Lungs are ventilated by pressure change in the thoracic cavity. Air moves from region of high pressure to region of low pressure. Volume is inversely proportional to pressure; when volume increases, pressure decreases and when volume decreases, pressure increases.
During inhalation
During inhalation, outer intercostal muscles contract and inner intercostal muscles relax, rib cage moves upwards and outwards, diaphragm contracts and flattens. The volume of thoracic cavity increases, air pressure decreases, thus air is drawn into the lungs.
During exhalation
During exhalation, outer intercostal muscles relax and inner intercostal muscles contract, rib cage moves downwards and inwards, diaphragm relaxes and curves upwards. The volume of thoracic cavity decreases, air pressure increases, thus air is forced out from the lungs.