How does iron help red blood cells carry oxygen?

Iron is essential for oxygen transport because it serves as the primary component of hemoglobin, the protein within red blood cells that gives blood its characteristic red color.123

Within each hemoglobin molecule, there are four heme groups, each containing an iron atom attached to a ringlike structure called porphyrin. This iron atom acts as the specific site that reversibly binds oxygen molecules as blood circulates between the lungs and body tissues. Because each hemoglobin molecule contains four iron atoms, it is capable of binding four molecules of oxygen.14

The complex protein structure of hemoglobin provides the necessary environment for these iron atoms to bind and release oxygen appropriately based on physiological conditions. When oxygen pressure is high, such as in the lungs, hemoglobin becomes saturated; when pressure drops in the tissues, the iron releases the oxygen so it can diffuse into the cells where it is needed for metabolism.13