What is the primary function of the respiratory system?

The primary function of the respiratory system is enabling breathing and gas exchange. This process is vital for the body's survival, as it allows oxygen from the atmosphere to enter the lungs, where it then diffuses into the bloodstream. Once in the bloodstream, oxygen is transported to cells throughout the body, facilitating cellular respiration, which is the process by which cells generate energy.

Simultaneously, the respiratory system plays a crucial role in expelling carbon dioxide, a waste product of metabolism, from the blood back into the atmosphere. This exchange of gases—oxygen in and carbon dioxide out—is essential for maintaining the body’s pH balance and overall homeostasis.

The other options describe functions related to different systems; for instance, providing nutrients to cells pertains to the digestive system, distributing hormones is a function of the endocrine system, and defending against pathogens is primarily the role of the immune system. Thus, enabling breathing and gas exchange is distinctly the primary function of the respiratory system.