Breathing

Breathing in and out is a simple application of Boyle's Law. In the diagram below, you can see that the lungs are enclosed in the chest cavity and that the bottom of the chest cavity is a large muscle named the diaphragm (pronounced di-a-fram). In the diagram, the diaphragm is in its relaxed position.

When you contract your diaphragm, it moves down, expanding the volume of the chest cavity. As Boyle's Law tells us, when the volume increases, the pressure decreases. So the air in your lungs is pushing out with less force than the air outside is pushing in, and air is pushed into your lungs.

When you relax your diaphragm, the muscle moves up, decreasing the volume of your chest cavity. As a result, the pressure increases and the air in your lungs pushes out harder than the atmosphere is pushing in, so air is pushed out.