What is the primary function of red blood cells?

The primary function of red blood cells, or erythrocytes, is to transport oxygen from the lungs to the tissues throughout the body. This is achieved through the protein hemoglobin, which binds to oxygen molecules in the lungs and releases them in areas where tissues require oxygen for metabolism. This role is crucial for maintaining cellular respiration, which is the process by which cells generate energy.

While other options relate to important functions in the body, they do not align with the main role of red blood cells. The defense against infection is primarily the responsibility of white blood cells. Coagulation, or blood clotting, is facilitated by platelets and various plasma proteins, not red blood cells. Finally, waste removal is mainly carried out by the kidneys and other organs, with some assistance from plasma components, rather than being a direct function of red blood cells. Thus, option B accurately describes the essential role of red blood cells in the circulatory system.