What is the primary function of red blood cells?

The primary function of red blood cells (RBCs), also known as erythrocytes, is to transport oxygen throughout the body. This is achieved through the protein hemoglobin, which binds to oxygen in the lungs and releases it to the tissues and organs that need it. RBCs have a unique biconcave shape that increases their surface area, facilitating efficient gas exchange. This is essential for maintaining the overall metabolic functions of the body's cells, as oxygen is critical for producing energy through cellular respiration.

While white blood cells are responsible for fighting infections, clotting factors and platelets assist in the clotting process, and other systems play roles in regulating body temperature, it is the red blood cells that are specifically designed to transport oxygen, making option B the correct answer. This critical task supports every bodily function by ensuring that all cells receive the oxygen they need to function properly.