If lung was allowed to recoil without the chest wall in expiration what would be the lung volume?
Ans. (C) Minimal value(Ref: Ganongs Review of Medical Physiology 25th ed; pg. 629)If the lung elastic recoil (collapsing force) is not opposed by the chest wall recoil (expanding force), the lung will collapse to minimal lung volume (which is lower than residual volume).Chest wall was removed means, lung outside pressure is ATM Pressure (O mm Hg), not the negative intrapleural pressure.So the condition is pneumothorax, like causing collapse of lung.Basic ConceptIf air is introduced into the intrapleural space (pneumothorax), the intrapleural pressure becomes equal to atmospheric pressure. Without the normal negative intrapleural pressure, the lungs will collapse (their natural tendency) and the chest wall will spring outward (its natural tendency).Collapse does not mean Zero volume, because medium size airway collapses before full alveolar collapses lading to small amount of air lung (generally 10% of TLC).Elastic Recoil and ComplianceElastic recoil is defined as the ability of stretched or inflated lung to return to its resting volume (FRC). Elastic recoil of the lung is directly related to lung stiffness, that is, the stiffer the lung, the greater is the elastic recoil (but lower compliance).The lung volume at which elastic recoil of lung and elastic recoil of chest wall are at equilibrium (i.e. equal elastic recoil force but in opposite directions) is represented by FRC. Because the lung and chest walls are recoiling equally but, in opposite directions at FRC, FRC is often referred to as the resting volume of the lungs.A change in the elastic properties of either the lungs or chest wall has a significant effect on FRC. For example, if the elastic recoil of the lung is increased (i.e. lower compliance). A new equilibrium is established between the lungs and chest wall, resulting in a decreased FRC.Conversely, if the elastic recoil of chest wall is increased, FRC is higher than normal. The elastic recoil of the chest wall at low lung volumes is a major determinant of RV in young peoples.The elastic recoil of the chest wall is such that if the chest were unopposed by the recoil of the lung, it would expand to about 70% of TLC. This volume represents the resting position of the chest wall unopposed by the lung.If the chest wall is mechanically expanded beyond its resting position, it recoils inward. At volumes <70% of TLC, the recoil of the chest wall is directed outward and is opposite to the elastic recoil of the lung. The outward elastic recoil of the chest wall is greatest at RV, whereas the inward elastic recoil of the lung is greatest at TLC.
Residual volume
FRC
Minimal value
Zero
Respiratory System