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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?
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a. CO2 bound as carbaminohemoglobin
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b. Dissolved CO2 in the plasma
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c. Dissolved CO2 within erythrocytes
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d. HCO3− transported in the plasma
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e. HCO3− transported within erythrocytes
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The most correct answer is d.
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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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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?
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a. Warming the blood from 20°C to 37°C before measurement
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b. Reducing this sample’s pH by 0.3 units using lactic acid
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c. Increasing the concentration of [2,3-DPG] in these erythrocytes
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d. Equilibrating this blood sample with 16% O2/12% CO2/72% N2
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e. Mixing her sample 1:1 with cord blood from a normal neonate
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The most correct answer is e.
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A decrease in P50 is equivalent to a leftward shift in the ODC and increased HbO2 affinity. Of the choices offered, only mixing bloods containing normal Hb-A with normal Hb-F will achieve this, while all remaining choices would decrease HbO2 affinity, whether by temperature (answer a), acid Bohr effect (answer b), augmented [2,3-DPG] (answer c), or CO2 Bohr effect (answer d).
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A patient is being admitted to the ICU of a hospital in Los Angeles. While a 3.0 mL arterial blood specimen is being drawn into a glass syringe, 0.8 mL air is accidentally introduced and not discovered until 30 minutes later as the sample is prepared for analysis. What effect will this bubble have on the blood gas results being reported?
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a. None if the specimen has been stored on ice until analyzed
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b. Increased PaCO2, decreased PaO2 with decreased pH
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c. Decreased PaCO2, increased PaO2 with increased pH
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d. Decreased PaCO2, increased PaO2 with no change in pH
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e. Increased PaCO2, increased PaO2 with decreased pH
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The most correct answer is c.
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Arterial blood drawn for gas analysis must be kept free of contaminating room air that contains high levels of O2 and almost no CO2. Failure to exclude such a large air bubble allows both gases to diffuse down their partial pressure gradients to raise the sample’s PaO2 and reduce its PaCO2, causing a corresponding alkalosis (↑ pH). While good lab practice is always to store such samples on ice until analyzed, even that precaution (against continued aerobic metabolism by any leukocytes and platelets, and anaerobic acidification by the RBCs) will not prevent these artifactual changes in PaO2 and PaCO2 caused by air bubbles.