore equals approximately (760 − 47) × 0.2093 = 149 mm Hg. Oxygen uptake and the addition of CO₂ lower alveolar pO₂ to about 100 mm Hg on room air at sea level. Diffusion barriers such as emphysematous surface loss or pulmonary edema fluid, airway obstruction, and perfusion failure from embolism, pulmonary hypertension, or heart failure all lower delivered oxygen further. Oxygen diffuses about 20-fold more slowly across the alveolar membrane than CO₂, so diffusion-limited disease produces hypoxemia with little pCO₂ change and often responds well to supplemental O₂. Hypoventilation-related hypoxemia may also improve with supplemental O₂, but oxygen cannot correct CO₂ retention; ventilatory support is required to correct the underlying ventilatory failure.²

Hemoglobin species

Each hemoglobin A tetramer reversibly binds four O₂ molecules. Four forms matter clinically:²

Four measured and derived oxygen parameters

O₂ saturation (SO₂) equals O₂Hb divided by the sum of O₂Hb and HHb, the percent of functional hemoglobin loaded. Blood gas analyzers usually calculate SO₂ from pO₂ through an assumed dissociation curve that presumes no dyshemoglobins, a false assumption in CO poisoning or methemoglobinemia.

Fractional oxyhemoglobin (FO₂Hb) equals O₂Hb divided by total hemoglobin, with COHb, MetHb, and sulfhemoglobin in the denominator. FO₂Hb and SO₂ coincide in health but diverge sharply once dyshemoglobins accumulate.

Pulse oximetry (SpO₂) uses noninvasive two-wavelength devices that read only O₂Hb against HHb. These devices overestimate effective oxygenation with COHb, and substantial MetHb tends to drive SpO₂ toward about 85% regardless of the true saturation. Multiwavelength units add those channels.

pO₂ measures pressure from dissolved O₂ only. Dissolved oxygen is a minor store, about 0.3 mL O₂/dL at a pO₂ of 100 mm Hg, against more than 1000 mL carried as O₂Hb across a 5-L blood volume. In a hyperbaric chamber, pressurized dissolved O₂ alone can sustain tissue if hemoglobin delivery fails.²

Oxygen capacity and content

With hemoglobin 13.5 g/dL and oxygen saturation of 90%, hemoglobin-bound capacity is 1.39 × 13.5 = 18.77 mL O₂/dL. At a pO₂ of 70 mm Hg, dissolved oxygen contributes about 70 × 0.00314 = 0.22 mL O₂/dL. Total oxygen content is therefore approximately 0.22 + (18.77 × 0.90) = 17.1 mL O₂/dL. Partial pressure and oxygen content are distinct measurements.

The oxyhemoglobin dissociation curve

Plotting percent SO₂ against pO₂ gives a sigmoid curve. Cooperative binding keeps saturation high until pO₂ falls near 60 mm Hg, below which O₂ unloads readily. The curve’s position is summarized by P₅₀, the pO₂ at 50% saturation, normally about 26 to 27 mm Hg. A right-shifted curve, higher P₅₀, means lower affinity and easier tissue unloading; a left shift means tighter binding and poorer delivery. Falling pH and rising pCO₂, together called the Bohr effect, shift the curve right, as do rising temperature and rising 2,3-diphosphoglycerate. These conditions occur in active tissue with high oxygen demand. Chronic anemia and high-altitude exposure raise erythrocyte 2,3-DPG, right-shifting the curve for more unloading per gram of hemoglobin. Stored blood loses 2,3-DPG over weeks, producing a transient left shift after massive transfusion. CO poisoning and high-oxygen-affinity hemoglobin variants shift the curve left, and low-affinity variants shift it right.²

Blood gas instrumentation

Analyzers measure pH, pCO₂, and pO₂ electrochemically, then calculate HCO₃^-, total CO₂, base excess, and SO₂. Many also measure CO-oximetry, hemoglobin, electrolytes, glucose, and lactate. Two transduction principles apply: amperometry measures current proportional to O₂ reduced at an electrode, and potentiometry measures voltage between measuring and reference electrodes to track ion activity. The electrochemical designs below correspond to the instrumentation treatment in the Laboratory Operations area; clinical blood-gas application is the subject here.³

Formats range from classic macroelectrodes to miniaturized thick- and thin-film disposable cartridges. Optical optodes use analyte-quenched or shifted fluorescent dyes instead of electrochemistry and resist the electrode drift that forces frequent recalibration.

