Acid–Base Balance Links (CO2/Bicarbonate) - Anatomy and Physiology

1. What is the chloride shift in systemic capillaries?

   Bicarbonate formed inside red blood cells moves out into plasma in exchange for chloride moving in, keeping the cell electrically neutral.

2. What ratio of bicarbonate to dissolved CO2 corresponds to a normal arterial pH of 7.4?

   About 20 to 1. Any change that keeps this ratio near 20:1 keeps pH near 7.4.

3. Which enzyme in red blood cells catalyzes the reaction of CO2 with water to form carbonic acid?

   Carbonic anhydrase. The carbonic acid it forms then dissociates into H+ and bicarbonate.

4. A person hypoventilates. In which direction do arterial PCO2 and pH change?

   PCO2 rises and pH falls. Retained CO2 pushes the carbonic acid equilibrium toward more H+.

5. In the pulmonary capillaries, in which direction does the carbonic anhydrase reaction run?

   Toward CO2 and water. Bicarbonate re-enters red cells, combines with H+, and the CO2 formed diffuses into the alveoli to be exhaled.

6. What is the Henderson-Hasselbalch equation for the bicarbonate buffer in blood (PCO2 in mmHg)?

   $\text{pH} = 6.1 + \log\frac{[\text{HCO}_3^-]}{0.03 \times P_{\text{CO}_2}}$. pH depends on the ratio of bicarbonate to dissolved CO2.

7. Why is the bicarbonate buffer so effective even though its pKa (6.1) is far from 7.4?

   It is an open system: the lungs adjust CO2 and the kidneys adjust bicarbonate independently, so both sides of the ratio can be regulated.

8. An anxious person hyperventilates. In which direction do arterial PCO2 and pH change?

   PCO2 falls and pH rises. Exhaling extra CO2 pulls the equilibrium away from H+ production.

9. In which form is most CO2 carried in the blood?

   As bicarbonate ions dissolved in plasma (about 70 percent). Smaller fractions travel bound to hemoglobin or dissolved as CO2.

10. What is the reversible reaction that links CO2 to bicarbonate in the blood?

    $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$. Adding CO2 drives it to the right and produces more H+.

11. Which organ eliminates the body's volatile acid (CO2), and which excretes fixed (nonvolatile) acids?

    The lungs eliminate CO2; the kidneys excrete fixed acids such as sulfuric and phosphoric acid.

12. What is the normal range of arterial PCO2?

    35 to 45 mmHg. Values above this indicate CO2 retention; values below indicate excess CO2 loss.

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