Red blood cells (RBCs) are primarily responsible for carrying oxygen in the bloodstream. These cells contain the protein hemoglobin, which binds to oxygen molecules in the lungs and transports them to tissues throughout the body. The structure and flexibility of red blood cells enable them to navigate through even the smallest capillaries, ensuring efficient oxygen delivery. Their lifespan is approximately 120 days, after which they are recycled by the spleen and liver, helping maintain healthy hemoglobin levels in circulation.