Breathing starts with a dome-shaped muscles at the bottom of the lungs called the diaphragm .. WHEN YOU BREATH IN the diapgram Contract's . when it contracts it flatters out and pulls downward . thin movement enlarge the space that the lungs are in. this larger space fulls air into the lungs when you breath out .. the diapgram expand readucing the amount of space for the lungs and forcing air out ..