What is the primary function of red blood cells?

Red blood cells, also known as erythrocytes, have a primary function that revolves around the transport of oxygen. They contain a protein called hemoglobin, which binds to oxygen in the lungs and carries it to the tissues throughout the body. This process is vital for cellular respiration, as oxygen is essential for the production of energy in cells. Additionally, red blood cells also assist in transporting a small amount of carbon dioxide, a waste product, from the tissues back to the lungs for exhalation.

The other options, while relevant to overall bodily functions, do not align with the primary role of red blood cells. The immune system primarily involves white blood cells in fighting off infections, body temperature regulation is managed through a variety of mechanisms within the body including the hypothalamus, and hormones are produced by various glands such as the endocrine system, not red blood cells themselves. Thus, the clear and vital role of red blood cells in oxygen transport defines their function within the circulatory system.