Acid-Base Balance and Blood Gases

pH is the negative logarithm of hydrogen ion activity, so each one-unit fall in pH represents a tenfold rise in hydrogen ion concentration, and even small pH changes have substantial physiological effects. An acid donates a proton in water and a base accepts one. Acid strength is expressed by the dissociation constant, where pKa is the negative logarithm of Ka. At a pH equal to the pKa, the protonated and unprotonated forms of a simple conjugate pair are present in equal amounts. Below the pKa, the protonated form dominates. A buffer pairs a weak acid with its conjugate base. The conjugate base absorbs added hydrogen ion and the weak acid absorbs added hydroxide, so a buffer resists pH change most effectively near its own pKa. Buffer capacity also depends on the buffer's total concentration. The bicarbonate system is the physiologic exception. Its pKa of 6.1 sits more than a unit below blood pH, yet the lungs continuously remove CO₂ and the kidneys regulate bicarbonate independently, so the system behaves as an open one. Its response still depends on adequate ventilation and renal function.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

The following values form a composite teaching set for arterial blood analyzed at 37 °C, at sea level, while breathing room air:^{2,62 Castro D, et al. Arterial blood gas analysis. 2026.6 MedlinePlus. Blood gases. 2024.}

Treat every interval above as an example. Bicarbonate and pO₂ intervals vary by laboratory and analyzer, and pO₂ falls with age, altitude, and inspired oxygen. Interpret patient results against your laboratory's verified intervals. For examination answers, use the ASCP BOC ranges: pCO₂ 35–44 mm Hg, HCO₃^- 23–29 mmol/L, and pO₂ >80 mm Hg. Serum total CO₂ from a chemistry panel is a separate measured analyte with its own interval. The electrolyte lessons in this area describe that measurement.

pH below 7.35 indicates acidemia, produced by a process called acidosis. A pH above 7.45 indicates alkalemia, produced by alkalosis. Acidemia and alkalemia describe the measured blood pH. Acidosis and alkalosis are the disorders that produce them. The respiratory component of the pH is pCO₂ and the metabolic component is bicarbonate. Changes in these two variables produce four primary disorders:^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Interpret a blood gas in six steps:^{22 Castro D, et al. Arterial blood gas analysis. 2026.}

1. Confirm specimen validity and collect the interpretation inputs: arterial or venous source, inspired oxygen at collection, draw time, and temperature if recorded.
2. Read pH and classify acidemia or alkalemia.
3. Identify the variable that explains the pH. Low bicarbonate supports metabolic acidosis and high bicarbonate metabolic alkalosis. High pCO₂ supports respiratory acidosis and low pCO₂ respiratory alkalosis.
4. Test the other, compensating variable against the expected-compensation table. A value outside the predicted range suggests a second, superimposed disorder.
5. Compute the anion gap whenever metabolic acidosis is present.
6. Correlate the pattern with the recorded source, inspired oxygen, and collection details before you report it.

The examples use the no-potassium anion-gap convention, AG = Na⁺ − (Cl^- + HCO₃^-), whose matching reference interval is about 7 to 16 mmol/L. The anion gap lesson develops both conventions and their matching intervals.

The following constructed examples use pKa′ 6.1 and the displayed coefficient 0.0307, with calculated pH rounded to two decimal places.

Example A: high-gap metabolic acidosis with appropriate compensation. Results are pH 7.31, 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. Winters 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, well above the 7 to 16 mmol/L interval for this convention. That supports a high-gap process, because isolated bicarbonate loss leaves the gap normal.

Example B: chronic respiratory acidosis with appropriate renal compensation. A blood gas shows pH 7.32, 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) × 3.5 to 5, or 10.5 to 15 mmol/L, giving an expected HCO₃^- of about 35 to 39 mmol/L. The measured 36 mmol/L falls within that range, a pattern consistent with a chronic compensated respiratory acidosis. In this example, the pH remains below 7.35. A milder chronic respiratory disorder can have expected compensation with pH inside the reference interval.

Example C: acute respiratory alkalosis. Results are pH 7.48, 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 acutely lowers bicarbonate by about 2 mmol/L, predicting 22 mmol/L. The agreement with the prediction supports 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, high for the no-potassium convention. Winters formula predicts pCO₂ = 1.5 × 14 + 8 = 29 ± 2 mm Hg, and the measured value is only 22 mm Hg. That additional fall in pCO₂ indicates a superimposed primary respiratory alkalosis. Example D shows why all six steps apply to every gas, including one with a normal pH.

