Is inspiration considered a passive or active process?

Inspiration is an active process primarily because it involves the contraction of specific muscles, such as the diaphragm and intercostal muscles, to increase the volume of the thoracic cavity. When these muscles contract, the diaphragm moves downward and the rib cage expands, creating a negative pressure in the chest cavity relative to the outside atmosphere. This negative pressure allows air to flow into the lungs. The active muscular effort required for this process distinguishes it from expiration, which can be primarily passive during quiet breathing. In this context, understanding that inspiration relies on muscle activity is key to recognizing it truly is an active event in the respiratory cycle.