When asked how we breathe, many people will vaguely respond that we do something with the diaphragm, and somehow that makes air come into the lungs. In fact it's much simpler than that.
Air is a gas, and gas fills spaces that are empty.
That simple exchange of air is what we call breathing. Because of this, not all the ribs move in the same way, or to the same degree. Figure 1 a Figure1b Figure 1. When the ribs return to normal position, the diaphragm relaxes and ascends, air is forced out, and we exhale. None of these movements, as we well know, is voluntary.