Final answer:
The saturation of oxygen in a patient's blood can differ from the inspired oxygen fraction due to factors like pH changes, CO₂ levels, and specific conditions like methemoglobinemia, affecting hemoglobin's oxygen-binding capacity.
Step-by-step explanation:
The saturation of oxygen (the percentage of hemoglobin sites occupied by oxygen) in a patient's blood may not always match the fraction of inspired oxygen (FO₂Hgb) due to various factors affecting the affinity of hemoglobin to bind oxygen. Factors that can create a disparity include changes in blood pH, carbon dioxide levels, temperature, and the presence of conditions like methemoglobinemia or hemoglobinopathies (e.g., sickle cell anemia, thalassemia).
Methemoglobin is a form of hemoglobin where the iron is in the ferric (Fe³⁺) state instead of the ferrous (Fe²⁺) state. This ferric state cannot bind oxygen, therefore reducing the oxygen-carrying capacity of hemoglobin. The presence of increased levels of methemoglobin or hemoglobinopathies affects the normal oxygen dissociation curve, changing the relationship between the sO₂ and the FO₂Hgb.