The pH of blood is calculated using the Henderson-Hasselbalch equation, which incorporates the bicarbonate ([HCO3-]) concentration and the partial pressure of carbon dioxide (PaCO2).
Understanding the Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation is a fundamental tool used in biochemistry and physiology to determine the pH of a buffer solution, such as blood. The equation provides a relationship between the pH, the pKa of the buffer, and the ratio of the concentrations of the deprotonated and protonated forms of the buffer. In the context of blood, the bicarbonate buffer system is the primary buffer, and the equation is expressed as:
pH = pKa + log ([HCO3-] / CO2)Where:
- pH represents the hydrogen ion concentration in the blood, indicating its acidity or alkalinity.
- pKa is the negative logarithm of the acid dissociation constant for carbonic acid, a constant that is approximately 6.1 for the bicarbonate system.
- [HCO3-] is the concentration of bicarbonate ions in the blood plasma.
- CO2 represents the partial pressure of carbon dioxide (PaCO2) dissolved in the blood. While CO2 is a gas, its concentration in blood is proportional to PaCO2 and can be used in the Henderson-Hasselbalch equation in a calculation using a solubility coefficient.
Practical Measurement
The good news is that PaCO2 and [HCO3-] are conveniently measured by a blood gas analyzer. This means we can avoid using manual calculations, simplifying blood pH determination in medical environments.
How Blood Gas Analyzers Work
- Sample Collection: A small blood sample is taken, usually from an artery, and placed in the analyzer.
- Electrode Measurement: The analyzer uses specialized electrodes to measure the partial pressure of carbon dioxide (PaCO2), as well as the concentrations of hydrogen ion and bicarbonate.
- Calculations: The measured values are input into the Henderson-Hasselbalch equation, which the machine automatically computes to give a pH reading.
- Output: The analyzer displays the results, including the pH of the blood, as well as PaCO2 and HCO3- values, plus additional calculated parameters.
Example and Insights
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Scenario: Suppose a blood sample is analyzed, and the blood gas analyzer reports a PaCO2 of 40 mmHg and a [HCO3-] concentration of 24 mEq/L. The pKa for the bicarbonate buffer system in blood is 6.1.
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Calculation: Using the Henderson-Hasselbalch equation, we would have:
pH = 6.1 + log (24/40)
pH = 6.1 + log(0.6)
pH ≈ 6.1 + (-0.22)
pH ≈ 7.37
(Note that the CO2 value must first be converted into mmol/L using a solubility coefficient.)
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Insight: A pH of 7.37 falls within the normal physiological range for blood pH, which is generally 7.35-7.45. Deviations from this range indicate conditions like acidosis (lower pH) or alkalosis (higher pH), which require clinical intervention.
Key Points:
- Simplified Calculation: Modern blood gas analyzers make the calculation straightforward by directly measuring the required parameters.
- Clinical Importance: Blood pH is a critical indicator of metabolic and respiratory conditions, requiring regular monitoring in critically ill patients.
- Henderson-Hasselbalch Significance: This equation highlights the importance of the bicarbonate buffer system in maintaining the acid-base balance in the blood, which helps keep the blood in its optimal pH range.
- Reference: A large number of them have reported data obtained using the Henderson-Hasselbach equation, where pH = pKa + log [HCO3-]/CO2 (Kingston and Bayly, 1998; Lindinger and Waller, 2008). The independent variables (PaCO2 and [HCO3-]) are easily measured by using a blood gas analyzer.
