Final answer:
Normally, 1.7 to 2.4% of total hemoglobin is present as methemoglobin, a form of hemoglobin with iron in the ferric state which cannot bind oxygen effectively.\
Step-by-step explanation:
The question pertains to the percentage of hemoglobin (Hgb) that exists as methemoglobin (MetHgb) in the bloodstream. Methemoglobin is a form of hemoglobin in which the iron within the heme group is in the ferric (Fe³+) state, as opposed to the normal ferrous (Fe²+) state. This ferric iron is incapable of binding oxygen, which disrupts the oxygen-carrying function of hemoglobin. Normally, 1.7 to 2.4% of total hemoglobin will be in the form of methemoglobin. This percentage can increase due to environmental factors, drug consumption, or genetic disorders, leading to symptoms like cyanosis and dyspnoea.