Calibration

Analyzers self-calibrate every 30 to 60 minutes against two gas mixtures bracketing the working range, for example a low-O₂ moderate-CO₂ mix and a higher-O₂ higher-CO₂ mix. The pH electrode calibrates against two NIST-traceable buffers stored sealed, because exposure to air lets CO₂ absorb and shift pH. Every chamber runs at 37 ± 0.1 °C because both the Nernstian slope and gas solubility are temperature-dependent. Results are conventionally reported at 37 °C. Temperature-corrected values, if generated for a hypo- or hyperthermic patient, are reported alongside the 37 °C values, because temperature-specific reference intervals are sparse.³

Calculated versus measured bicarbonate

HCO₃^- on a blood gas panel is calculated from measured pH and pCO₂ through the Henderson-Hasselbalch relationship, assuming pKa′ 6.1 and a solubility coefficient of 0.0307, both of which drift with temperature and with severe matrix abnormalities such as lipemia. A serum chemistry panel’s total CO₂ is measured directly and is dominated by bicarbonate. The two agree closely in health but use different methods. That difference helps explain discordance in specimens with matrix effects. The electrolyte modules in this area describe serum total CO₂ measurement.

CO-oximetry

Multiple wavelengths applied to hemolyzed whole blood resolve hemoglobin species by their characteristic absorbance spectra. Two wavelengths separate O₂Hb and HHb, four add COHb and MetHb, and five add sulfhemoglobin. Measured O₂Hb directly quantifies oxyhemoglobin, while calculated SO₂ estimates saturation from pO₂ and an assumed dissociation curve. In carbon monoxide poisoning, calculated SO₂ can remain deceptively normal despite a substantially decreased measured O₂Hb. The toxicology modules cover CO exposure testing.³

Specimen collection and quality assurance

Preanalytic errors outnumber analytic errors in blood-gas testing. A painful arterial puncture can provoke hyperventilation that lowers the patient’s own pCO₂ and raises pH, so technique and patient reassurance affect reported values. CLSI C46-A2 (2009), archived but still listed by CLSI as technically valid as of June 2019, covers blood-gas and pH specimen collection and preanalytical handling.⁴

Arterial blood is preferred for assessing ventilation and oxygenation, but peripheral venous blood is an acceptable substitute whenever ventilation or oxygenation status is not the question, provided the source is documented and venous reference intervals are applied. Capillary specimens correlate acceptably with arterial pH and pCO₂ but poorly with pO₂ because of air exposure. Good technique requires warming the site to arterialize the bed and a free-flowing fill without milking, which contaminates the sample with tissue fluid. Before a radial puncture, collateral ulnar flow to the hand can be screened with a modified Allen test.⁴

Collection device and handling. The standard device is a 1 to 3 mL self-filling syringe preloaded with dry, lyophilized lithium heparin. Liquid heparin dilutes the specimen and traps air. Evacuated tubes are unacceptable. Mix thoroughly immediately to dissolve heparin and again just before analysis to resuspend settled cells and prevent microclots. Expel trapped air immediately; air contamination drives specimen pO₂ toward the pO₂ of room air and can raise or lower the result depending on the patient value. Document inspired oxygen, room air or supplemental, and body temperature at draw time. Analyze plastic-syringe blood-gas specimens at room temperature within 30 minutes. For longer delays, follow the validated blood-gas protocol; ice a combined blood-gas and electrolyte specimen only when that handling has been validated, because cooling can alter potassium. At room temperature, ongoing cellular metabolism consumes O₂ and glucose and produces CO₂ and lactate, lowering pH and raising pCO₂ over time. Marked leukocytosis or thrombocytosis can outpace even prompt handling, producing pseudohypoxemia, and no routine QC scheme detects these preanalytic losses after the fact.⁴

Quality assurance. Much of the testing cycle, including ordering, collection, transport, and storage, lies outside direct laboratory control, so personnel who handle the specimen require prospective education. Blood-gas analysis is clinically urgent and repeat arterial collection may be difficult, so preventing preanalytic errors is preferable to detecting them. Quality assurance uses these approaches:⁴

References

1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
2. Todorovic J, Terzic S, Zikic T, Stankovic M. Acid-base and oxygen transport physiology. In: StatPearls. Treasure Island, FL: StatPearls Publishing; 2024. NCBI Bookshelf. Accessed August 31, 2026.
3. Kellum JA, Elbers PWG, eds. Stewart's Textbook of Acid-Base. 2nd ed. AcidBase.org; 2009. Blood-gas electrode principles and CO-oximetry; electrochemical engineering details in Laboratory Operations.
4. Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved Guideline, Second Edition. CLSI document C46-A2. Wayne, PA: CLSI; 2009. Archived; CLSI lists the document as technically valid as of June 2019.