A key molecule, containing iron, that our Red Blood Cells (RBCs) are made of. The oxygen we breathe in latches onto hemoglobin, so RBCs can transport it to all the cells of the body. If we don’t have enough hemoglobin, we can’t use the oxygen our lungs breathe in. We can measure the amount of hemoglobin in our RBCs; see Anemia for more explanation.