What is the active component of respiration?

The correct choice identifies inspiration as the active component of respiration. During inspiration, the diaphragm and intercostal muscles contract, expanding the thoracic cavity and reducing the pressure in the lungs. This creates a gradient that allows air to flow into the lungs. The effort involved in the muscular contractions makes inspiration an active process, as it requires energy and the involvement of the central nervous system to initiate and control the breathing action.

In contrast, exhalation is often a passive process, particularly under resting conditions, where the elastic recoil of the lungs and chest wall helps expel air without muscular effort. Circulation is related to the movement of blood and gases but does not specifically pertain to the act of breathing itself. Ventilation refers to the overall process of air moving in and out of the lungs, which includes both inspiration and exhalation, but it does not pinpoint a specific active component. Thus, inspiration is distinctly recognized as the active phase of respiration.