Describe the process of expiration
The muscles involved in respiration relax during expiration - the lungs pull the chest wall and diaphragm back. This passive process is aided by the intercostal muscles and muscles of the chest wall. Movement of the abdominal contents against the diaphragm during expiration is important in quadrupeds, especially when running. The net effect of expiration is that the chest wall is returned down onto the lungs and air is expelled.