During inhalation, our rib cage moves up and out. The primary organ of our respiration system is lungs. Other components of the respiratory system convey air to the lungs, such as the trachea (the windpipe) which branches into smaller structures called bronchi.
The process of respiration (breathing) is divided into two distinct phases.
The diaphragm contracts and pulls downward during Inspiration while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs. But during Expiration, the diaphragm relaxes, and the volume of the thoracic cavity reduces, while the pressure within it increases. As a result, the lungs contract and air is forced out.