What is the primary function of the respiratory system?

The primary function of the respiratory system is breathing, which encompasses the processes of inhaling oxygen and exhaling carbon dioxide. During inhalation, oxygen from the air enters the lungs, where it diffuses into the bloodstream and is transported to cells throughout the body for cellular respiration. This process is vital because oxygen is necessary for the production of energy in cells, and removing carbon dioxide, a waste product of metabolism, is equally important for maintaining the body's pH balance and homeostasis.

In contrast, the other choices represent functions of different systems in the body. Nutrient absorption occurs in the digestive system, blood circulation is the responsibility of the cardiovascular system, and food digestion takes place in the digestive tract. Each of these systems plays a critical role in maintaining overall health but does not describe the primary function of the respiratory system.