When inspiratory muscles, such as the diaphragm relax, the thoracic cavity and lungs decrease in volume.
Blue indicates that the diaphragm is relaxed. With diaphragm relaxation, the lung volume decreases. This process is expiration.
The decrease in volume is due to the elasticity of the lungs, which will be discussed later.
|lung volume (within one quiet respiratory cycle)||max||max→min||min|