Which mineral is central to the oxygen-carrying function of blood proteins?

• A. Calcium
• B. Sodium
• C. Potassium
• D. Iron

Answer: (D)

Oxygen transport hinges on iron being part of the heme groups in hemoglobin (and in myoglobin). Each hemoglobin molecule has four heme units, and the iron atom in each heme binds one molecule of oxygen reversibly. In the lungs, iron binds O2 to form oxyhemoglobin; in tissues, the oxygen is released as iron returns to its usable state. This reversible binding allows red blood cells to pick up oxygen where it’s plentiful and deliver it where it’s needed. If iron is deficient, hemoglobin synthesis and function are impaired, reducing the blood’s oxygen-carrying capacity and leading to symptoms like fatigue.

The other minerals don’t directly participate in binding oxygen. Calcium is key for signaling, muscle contraction, and bone health; sodium and potassium regulate fluid balance and electrical activity in cells.