The main function of red blood cells (RBCs), or erythrocytes, is to transport oxygen from the lungs to tissues and organs throughout the body. They achieve this crucial task through hemoglobin, a protein that binds to oxygen molecules and carries them in the bloodstream. Red blood cells also play a role in removing carbon dioxide, a waste product produced by cellular metabolism, by transporting it back to the lungs for exhalation. The normal lifespan of red blood cells is about 120 days, after which they are recycled by the spleen and liver. Their efficiency is fundamental to maintaining overall health and energy levels.