Which of the following is NOT a function of the respiratory system?

The correct answer identifies circulating nutrients in the blood as not a function of the respiratory system. The respiratory system primarily focuses on facilitating gas exchange, where oxygen is inhaled and carbon dioxide is exhaled. This process is vital for maintaining appropriate oxygen levels in the blood for cellular metabolism and removing waste products.

In addition to gas exchange, the respiratory system plays a role in filtering out foreign particles, which helps protect the lungs from pathogens and irritants. It also regulates the pH of the blood through the management of carbon dioxide levels. When carbon dioxide levels increase, it can lead to a decrease in blood pH (making it more acidic), and the respiratory system can respond by increasing the rate and depth of breathing to expel more carbon dioxide.

Circulating nutrients, however, is primarily a function of the circulatory system, which transports nutrients, oxygen, and other essential substances throughout the body via the bloodstream. This is not a role of the respiratory system, which is focused on the process of breathing and gas exchange.