Explain breathing out?

We breathe in and out by changing the volume of our chest cavity. To breathe in, we must enlarge the chest cavity to create suction which draws air into the lungs. To enlarge the chest cavity, we contract muscles between our ribs, called intercostals, and our diaphragms. The diaphragm is a large flat muscle that forms the floor of the chest cavity. Contracting the diaphragm results in the floor of the chest cavity moving downward. Contracting the intercostals results in the ribs moving up and outward. Both these actions increase the volume of the chest cavity.

To breathe out, we simply relax the intercostals and the diaphragm. This causes the volume of the chest cavity to decrease, which pushes the air out of the lungs. When a healthy person is at rest, breathing out requires no work. However, sometimes we need to exhale quickly (such as when we are exercising) or forcefully (such as when we are sick). In this case, the abdominal muscles can be contracted which push the internal organs up against the diaphragm. There are also internal intercostal muscles which can actively pull the ribs down. These two actions cause breathing out to occur with more force and in less time than with simple muscle relaxation.