Distensibility of the lungs. High lung compliance means the lungs will expand easily. Compliance is decreased in pulmonary edema, insufficient surfactant, and scar tissue.
Elastic recoil
Lung's intrinsic tendency to deflate following inflation (inspiration), due to pulmonary elastic fibres and surface tension
Pulmonary surfactant
Lipoprotein complex (70% lipid, mainly dipalmitoylphosphatidylcholine; 30% protein) produced by Type II alveolar epithelial cells, lowers surface tension so breathing can occur without too much effort, increases lung compliance and reduces the tendency of the lungs to recoil inward, makes the alveoli stable against collapse
As alveoli radii change
Surface tension varies. At end of expiration, alveoli radii reduced, surfactant more compressed and more active, reducing surface tension. At end of inspiration, alveoli expanded, surfactant more diluted and less active, increasing surface tension.
Panting
A controlled increase in respiratory frequency accompanied by a decrease in tidal volume, to increase ventilation of the upper respiratory tract, preserve alveolar ventilation and increase evaporative heat loss
Inspiration
Active process, air moves into the lungs due to a decrease in air pressure within the lungs
Normal expiration
Passive process, air moves out of the lungs due to an increase in air pressure within the lungs
Forceful exhalation
The abdominal and internal intercostals muscles contract
Intrapulmonary/alveolar pressure
The pressure of the air within the alveoli, changes during breathing, eventually equalizes with atmospheric pressure
Intrapleural/pleural pressure
The pressure of the air within the pleural cavity, between the visceral and parietal pleurae, changes during breathing, is always lower than or negative to the intrapulmonary pressure
Transpulmonary pressure
Represents the pressure that effectively promotes air flow and distends the lungs, the difference between the intrapleural pressure and the intrapulmonary pressure
Intrapleural pressure is normally negative
Due to inward lung and outward chest wall recoil
In pneumothorax
Air enters the pleural space, intrapleural pressure increases and lung volume decreases, can cause partial or complete lung collapse