The main function of red blood cells, or erythrocytes, in the human body is to transport oxygen from the lungs to body tissues and return carbon dioxide from the tissues to the lungs. Hemoglobin, a protein contained within red blood cells, binds to oxygen and enhances its delivery throughout the body. This process is critical for cellular respiration, allowing cells to produce the energy needed for various bodily functions. Maintaining adequate levels of red blood cells is essential for overall health.