About the Venous Blood pH Calculator
This calculator applies the Henderson-Hasselbalch equation to a venous bicarbonate (HCO3-) and PCO2 value to compute venous blood pH. It is intended for educational and informational use. Medical calculations require clinical context — lab values, patient history, and professional judgment — that a calculator cannot provide. Always review outputs with a qualified clinician before making medical decisions.
The Henderson-Hasselbalch equation
Blood pH is governed by the bicarbonate buffer system, and the standard equation relating its components is: pH = 6.1 + log10(HCO3- / (0.03 x PCO2)). Here, HCO3- is the bicarbonate concentration in mmol/L, PCO2 is the partial pressure of carbon dioxide in mmHg, 6.1 is the pKa of the bicarbonate-carbonic acid buffer system, and 0.03 mmol/L per mmHg is the solubility coefficient that converts dissolved CO2 into an equivalent carbonic acid concentration. The same equation applies to arterial or venous samples; only the input values differ.
Why venous pH runs lower than arterial pH
As blood passes through tissue, cells release carbon dioxide and acidic metabolic byproducts into it, so venous blood carries more CO2 and is slightly more acidic than the arterial blood that fed the same tissue bed. A commonly cited normal range for venous whole blood pH is about 7.31 to 7.41, compared with about 7.35 to 7.45 for arterial blood — venous pH typically running roughly 0.02 to 0.04 units lower than a matched arterial sample drawn at the same time. The gap can be larger for central or mixed venous samples, which reflect blood returning from a wider mix of tissue beds, including working muscle and organs with high metabolic demand.
Reading the other outputs
Hydrogen ion concentration, [H+], is the exact inverse of pH: [H+] (nmol/L) = 10^(9 - pH). It is included because clinicians often think in terms of [H+] when tracking acid-base trends, since it moves in a roughly linear way near the normal range while pH itself is logarithmic. The HCO3- / PCO2 ratio is the value inside the logarithm; a ratio near 20:1 corresponds to a pH near 7.4, so ratios below 20:1 point toward acidemia and ratios above 20:1 point toward alkalemia, whether the underlying driver is metabolic (HCO3-) or respiratory (PCO2).
Important disclaimer
This tool performs the Henderson-Hasselbalch arithmetic on the values you provide. It does not diagnose or classify acid-base disorders, and it does not constitute medical advice. Do not use it to diagnose, treat, or manage any medical condition without consulting a licensed healthcare provider.