What is the primary function of red blood cells?

The primary function of red blood cells (RBCs) is to transport oxygen throughout the body. These cells contain hemoglobin, a protein that binds to oxygen in the lungs and carries it to tissues and organs, where it is released to support cellular metabolism. The efficient transport of oxygen is crucial for the survival of all bodily cells, as oxygen is necessary for producing energy through aerobic respiration. Additionally, red blood cells also play a role in transporting carbon dioxide, a waste product of metabolism, back to the lungs for exhalation.

The other options highlight functions that are important, but they are not the primary role of red blood cells. Fighting infections is primarily the role of white blood cells. Transporting nutrients is mainly the function of plasma and other components of blood. Regulating body temperature is a complex process involving various body systems, including the integumentary system, and is not a direct function of red blood cells. Therefore, the correct answer accurately identifies the fundamental role of red blood cells in the circulatory system.