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. -Blood gas analysis results are listed below. From the data, determine the HCO₃⁻ concentration in the blood sample resulting from the decomposition of carbonic acid.

Question

Quizplus · Accepted Answer

11ecba2d_1191_1cfe_a3bf_3b3c231da93a_MD0008_00 Buffers consist of a weak acid and its conjugate base. They help maintain H⁺ concentration when the pH of a solution is within the "buffering region," or within 1 pH unit of the buffer's pK_a (the pH at which half of the buffer molecules are protonated and half are deprotonated). The Henderson-Hasselbalch equation relates the pH of the buffered solution to pK_a of the buffer and the ratio of the conjugate base to the acid. For the HCO₃⁻ buffer system that functions at physiologic pH, the acid is carbonic acid (H₂CO₃) and its conjugate base is HCO₃⁻. According to the passage, the pK_a is 6.35. 11ecba2d_1191_1cff_a3bf_fb853a470898_MD0008_00 The question asks for the concentration of HCO₃⁻ given a pH of 7.35 and a PaCO₂ of 40 mm Hg. As described in the passage, the H₂CO₃ concentration in mEq/L can also be calculated from PaCO₂ in mm Hg. 11ecba2d_1191_4410_a3bf_7f7ce783c45a_MD0008_00 The HCO₃⁻ concentration can then be calculated by incorporating the pH and the H₂CO₃ concentration into the Henderson-Hasselbalch equation: 11ecba2d_1191_4411_a3bf_99a184c2054b_MD0008_00 11ecba2d_1191_4412_a3bf_f18666f4192b_MD0008_00 Removal of the logarithm (base 10) is achieved by taking the inverse logarithm of both sides of the equation (ie raising 10 to the power of each side): 11ecba2d_1191_4413_a3bf_2dea93a8eee6_MD0008_00 11ecba2d_1191_4414_a3bf_8b8765f22832_MD0008_00 Solving for [HCO₃⁻] yields: 11ecba2d_1191_6b25_a3bf_b1a5b58f1f52_MD0008_00 (Choice A) This value is the result of using the Henderson-Hasselbalch equation with the positions of H₂CO₃ and HCO₃⁻ inverted. The appropriate ratio is the conjugate base divided by the conjugate acid. (Choice B) This value is the H₂CO₃ concentration. The Henderson-Hasselbalch equation shows that there is ten times as much HCO₃⁻ as H₂CO₃. (Choice C) This value corresponds to a ratio of 2:1 for the concentrations of HCO₃⁻ to H₂CO₃. Because the log of [HCO₃⁻]/[H₂CO₃] is equal to 1, the quotient of [HCO₃⁻]/[H₂CO₃] is equal to 10¹, which corresponds to an HCO₃⁻concentration of 12.0 meq/L. Educational objective: Consisting of a weak acid and its conjugate base, buffers help maintain the pH of a solution near the pK_a. The Henderson-Hasselbalch equation is used to relate pH, pK_a, and the concentration ratio of the acid and conjugate base.

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

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