Chapter 3: Mechanisms of O₂ and CO₂ Transport by Erythrocytes

Chapter 3. Mechanisms of O₂ and CO₂ Transport by Erythrocytes

Paired blood samples from left radial arteries and veins are drawn on healthy subjects who are 23-61 years old. Each individual breathes room air while resting quietly as samples are obtained. Lab results indicate their total CO₂ contents average 21.5 mM in arterial samples and 24.3 mM in venous samples. Which of the following provides the single largest amount of CO₂ excreted from their venous bloods?

a. CO₂ bound as carbaminohemoglobin

b. Dissolved CO₂ in the plasma

c. Dissolved CO₂ within erythrocytes

d. HCO₃⁻ transported in the plasma

e. HCO₃⁻ transported within erythrocytes

The most correct answer is d.

While all listed choices contribute to CO₂ excretion from alveolar capillaries, the largest single fraction is plasma HCO₃⁻ the instant before being exhaled (Table 3.1). Such data support a prominent role for an isoform of carbonic anhydrase expressed by capillary endothelia that can facilitate direct formation of H₂CO₃ from plasma H⁺ and HCO₃^–, yielding a rapid rise in plasma dissolved CO₂ that then diffuses into alveolar airspaces.

The whole-blood P₅₀ for a healthy 25-year-old woman is found to be 27 mm Hg at pH = 7.40 when fully equilibrated with a range of gas mixtures from 0% O₂/5% CO₂/95% N₂ through 16% O₂/5% CO₂/79% N₂. Which of the following manipulations would cause the P₅₀ of this blood sample to decrease?

a. Warming the blood ...