[HbCO[HbO2 MPCOPO2

where the equilibrium constant M is 200 to 230 for human Hb. This relationship shows how a relatively small partial pressure of CO can form significant amounts of COHb. In addition to decreasing the Hb binding sites available for oxygen, CO also shifts the oxyhemoglobin dissociation curve to the left, increasing the affinity of Hb for any oxygen molecules bound to it. This effect further decreases oxygen delivery to the tissues.

Clinical experience and experiments in animals indicate that the binding of CO to Hb and the resulting tissue hypoxia do not completely explain all the toxicity seen after CO poisoning (CoburQ 1.979). Indeed, the correlation between COHb level and the severity of poisoning is poor. Thus additional mechanisms have been proposed to explain the toxic effects of CO. CO binding to intracellular proteins containing iron (hemoproteins) plays a role in some cases of CO poisoning. Myoglobin, cytochrome a-a3, and guanylate cyclase are candidates for the cellular effects of CO. CO uptake by binding to intracellular proteins is increased with hypoxia. The role of CO binding to cellular hemoproteins in clinical CO poisoning is unclear, but up to half the total body store of CO after an exposure may be in the tissues. Leukocyte-mediated vascular (endothelial) injury may also occur after CO poisoning ( Thom 1993).

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