Elastic recoil is the tendency of the lungs to recover as people breathe in and out, preventing the lungs from collapsing on exhalation and allowing them to fill on inhalation. This characteristic can be seen with other air-filled objects like balloons and the bladders used by undersea organisms to control submersion. In people who are sick, the elastic recoil may be decreased, putting patients at risk of airway collapse and other difficulties with breathing.
When people inhale, their lungs fill rapidly with air. Surfactant, a lubricant in the lungs, changes the surface tension in the lungs and allows the alveoli, tiny bubble-like structures lining the lungs, to expand. At the same time, the pressure inside the pleural cavity, the space surrounding the lungs, decreases to allow the lungs to enlarge with inhalation. If the pressure remained constant, the lungs would be crushed inside the chest when people breathed in.
On exhalation, the lungs empty of air, the surfactant allows the alveoli to get smaller, and the interpleural pressure increases. The lungs go through this cycle every time people breathe. Variations in pressure can make it difficult to breathe because people cannot expand the lungs fully. Likewise, there is also a risk of lung collapse if the relationship between the interpleural pressure and the surface tension inside the lungs is disturbed.
Elastic recoil is made possible by highly elastic fibrous cells in the structure of the lungs. These cells can easily stretch to accommodate the lungs as they expand, and compress when the lungs shrink down on exhalation. Other cells in the body are more rigid and less capable of permitting elastic recoil. People with chronic lung diseases leading to scarring and the formation of plaques in the lungs may have impaired elastic recoil as their lungs are no longer as flexible. Essentially, the lungs are like a balloon inside the chest, expanding and contracting as air moves in and out.
Patients with airway constriction who can only take in small breaths can experience problems with elastic recoil, as the lungs may not expand enough on inhalation and a collapsed lung can develop. When patients are placed on assisted ventilation because they cannot breathe independently, the ventilating device is carefully adjusted to permit the patient's lungs to expand fully, without causing damage to the lungs by forcing too much air into the lungs or delivering air with too much pressure and disrupting the balance of tension and pressure inside the chest.