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

The respiratory system primarily facilitates gas exchange, allowing oxygen to enter the bloodstream and carbon dioxide to be expelled. This process is crucial for maintaining cellular respiration and overall metabolic function.

Additionally, the respiratory system plays a role in the regulation of blood pH through the management of carbon dioxide levels. When CO2 is dissolved in blood, it forms carbonic acid, which can affect the acidity of the blood. By adjusting the rate and depth of breathing, the respiratory system can help maintain acid-base balance.

Protection from pathogens is another important function of the respiratory system. Structures such as mucous membranes and cilia in the respiratory tract trap and help remove particles, including pathogens, preventing them from entering the lungs and causing infections.

In contrast, the production of hormones is not a primary function of the respiratory system. Hormones are typically produced by endocrine glands, and while the respiratory system interacts with various hormones (such as those regulating metabolism and respiration), it does not synthesize hormones itself. Thus, the correct choice highlights an aspect that is outside the core functions of the respiratory system.