In looking at passive respiration we can find out how the body is programmed to breathe for everyday life. We breathe in as a response to signals from the brain. The trigger for the signal is the need for oxygen. On receiving the signal the diaphragm contracts and pulls down, decreasing the air pressure in the chest so that the lungs fill with air. Other muscles also help by expanding the chest cavity; these are the muscles that raise the ribs and increase the width and depth of the ribcage. The diaphragm, however, is responsible for 60-80% of the work during inspiration.