Blood pH: The Bicarbonate Buffer
Your blood stays between pH 7.35 and 7.45 thanks to the bicarbonate buffer: the lungs set the CO₂, the kidneys the bicarbonate. Find the pH from the bicarbonate and the pCO₂, or the value that goes with a given pH, and see which textbook pattern of acidosis or alkalosis the numbers fit.
How Does the Bicarbonate Buffer Work?
CO₂ from your cells dissolves in the blood and forms carbonic acid, which splits into H⁺ and bicarbonate: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The pH depends on the ratio of bicarbonate to dissolved CO₂, normally about 20 to 1. That is far from the 1 : 1 of an ideal buffer, yet it works very well, because the body can change both sides: the lungs breathe CO₂ out within minutes, and the kidneys keep or remove bicarbonate over hours to days.
A change that starts with the CO₂ is called respiratory; one that starts with the bicarbonate is metabolic. The other organ then tries to restore the ratio: this is compensation. Fast breathing during a metabolic acidosis is an example.
Davenport Diagram
pHWorked Techniques
0.0307 × pCO₂ (in mmHg) is the dissolved CO₂ in mmol/L, the acid side of the buffer.
In a metabolic acidosis the lungs blow off CO₂. If the measured pCO₂ is in this range, the breathing response is as expected; above or below it points to a second, respiratory problem.
Sources & Verification
- J. E. Hall & M. E. Hall, Guyton and Hall Textbook of Medical Physiology, 14th ed., Elsevier, 2021 — Ch. 31 (Acid–base regulation; the bicarbonate buffer, pK 6.1).
- O. Siggaard-Andersen, The Acid–Base Status of the Blood, 4th ed., Munksgaard, 1974 — solubility of CO₂ in plasma at 37 °C, 0.0307 mmol/L per mmHg (0.230 per kPa).
- H. W. Davenport, The ABC of Acid–Base Chemistry, 6th ed., University of Chicago Press, 1974 — the pH–bicarbonate diagram.
- R. W. Albert, A. R. Dell & R. W. Winters, “Quantitative displacement of acid-base equilibrium in metabolic acidosis”, Ann. Intern. Med. 66(2), 1967, pp. 312–322 — Winter’s formula.
Frequently Asked Questions
How do you calculate blood pH from bicarbonate and pCO₂?
Use the Henderson–Hasselbalch equation for the bicarbonate buffer: pH = 6.1 + log([HCO₃⁻] / (0.0307 × pCO₂)), with HCO₃⁻ in mmol/L and pCO₂ in mmHg. Example with normal values, 24 mmol/L and 40 mmHg:
1. Dissolved CO₂ = 0.0307 × 40 = 1.228 mmol/L.
2. Ratio = 24 / 1.228 = 19.54, and log 19.54 = 1.291.
3. pH = 6.1 + 1.291 = 7.39. With pCO₂ in kPa, use 0.230 instead of 0.0307. Many textbooks round 0.0307 to 0.03, which gives 7.40 for the same values.
What is the normal ratio of bicarbonate to CO₂ in blood?
About 20 to 1 (19.5 to 1 for a bicarbonate of 24 mmol/L and a pCO₂ of 40 mmHg), which gives a pH of about 7.4. The ratio, not the amounts themselves, sets the pH: if the bicarbonate and the CO₂ both double, the pH stays the same. That is why the body can correct a change in one by changing the other.
What is the difference between respiratory and metabolic acidosis?
In a respiratory acidosis the problem starts with the CO₂: breathing too little lets CO₂ build up and the pH falls. In a metabolic acidosis it starts with the bicarbonate: acids are added or bicarbonate is lost, so the bicarbonate falls. Alkalosis is the reverse: too little CO₂ (hyperventilation) or too much bicarbonate. The calculator shows which textbook pattern your numbers fit.
What is Winter’s formula?
A rule for the breathing response to a metabolic acidosis: the expected pCO₂ is 1.5 × [HCO₃⁻] + 8 ± 2 mmHg. With a bicarbonate of 12 mmol/L the expected pCO₂ is 24 to 28 mmHg. A measured pCO₂ above that range suggests that breathing is not keeping up (an extra respiratory acidosis); below it, an extra respiratory alkalosis.
Why is the pK 6.1 and not 6.35?
6.35 is the pKa of carbonic acid (dissolved CO₂) in water at 25 °C. In blood plasma, at 37 °C and at the salt concentration of the body, the apparent value is about 6.1. That is the value used in physiology, together with 0.0307 mmol/L per mmHg for how much CO₂ dissolves in plasma at 37 °C.
Can I use this to interpret my own blood gas results?
No. This page is for learning how the bicarbonate buffer works. The patterns are simplified textbook rules, and real blood gas results are interpreted by doctors together with other measurements and the full clinical picture. Ask your doctor about your own results.