The lifespan of a red blood cell is typically about 120 days. During this time, red blood cells (erythrocytes) carry oxygen from the lungs to tissues and return carbon dioxide to the lungs for exhalation. After approximately 120 days, they become less efficient, and the body removes them from circulation, primarily in the spleen and liver. The production of new red blood cells occurs in the bone marrow, where stem cells differentiate into erythrocytes, ensuring a consistent supply to meet the body's oxygen demands.