What is the function of red blood cells in the circulatory system?

Red blood cells have a crucial role in the circulatory system primarily focused on the transportation of oxygen to body tissues. This function is facilitated by a protein called hemoglobin, which is abundant in red blood cells. Hemoglobin binds with oxygen in the lungs and carries it through the bloodstream to various parts of the body, ensuring that tissues receive the necessary oxygen for cellular respiration and energy production.

While red blood cells hypothetically have minimal roles in transporting carbon dioxide back to the lungs, their primary and most important function is to deliver oxygen. Options regarding fighting infections or regulating blood sugar levels pertain to other components of the immune system and metabolic processes, respectively, and do not involve red blood cells at all. Hence, the option that describes the transport of oxygen accurately encapsulates the essential role red blood cells play in maintaining the body's oxygen levels for various physiological functions.