Breathing is only one of many processes involved in respiration. It begins when the diaphragm, the dome-shaped muscle below the chest cavity, contracts and moves downward. The rib cage then moves up and and outward. The chest cavity enlarges. At this time, air pressure outside is greater than inside the body causing the air to rush in. This part of breathing process is called inhalation or inspiration.
When the air pressure has been equalized, it causes the diaphragm to relax and move upward or return to its normal position. The chest cavity is therefore reduced in size, making the air pressure inside the chest cavity greater than the air outside the body. Air is then pushed out of the lungs, equalizing the pressure. This part of the breathing process is called exhalation or expiration.