Overall process of respiration comprises of two steps: Inspiration and expiration.

Inspiration: During this process we inhale the oxygen-rich air through the nostrils which passes through the trachea and then through bronchioles finally reaches the alveoli. What happens during this process is that diaphragm becomes flattened and rib cage expands. Due to this the size of the thoracic cavity increases. The pressure during this process is low inside and is high outside and as a result we inhale the oxygen-rich air and this process is called inhalation.

Expiration: During this process we exhale the CO₂ rich air outside the body. The diaphragm becomes flattened and the size of the thoracic cavity decreases. Due to this the pressure inside the lungs is high as compared to atmospheric pressure and as a result the air is exhaled outside the lungs.