Breathing is the process of taking in oxygen-rich air and giving out carbon dioxide-rich air.
Breathing in is 'inhalation' and breathing out is 'exhalation'.
Inhalation involves bringing in air from outside the body into the lungs. When we breathe in, the size of our chest increases. This happens because when air enters, the lungs expand and the ribs move outwards. Simultaneously, the diaphragm contracts and becomes flat. Thus, the size of our chest increases.
Exhalation involves removal of CO 2 from the body. When we breathe out, the size of our chest decreases. This happens because when air moves out of our lungs, the lungs contract,the ribs move back, and the diaphragm curves upwards into the chest decreasing the size of the chest.