We know that the breathing is the process of inhaling and exhaling .the organs involved are mainly the lungs.You cannot see your lungs,it's easy to feel them in action.Put your hands on your chest and breathe in very deeply.You will feel your chest getting slightly bigger.Now breathe out the air,and feel your chest return to its regular size.You have just felt the power of your lungs.The lungs themselves can neither draw in air nor push it out.Instead the chest wall muscles and another flexible muscle called diaphragm helps the lungs in moving air into and out of them.
The chest wall is made up of ribs,muscles,and the skin.The ribs are attached at an angle to the spine.When we inhale,the chest wall moves up and expands.This causes an increase in the volume of the chest cavity.
The diaphragm may be imagined as the floor if you think of the chest cavity as a room.When the diaphragm is relaxed when we breathe out,it is in the shape of the dome with the convex side of the dome extending into the chest cavity.When the diaphragm contracts during inhalation it flattens out a bit or the dome moves downward.As a result, the volume of the chest cavity is increased.
When the diaphragm flattens and the volume of the chest cavity is increased ,its internal pressure decreases and the air from the outside rushes into the lungs.This is inspiration.
Then the reverse occurs.The chest wall is lowered and moves inward and the diaphragm relaxes and assumes its dome shape.These changes increase the pressure on the lungs;their elastic tissue contracts and squeezes the air out through the nose to the external atmosphere.This is expiration.