Passage The bicarbonate (HCO 3 − ) buffer system (Reaction 1) helps to maintain acid-base homeostasis and a blood pH near 7.4. Therefore, concentrations of carbon dioxide (CO 2 ) and HCO 3 − must be tightly regulated through adaptations in respiratory and renal physiology. For patients with suspected acid-base imbalance, these concentrations can be monitored by blood gas analysis performed on arterial or venous blood. CO2g+H2Ol⇔carbonic anhydraseH2CO3aq⇔pKa=6.35HCO3-aq+H+aqReaction 1In most automated blood gas analyzers, ionized hydrogen, CO 2 , and oxygen (O 2 ) in the sample diffuse through semipermeable membranes and are measured at separate electrodes (Figure 1). The pH-sensitive glass electrode (E1) is separated from the blood sample by a membrane permeable to hydrogen ions. The sample pH is determined according to the voltage difference between E1 and a reference electrode maintained in a solution of standard pH. CO 2 diffuses across a gas-permeable membrane to E2, where it reacts to generate HCO 3 − and free hydrogen ions. In a modification of the mechanism used in E1, partial pressure of CO 2 (PaCO 2 ) is calculated indirectly from the change in pH, as determined by the potential difference between E2 and its reference electrode.E3, also called the Clark electrode, is an electrochemical cell containing a silver anode and a platinum cathode. O 2 diffuses through another gas-permeable membrane and reacts with hydrogen ions to form water. The current measured at E3 is then used to calculate PaO 2 in the blood sample. Figure 1 Automated blood gas analyzerThe concentration of carbonic acid (H 2 CO 3 ), reported in milliequivalents per liter (mEq/L), can be calculated from PaCO 2 by Equation 1: H2CO3mEqL=0.03×PaCO2mm HgEquation 1Subsequently, using the concentration of H 2 CO 3 obtained from Equation 1, the HCO 3 − concentration resulting from decomposition of carbonic acid can then be calculated by applying the Henderson-Hasselbalch equation. -The reference electrode for E2 should be maintained at a known, stable concentration of:

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11ecba2d_118f_e47b_a3bf_cf89d76550c5_MD0008_00 A reference electrode is an electrode that has a known electric potential and standard concentration of reacting species. The passage states that the mechanism used at E2 to measure CO₂ is a modification of the mechanism used at E1 to measure pH: E1 is a concentration cell, meaning both electrodes contain the same chemical species. In this case the species is H⁺, which diffuses from the blood sample into E1. The pH is calculated from the voltage difference between E1 and its reference electrode, which is maintained at a standard pH. pH is directly related to the concentration of H⁺ by the equation pH = −log[H⁺]. CO₂ diffuses from the blood sample into a reaction solution at E2. The reaction produces H⁺, and the change in pH is determined by the voltage difference between E2 and its reference electrode. Therefore, the reference electrode for E2 should also be maintained at a standard concentration of H⁺. (Choices B and C) As stated in the passage, CO₂ reacts to produce H⁺, and it is this change, not the change in CO₂ or HCO₃⁻, that is measured. Therefore, the reference electrode should be maintained at a standard concentration of H⁺. (Choice D) O₂ gas is not involved in the mechanism used at E2. Educational objective: A reference electrode contains a known electric potential and concentration of ions. When both electrodes in a cell contain the same chemical species (a concentration cell), the electric potential difference (voltage) between the electrodes depends only on the relative concentrations of these species.

Passage The Bicarbonate (HCO3−) Buffer System (Reaction 1) Helps to Maintain

Passage The Bicarbonate (HCO₃⁻) Buffer System (Reaction 1) Helps to Maintain