Ventilation and oxygenation are distinct processes. pCO₂ reflects alveolar ventilation relative to CO₂ production. pO₂ reflects oxygen diffusion and gas exchange at the alveolus. Neither value alone equals oxygen delivery to tissue, which also depends on hemoglobin content, saturation, and perfusion. A normal pO₂ in a patient receiving supplemental oxygen therefore does not exclude ventilatory failure, so always read pO₂ with pCO₂ and the recorded inspired oxygen. Tissue oxygenation requires atmospheric O₂, adequate ventilation and alveolar gas exchange, hemoglobin able to bind O₂, sufficient hemoglobin mass, adequate perfusion, and O₂ release to tissue. Failure of any component can impair tissue oxygenation, a state called hypoxia. Hypoxemia, abnormally low arterial oxygen tension, is one cause of hypoxia. Anemia, dyshemoglobins, impaired unloading, and poor perfusion cause hypoxia even with a normal pO₂.^{44 Rhodes CE, et al. Physiology, oxygen transport. 2022.}

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. Arterial pO₂ then sits slightly lower still, at 85 to 105 mm Hg in young adults, because of the small alveolar-to-arterial gradient. 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's effective transfer across the alveolar-capillary membrane is roughly one-twentieth that of CO₂ at the same pressure gradient, so diffusion limitation lowers arterial pO₂ well before pCO₂ rises. Isolated diffusion limitation is uncommon, and coexisting hypoventilation raises pCO₂.^{44 Rhodes CE, et al. Physiology, oxygen transport. 2022.}

Hemoglobin species

Each hemoglobin A tetramer reversibly binds four O₂ molecules. Four forms matter in routine practice, and sulfhemoglobin is an additional measured species:^{44 Rhodes CE, et al. Physiology, oxygen transport. 2022.}

Oxygen parameters on the report

O₂ saturation (SO₂) is O₂Hb divided by the sum of O₂Hb and HHb, ×100%, representing the percentage of functional hemoglobin loaded. Analyzers without a co-oximeter calculate SO₂ from pO₂, together with pH, pCO₂, and temperature, through an assumed dissociation curve that presumes no dyshemoglobins. Co-oximeters measure the hemoglobin fractions directly.

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. When you suspect a saturation gap or a dyshemoglobin, report the measured fractions, because calculated saturation cannot account for a dyshemoglobin.

Pulse oximetry (SpO₂) uses noninvasive two-wavelength devices that infer an O₂Hb-to-HHb ratio from the absorption of red and infrared light. These devices read falsely normal or high with COHb, and significant MetHb tends to drive the reading toward about 85% regardless of the true saturation, an artifact that varies by device and MetHb concentration. Multiwavelength units add channels for the additional hemoglobin species.

pO₂ measures the partial pressure of dissolved O₂ only. Dissolved oxygen is a minor store, about 0.3 mL O₂/dL at a pO₂ of 100 mm Hg, whereas 5 L of fully saturated blood at hemoglobin 15 g/dL carries more than 1,000 mL bound to hemoglobin. A high pO₂ therefore does not guarantee adequate oxygen content when hemoglobin mass is low or functional hemoglobin is occupied. Oxygen content needs hemoglobin and saturation, so review them together with pO₂.^{44 Rhodes CE, et al. Physiology, oxygen transport. 2022.}

Oxygen capacity and content

Suppose hemoglobin is 13.5 g/dL, oxygen saturation is 90%, and pO₂ is 70 mm Hg.

Hemoglobin-bound capacity at full saturation = 1.39 × 13.5 = 18.77 mL O₂/dL

Many laboratories use the conventional clinical constant 1.34, which gives a capacity of 18.1 mL/dL and a correspondingly lower content. The theoretical 1.39 and the empirical 1.34 differ because not every hemoglobin site is functionally available.

Dissolved oxygen = 70 × 0.00314 = 0.22 mL O₂/dL

Total oxygen content (1.39 constant) = 0.22 + (18.77 × 0.90) = 17.1 mL O₂/dL

Partial pressure and oxygen content are different measurements. An interpretation of pO₂ without hemoglobin and saturation gives an incomplete assessment of oxygenation.

The oxyhemoglobin dissociation curve

Plotting percent SO₂ against pO₂ gives a sigmoid curve. Cooperative binding keeps saturation high between roughly 60 and 100 mm Hg and releases O₂ steeply as pO₂ falls through the 60 to 20 mm Hg range. The curve's position is summarized by P50, the pO₂ at 50% saturation, conventionally reported at 37 °C, pH 7.4, and pCO₂ 40 mm Hg, normally about 26 to 27 mm Hg. A right-shifted curve, with a 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. H⁺, CO₂, and temperature change within minutes in active tissue with high oxygen demand. The 2,3-DPG response develops over hours to days. Chronic anemia and high-altitude exposure raise erythrocyte 2,3-DPG, right-shifting the curve so each gram of hemoglobin unloads more O₂. Refrigerated storage lowers erythrocyte 2,3-DPG within days to weeks, and the deficit recovers over roughly one to three days after transfusion. The clinical significance of the transient left shift after massive transfusion is uncertain. CO poisoning and high-oxygen-affinity hemoglobin variants shift the curve left, and low-affinity variants shift it right.^{44 Rhodes CE, et al. Physiology, oxygen transport. 2022.}

