What is the primary function of red blood cells?

Red blood cells, also known as erythrocytes, have the primary function of transporting oxygen from the lungs to the body's tissues and carbon dioxide from the tissues back to the lungs. This crucial role is facilitated by hemoglobin, a protein within red blood cells that binds to oxygen molecules. Red blood cells are specially adapted for this function; they have a biconcave shape that increases their surface area for oxygen absorption and allows for efficient gas exchange as they travel through capillaries.

In contrast, the other functions mentioned, such as fighting infection, regulating body temperature, and facilitating blood clotting, are primarily the roles of other components of the blood. White blood cells are involved in immune responses against infections, while plasma proteins and platelets are responsible for blood clotting. Body temperature regulation is more about the circulatory system's overall function and is not a primary role of red blood cells. This distinction highlights the specific and essential role of red blood cells in oxygen transport.