Phases of breathing

The act of breathing consists of two phases, inspiration and expiration
Inspiration (Inhalation)- Diaphragm and intercostal muscles contract. The diaphragm moves downwards. The intercostals muscles make the rib cage move upwards. These two processes increase the volume of the thoracic cavity and also reduces the air pressure to below atmospheric pressure allowing air to rush into the airways then into the alveoli. (Waugh & Grant 2004).
Expiration (Exhalation)-is the opposite of inspiration as in the diaphragm and intercostal muscles relax, this allows the diaphragm to move upwards and the intercostal muscles let the rib cage relax to its resting state. The volume within the thoracic cavity now decreases. This decrease in volume now causes an increase in pressure above atmospheric pressure which forces air out up the airway (Waugh & Grant 2004).