Which of the following statements best describes the respiratory system?

The statement that best describes the respiratory system is that it takes in oxygen and delivers it to the blood. The primary function of the respiratory system is to facilitate gas exchange, specifically to bring oxygen into the body and transport carbon dioxide out of it. When air is inhaled, oxygen passes through the respiratory passages into the lungs, where it diffuses across the alveolar membranes into the bloodstream. This is essential for cellular respiration, as oxygen is needed for cells to produce energy.

The other options relate to different body systems. The circulatory system, not the respiratory system, is responsible for carrying oxygen and nutrients to the cells and removing waste products. The nervous system controls voluntary and involuntary activities, and the endocrine system is responsible for hormone secretion that assists the body with its various functions. Each of these systems plays a critical role, but they do not define the primary function of the respiratory system as effectively as the correct choice.