The respiratory system is made up of the organs in our body that help us to breathe. We breathe to deliver oxygen to the body and to take away carbon dioxide. Breathing starts with a dome-shaped muscle at the bottom of the lungs called the diaphragm. When one breathes in, the diaphragm contracts, and breathes out, the diaphragm expands reducing the amount of space for the lungs and forcing air out. The diaphragm is the main muscle used in breathing.