Arterial Blood pH Calculator

Created by Małgorzata Koperska, MD
Reviewed by Bogna Szyk
Last updated: Feb 15, 2022

The arterial blood pH calculator estimates the acidity of the arterial blood using the Henderson-Hasselbalch equation. Bicarbonate (HCO₃) and arterial carbon dioxide partial pressure (PaCO₂) levels are included in the formula developed in 1908 by Dr. Lawrence Joseph Henderson and then improved by Dr. Karl Albert Hasselbalch, who amplified it to help with the calculation of the arterial blood gas.

If you'd like to estimate venous blood pH and learn more about it, use our venous blood pH calculator.

What is arterial blood gas?

You can find it surprising, but there is a significant amount of gas dissolved in your blood serum. It can be assessed by examining an arterial blood sample. The parameters measured in the arterial blood gas test are:

  • Arterial oxygen partial pressure (PaO₂);
  • Arterial carbon dioxide partial pressure (PaCO₂);
  • Bicarbonate (HCO₃⁻);
  • Total CO2 (tCO₂);
  • Base excess (BE).

What can you calculate using arterial blood gas levels?

Some of the most important information that can be gained from the blood gas values is blood pH. pH is a measure of acidity or alkalinity of any solution. Normal pH value ranges for arterial blood are 7.35 - 7.45, while normal pH of venous blood is 7.31-7.41. It means that venous blood is more acidic than arterial. It's due to the fact that there is more of the acidic carbon dioxide (CO₂) in the venous blood.

If the calculated pH is lower than the norm, it indicates acidosis, and if it's higher - informs us about alkalosis. These states of acid-base imbalance can be symptoms of respiratory or metabolic disorders. The body's acid-base homeostasis is a very complicated system - you can read more in the article published in Journal of Clinical & Diagnostic Research.

Calculating arterial blood pH with the Henderson-Hasselbalch equation

The formula developed by Dr. Henderson described the use of carbonic acid as a buffer solution. Then, Dr. Hasselbalch re-expressed that formula in a more complicated manner to study acid-base disorders, which resulted in the Henderson–Hasselbalch equation. It describes the pH as a measure of acidity in biological and chemical systems.

For medical use, it calculates the pH of the blood by inserting the HCO₃ (in mEq/L or mmol/L) and PaCO₂ (in mmHg or torr) values in the following formula:

pH = 6.1 + log10 ( HCO₃ / ( 0.0308 * PaCO₂ ) )

Małgorzata Koperska, MD
Bicarbonate (HCO₃)
Carbon dioxide partial pressure (PaCO₂)
Arterial blood pH
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