The primary role of hemoglobin in red blood cells is to transport oxygen from the lungs to tissues and facilitate the return of carbon dioxide from tissues to the lungs for exhalation. Hemoglobin is a protein made up of four subunits, each containing a heme group that binds to oxygen. This ability to carry oxygen is critical for cellular respiration, as cells require oxygen for energy production. Hemoglobin also helps maintain pH balance in the blood by binding to protons, illustrating its multifaceted role in human physiology.