What is the primary function of red blood cells in the body?

The primary function of red blood cells is to transport oxygen from the lungs to the tissues of the body and to carry carbon dioxide from the tissues back to the lungs for exhalation. This critical role is carried out by hemoglobin, a protein within red blood cells that binds to oxygen molecules. As the blood circulates through the body, red blood cells ensure that all cells receive the oxygen they need for metabolic processes, while also removing carbon dioxide, which is a waste product of cellular respiration.

In the context of the other functions listed, transporting nutrients to the cells is primarily the job of plasma and other blood components, while fighting infection is mainly the responsibility of white blood cells. Regulation of body temperature involves various mechanisms, including blood flow regulation, but is not a direct function of red blood cells. Hence, the role of red blood cells is uniquely focused on the transport of these gases, making this option the most accurate answer.