What primarily happens during methemoglobinemia?

In methemoglobinemia, the primary issue is the conversion of normal hemoglobin (which contains ferrous iron) to methemoglobin, where the iron is in the ferric state. This change impairs the hemoglobin's ability to bind and transport oxygen effectively. As a result, the presence of methemoglobin reduces the oxygen-carrying capacity of the blood, leading to decreased oxygen delivery to the tissues. Although patients with methemoglobinemia may show a normal or increased hemoglobin concentration, the function of that hemoglobin is compromised, which is why the ability to bind oxygen is decreased. This condition can lead to symptoms of oxygen deprivation, even if other parameters might appear normal in a blood test, emphasizing the importance of understanding the functional aspects of hemoglobin rather than just its concentration.