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Step-by-Step Explanation
Step 1: Understanding Carboxyhaemoglobin (COHb)
Carboxyhaemoglobin (COHb) is a compound formed when haemoglobin (the oxygen-carrying protein in red blood cells) combines with carbon monoxide (CO). Compared to its affinity for oxygen ($O_2$), haemoglobin has an affinity approximately 250 times greater for carbon monoxide. This makes COHb formation very efficient and dangerous when carbon monoxide is present.
Step 2: How Carboxyhaemoglobin Forms
When a person inhales air containing carbon monoxide, the gas readily binds with the ferrous ion ($\text{Fe}^{2+}$) in haemoglobin. In this reaction, iron can be oxidized, forming a stable complex with CO, often denoted as COHb. This reduces the amount of haemoglobin available to bind to oxygen.
Step 3: Effects of Carboxyhaemoglobin on the Body
Reduced $O_2$ Transport: Because haemoglobin is occupied by CO, it cannot carry as much oxygen. This deprives tissues and organs of oxygen (hypoxia).
Shift in the Oxygen Dissociation Curve: The haemoglobin that remains bound to oxygen (Hb$O_2$) exhibits an increased affinity for oxygen, which impairs the release of oxygen to tissues. This is referred to as a left shift in the dissociation curve.
Serious Health Consequences: High COHb levels can lead to symptoms like headache, dizziness, unconsciousness, and can be fatal in extreme cases if not treated promptly.
Step 4: Why Carbon Monoxide is the Correct Conclusion
Among the given options, carbon monoxide is the only one capable of forming carboxyhaemoglobin. The unusually high level of COHb in the patient’s blood indicates inhalation of CO-polluted air, making carbon monoxide the most likely culprit.
Answer
Carbon monoxide is the substance leading to high carboxyhaemoglobin levels in the patient’s blood.
