Normal Blood pH Range
The normal arterial blood pH range is 7.35 to 7.45, with an optimal value typically around 7.40. 1
Physiological Context
The pH of 7.40 represents a carefully maintained balance that reflects intracellular pH close to neutrality, which offers evolutionary advantages by keeping biosynthetic pathway intermediates ionized and preventing their penetration across biological membranes. 2 This tight regulation is critical because:
- Blood pH below 7.35 indicates acidemia, while pH above 7.45 indicates alkalemia 1
- The body maintains this narrow range through bicarbonate buffering, with normal serum bicarbonate being 22-26 mmol/L 1
- Venous pH is typically 0.03 units lower than arterial pH, making venous pH around 7.32-7.42 3
Critical pH Thresholds
Lower Limits
- pH <7.35 with bicarbonate <22 mmol/L defines metabolic acidosis 1
- pH <7.20 in pleural fluid (with normal blood pH) indicates serious pathology such as pleural infection, rheumatoid arthritis, esophageal rupture, or malignancy 3
- pH <7.0 in diabetic ketoacidosis may warrant bicarbonate therapy, though this remains controversial 3
- Survival has been documented with pH values as low as 6.71-6.74 in severe toxic alcohol poisoning, though this represents extreme pathology requiring intensive intervention 4
Upper Limits
- pH >7.45 indicates alkalemia 1
- The physiologically compatible pH range for survival extends from approximately 6.8 to 7.8, beyond which protein denaturation occurs 4
Clinical Measurement Considerations
For accurate assessment, arterial blood gas (ABG) analysis is the gold standard, providing pH, PaCO2, and bicarbonate levels simultaneously. 1 Key points include:
- Venous blood gas can be used for screening, but arterial samples better represent systemic acid-base status 3
- In umbilical cord blood at birth, the lower limit of normal arterial pH (mean -2 SD) is 7.10 5
- Blood samples for pH analysis should be processed within 15 minutes if not stored in ice to prevent falsely low readings from ongoing cellular metabolism 5
Common Pitfalls to Avoid
- Do not confuse serum CO2 on a basic metabolic panel with arterial pH—the BMP CO2 reflects total bicarbonate content (normal 22-26 mEq/L), not pH or PaCO2 1
- In patients with chronic respiratory conditions (COPD, chest wall deformities), elevated bicarbonate may represent renal compensation for chronic CO2 retention rather than primary metabolic alkalosis—ABG is needed to differentiate 3, 1
- Temperature affects pH measurement; normal values assume body temperature of 37°C 6