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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.

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Table 3.1. O2 and CO2 contents for paired arterial and mixed venous blood estimated at 37°C with Hct = 42% and [Hb] = 14 g/dL
Parameter Arterial Mixed Venous A – V Difference
Whole blood, 1.0 L
PO2, mm Hg (% Oxy-Hb) 100 (100%) 40 (75%) − 60 (25%)
PCO2, mm Hg 39 45 + 6
Total O2, mM 8.82 6.62 − 2.20
Total CO2, mM 22.40 24.23 + 1.83
Erythrocytes, 0.420 L
Total CO2, mM 6.54 7.30 + 0.76
Bicarbonate, mM 5.39 5.88 + 0.49
Dissolved CO2, mM 0.41 0.47 + 0.06
Carbamino CO2, mM 0.74 0.95 + 0.21
Plasma, 0.580 L
Total CO2, mM 15.86 16.93 + 1.07
Bicarbonate, mM 15.16 16.13 + 0.97
Dissolved CO2, mM 0.70 0.80 + 0.10

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 ...

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