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The 56-day interval between whole blood donations is not just to build up our red blood cell (RBC) and hemoglobin levels again but also to give the body enough time to collect the amount of iron it lost in the donation. Our bodies can make their own hemoglobin and RBCs in the bone marrow and liver and do so constantly, but it can’t make iron.
We get iron from the foods we eat or in supplements. And it’s the iron at the center of the hemoglobin molecules that binds to oxygen (and carbon dioxide). That’s how RBCs transport these gases throughout the body.