The process by which oxygen-rich air enters the lungs is called inspiration or inhalation. During this the diaphragm and intercostal muscles contract. The peritoneum rises upwards and the diaphragm descends. As a result, the volume of the thoracic cavity increases. The lungs expand. Air pressure inside the lungs is low. But the air pressure in the outside environment is normal. As a result, oxygen-rich air from the outside environment enters the lungs.