Respiration consists of two actions, Inhalation and Exhalation.

Inhalation
The atmospheric air is inhaled inside the Lungs in this action, which is called Purak in Yoga. The volume of Lungs increased due to the following three movements:-
1. The Diaphragm is pushed down..
2. The Ribs are lifted upwards.
3. The Ribs are opened outside as    shutters of the window.

As the volume of Lungs increases, air pressure drops creating a vacuum. The air is inhaled in from outside. As the air is filled in the lungs, the pressure is raised to atmospheric pressure. This completes the Inhalation process.