Figure 9.02. Comparison of the oxygen dissociation curves of normal blood, blood containing 20%, 40% and 60% carboxyhemoglobin (COHb), and blood from a severely anemic patient.
Note the altered shape of the curves for blood containing COHb, more hyperbolic than sigmoidal. The two lower curves compare an anemic patient with only 8 vols % O2 due to his hemoglobin content being just 40% of normal, and a patient with 8 vols % O2 due to 60% of his Hb being bound to CO. The anemic patient can unload about 70% of his O2 from the blood when PO2 falls to 20 mmHg, while the CO-poisoned patient can unload only about 25% because of the left shift of HbO2 curve. Thus, CO not only decreases oxygen carrying capacity, but also slows oxygen unloading to the tissues.
For this reason CO is a deadly poison. Actually people do quite well with anemias as severe as shown above, but people rapidly die at COHb levels at and below 60%.
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