Hemoglobin is the substance in the body that carries oxygen to cells. This protein, found in red blood cells, binds to oxygen molecules in the lungs and transports them through the bloodstream to tissues and organs throughout the body. Hemoglobin also plays a role in returning carbon dioxide, a waste product of metabolism, back to the lungs for exhalation. The ability of hemoglobin to change its structure allows it to pick up oxygen efficiently and release it where needed, making it vital for sustaining life and maintaining cellular functions.