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Chapter 35. Gas Transport & pH

Arterial and venous blood was drawn from an individual. When comparing the two draws using a sensitive pH meter, it was noticed that the pH of the arterial blood was 7.40 while the pH of the venous blood was 7.36. How do CO2 differences in arterial versus venous blood account for these normal readings?

A. CO2 released from metabolism is acidic.

B. Increased carbamino compounds are acidic and alter the pH.

C. CO2 bound to hemoglobin releases H+ and increase acidity.

D. Cl shift in RBCs increases acidity in the plasma.

E. Increased CO2 in venous blood results in increased HCO3 and H+, which decreases pH.

The correct answer is E. CO2 is higher in venous blood (extra 3.7 mL/dL). The action of carbonic anhydrase quickly converts CO2 and H2O into H2CO3, which can dissociate into HCO3 and H+. The net increase in CO2 naturally results in more acidic blood. The process is reversed when the blood passes through the lung and the additional CO2 is moved from the blood to the alveoli. While it is true that CO2 is released from metabolism that occurs in the tissue (A) and that additional carbamino groups (B) including with hemoglobin (C), none of these are acidic (eg, H+ generating) and thus can be ruled out. The Cl shift (D) refers to the electroneutral exchange of HCO3 with Cl across the RBC membrane; this does not alter H+ concentration and can be ruled out.

In a laboratory experiment, a researcher was able to take whole blood, separate out various components, and compare the ability to hold onto O2. Each of the modified blood samples was exposed to O2 as a gas with a Po2 of 100 mm Hg and allowed to reach equilibrium. Which of the following samples showed the poorest ability to hold onto O2?

A. Whole blood with a slightly raised pH

B. Whole blood devoid of hemoglobin

C. Whole blood with raised CO2 content

D. Whole blood at a raised temperature

E. Whole blood devoid of 2,3-DPG

The correct answer is B. Upon equilibrium with a Po2 of 100 ...

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