Summary of Breathing cycle
The breathing cycle explains the change in pressures across the lungs and alveoli during inspiration and expiration. During inspiration, the diaphragm contracts and this increases lung volume making the pleural pressure even more negative. This decreases the alveolar pressure which allows atmospheric air to rush in and fill the lung. During normal expiration, the elastic properties of the lung and chest wall cause the air to be expelled out passively. During quiet breathing, the amount of air that is inspired and expired is referred to as the tidal volume and the amount of air remaining after a quiet expiration is the functional reserve capacity.