Exhalation (or expiration) is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing.
This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume. As the thoracic diaphragm contracts during inhaling, its relaxation causes it and the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air.
Exhaled air is rich in carbon dioxide, a waste product of cellular respiration during the production of energy.
During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.