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. -If a 0.2-mL blood sample with a PaCO₂ of 40 mm Hg diffuses across a membrane into the E2 electrode containing 0.3 mL of solution, the final PaCO₂ in the blood sample will be:

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11ecba2d_1192_f1c7_a3bf_2da69832c560_MD0008_00 When a membrane permeable to an uncharged species is used to separate two solutions, the species diffuses down its concentration gradient until its concentration is equal on both sides. For instance, if 0.2 mL of solution with a PaCO₂ of 40 mm Hg diffuses into 0.3 mL of solution, diffusion continues until the CO₂ is dissolved at the same concentration in both volumes on each side of the electrode membrane. Essentially, this diffusion scenario can be viewed as a dilution of the CO₂. Initially, all of the CO₂ is confined to the 0.2 mL volume of the blood sample. After diffusion, the CO₂ is distributed within a total volume of 0.5 mL (0.2 mL sample + 0.3 mL electrode solution). As such, the final concentration after diffusion can be calculated as follows: 11ecba2d_1192_f1c8_a3bf_173d9fc30c53_MD0008_00 11ecba2d_1192_f1c9_a3bf_b30ba4473210_MD0008_00 11ecba2d_1192_f1ca_a3bf_a7b08d58543d_MD0008_00 In this example, CO₂ diffuses from blood into a reaction solution, where some of the CO₂ is removed as it is converted to HCO₃⁻. As CO₂ is removed from the reaction solution, it will continue to diffuse from the blood into the reaction solution. Therefore, the final CO₂ in the blood sample will be less than 16 mm Hg. (Choice A) The CO₂ in the blood will equilibrate with the HCO₃⁻ solution. It is not completely consumed, so its concentration will not reach zero. (Choices C and D) If CO₂ did not react with the second solution, its equilibrium concentration would be exactly 16 mm Hg. Because CO₂ is removed during the reaction, the final concentration will be less than 16 mm Hg. Educational objective: An uncharged species will diffuse across a semipermeable membrane until its concentration is equal on both sides. If the species is removed from one side of the membrane, diffusion will continue in that direction until equilibrium is regained.

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

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