What happens during inhalation?
Intercostal muscles contract, lifting the ribcage up and out. At the same time, the diaphragm contracts and pulls down. This increases the size of the chest cavity, causing the lungs to expand. As a result of this expansion, the air pressure within the lungs is lower than the air pressure in the external environment. Air enters the lungs via the trachea, moving from a region of high pressure to a region of low pressure.