Explain respiration

The respiratory movement involves two phases - Inspiration and Expiration. Inspiration :
Is the breadth-1g in of fresh air. It is an active mechanical process, brought about by peripheral diaphragm and a group of intercoastal muscles which are collectively known as Inspiratory muscles. During inspiration the external intercoastal muscle (muscle between ribs) and the muscles of diaphragm contract. Contraction of the external intercoastal muscles raises up the ribs and sternum. Which in turn result in the expansion of thoracic cavity from behind, forward and from side to side. As a result ofmuscular contraction, the dome shaped diaphragm becomes straight. This increases the thoracic cavity from above down wards. The expansion of the thoracic cavity in all the three dimensions leads to the dilation of lungs. As a result, volume increases and pressure decreases inside the lungs. This causes the passing in of fresh air from outside.
Expiration :
Is the breathing out or exhaling of used air. It is a passive process. It involves the returning of diaphragm, ribs and sternum to their original position by relaxation of muscles. As a result, the thoracic cavity reduces in dimension and the thoracic walls press against the lungs. Consequently the lungs get compressed and the used air is forced out from them.