What role do erythrocytes play in the circulatory system?

Erythrocytes, more commonly known as red blood cells, play a crucial role in the circulatory system primarily by carrying oxygen from the lungs to the tissues and organs of the body. This function is vital for cellular respiration, which is the process by which cells generate energy using oxygen. The hemoglobin within erythrocytes binds oxygen efficiently in the lungs, where the oxygen concentration is high, and releases it in areas where the tissues have lower oxygen concentrations, thus facilitating oxygen delivery where it is most needed.

While blood clotting and the transport of nutrients are important functions of the circulatory system, these roles are primarily fulfilled by platelets and plasma proteins, or by plasma itself, respectively. Additionally, the regulation of body temperature involves various mechanisms, primarily through the actions of blood vessels and sweat glands, rather than being a direct function of erythrocytes. Therefore, the primary and defining role of erythrocytes is to transport oxygen, making it the correct answer.