How do the muscular and respiratory systems work together?

See below: Both systems help us breathe. Muscles are the diaphragm and intercostal muscles - for normal breathing. As the diaphragm drops down and the ribs expand, the lungs also expand, taking in air, and oxygen along with. When the diaphragm rises, and the intercostals relax, the chest wall recoils, causing the lungs to recoil causing air to move out - and CO2 along with it.