Red blood cells (RBCs), also known as erythrocytes, play a critical role in the human body by transporting oxygen from the lungs to tissues and carrying carbon dioxide from the tissues back to the lungs for exhalation. RBCs contain hemoglobin, a protein that binds oxygen molecules, allowing efficient gas exchange. The life cycle of red blood cells is about 120 days, after which they are recycled by the spleen and liver. This function is vital for maintaining proper metabolism, energy levels, and overall health, as oxygen is essential for cellular respiration and energy production in the body.