The main function of hemoglobin in red blood cells is to transport oxygen from the lungs to various tissues throughout the body and to facilitate the return of carbon dioxide back to the lungs for exhalation. Hemoglobin, a protein composed of iron-containing heme groups, binds oxygen molecules during respiration. This vital role in gas exchange ensures that cells receive the oxygen needed for energy production while helping to maintain proper acid-base balance in the bloodstream.