What is the primary function of the respiratory system?

The primary function of the respiratory system is to exchange gases, specifically oxygen and carbon dioxide, between the atmosphere and the bloodstream. This process occurs primarily in the alveoli of the lungs, where oxygen from inhaled air diffuses into the blood while carbon dioxide, a waste product of metabolism, diffuses out of the blood to be exhaled. This gas exchange is crucial for maintaining the body's oxygen levels, which are necessary for cellular respiration, and for removing carbon dioxide, which is critical for maintaining acid-base balance in the body.

The other options refer to functions of different systems: the circulatory system is responsible for the circulation of blood, the digestive system processes food and absorbs nutrients, while the thermoregulatory processes are mostly handled by the integumentary system and others, not specifically focused on respiration. Thus, the correct answer emphasizes the core role of the respiratory system in facilitating vital gas exchanges essential for life.