The process by which air rich in carbon dioxide is released from the lungs is called exhalation. During this time the diaphragm and intercostal muscles are stretched. The scrotum moves downward and the diaphragm moves upwards. As a result, the volume of the thoracic cavity decreases. The lungs are compressed. Air pressure inside the lungs increases (3-4 mmHg more). Air pressure in the lungs but the air pressure in the outside environment is normal. As a result, air rich in carbon dioxide is released from the lungs.