Why is inhalation discribed as an active process?

Inhalation is described as an active process because it requires the contraction of the diaphragm and intercostal muscles, which create a negative pressure in the thoracic cavity. This negative pressure allows air to flow into the lungs. Unlike passive processes, such as exhalation at rest, inhalation actively involves muscle work to expand the chest and draw air in. This effort is essential for adequate ventilation and gas exchange.