Analyzers measure pH, pCO₂, and pO₂ electrochemically, then calculate HCO₃^-, total CO₂, base excess, and SO₂. Many add co-oximetry, hemoglobin, electrolytes, glucose, and lactate. The menu is analyzer-specific. Two transduction principles apply. Potentiometry measures voltage between a measuring and a reference electrode to track ion activity, and amperometry measures current proportional to O₂ reduced at an electrode. The Laboratory Operations area covers electrochemical design, and this lesson adds the blood-gas application.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

Preanalytic errors outnumber analytic errors in blood-gas testing. A painful arterial puncture can alter the patient's respiratory pattern enough to change the reported pCO₂ and pH, so collection technique and patient reassurance affect the result. CLSI C46-A2 (2009) covers blood-gas and pH specimen collection and preanalytic handling. It is an archived document that CLSI retains because it remains technically valid, and no newer C46 edition has replaced it.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

Arterial blood is required when ventilation or oxygenation must be assessed, because venous and capillary pO₂ cannot substitute for arterial pO₂. Peripheral venous blood is acceptable for selected acid-base screening that does not require assessment of ventilation or oxygenation, under locally validated limits, with the specimen source documented and venous reference intervals applied. Venous pCO₂ runs several mm Hg higher and pH several hundredths lower than arterial, with limits of agreement that vary by site and perfusion. Arterialized capillary blood, drawn after warming the site with a free-flowing fill, approximates arterial pH and pCO₂ in many patients. Its pO₂ and saturation do not approximate arterial values, because arterialization, perfusion, and site variation affect them in addition to any air exposure. A modified Allen test screens collateral ulnar flow before radial puncture under some collection policies.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

Collection device and handling. The standard arterial device is a self-filling plastic syringe, commonly 1 to 3 mL, preloaded with dry, lyophilized, electrolyte-balanced (calcium-balanced) lithium heparin. Device and anticoagulant requirements are analyzer-specific. Excess liquid heparin dilutes the specimen and shifts gas and electrolyte results, so follow the manufacturer's draw-to-heparin ratio. Do not use the evacuated tubes used for chemistry panels for arterial gas specimens. Mix thoroughly immediately to dissolve the heparin and again just before analysis to resuspend settled cells and prevent microclots, and cap the syringe. Expel trapped air immediately. Air bubbles drive specimen pO₂ toward the 150 mm Hg of room air and let pCO₂ fall toward zero, so the direction of error depends on the patient value. Document the specimen source, the inspired oxygen condition (room air or supplemental), and body temperature at draw time. Analyze at room temperature within 30 minutes as an upper handling limit, sooner when pO₂ or saturation is the critical question. Pneumatic-tube transport requires validation because vibration and bubbles worsen gas shifts. Ice a combined blood-gas and electrolyte specimen only when the protocol validates that handling for the analyzer, because cooling alters potassium in plastic syringes. For longer delays, follow the validated protocol. 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 and produce pseudohypoxemia. Reject specimens with microclots, insufficient volume, suspected line or intravenous-fluid contamination, or unexplained gas values, and request recollection.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

Reading results critically. A result can be analytically or physiologically implausible in ways the laboratory checks before release: a pH inconsistent with the pCO₂ and calculated bicarbonate, a pO₂ far above or below the documented inspired oxygen, a large gap between calculated saturation and measured oxyhemoglobin fraction, or a result inconsistent with the patient's previous values. Each pattern calls for the same response sequence: keep the result unreleased, check quality control and the specimen condition, reanalyze or compare against another method or specimen, recollect when the specimen is the likely cause, and report with the specimen's limitations noted when it is usable.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

Communication and point-of-care testing. Blood gases are urgent results, and many jurisdictions and accreditation standards require immediate notification of critical acid-base or oxygenation findings, with read-back and documentation. The specific pH, pCO₂, and pO₂ thresholds come from your laboratory's policy, because no universal list exists. Include the specimen source and inspired oxygen in the notification so the care team can interpret the values. Point-of-care (POC) analyzers bring testing to the bedside, and their cartridge or optode methods, calibration algorithms, co-oximetry menus, and calculated intervals can differ from the central laboratory's systems. Compare POC and central results locally before treating them as interchangeable. POC testing adds operator training and competency requirements, per-device quality-control schedules, and connectivity for result capture. The same policy defines an escalation path for when POC and central results disagree.^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}

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:^{55 Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved… 2009.}