Hi there!

Breathing comprises of Inhalation and Exhalation.

The various changes taking place during these two events are as follows:

During Inhalation:

The diaphragm muscle contracts and becomes flattened.

The intercostal muscles (muscle present between the ribs) contract pushing the ribcage forward and upwards.

Overall volume of the chest cavity increases and the pressure inside the lungs decreases.

As a result, the air rushes into the lungs and gets filled inside. 

During Exhalation:

The diaphragm muscle relaxes and becomes dome-shaped.

The intercostal muscles relax pushing the ribcage backward and downwards.

Overall volume of the chest cavity decreases and the pressure inside the lungs increases.

As a result, the air rushes out of the lungs and is expelled out. 

Thanks