Relaxation volume of lung is documented as ?
Functional residual capacity
Residual volume
Vital capasity
Closing volume
Ans. is 'a' i.e., Functional residual capacity Relaxation volume The relaxation volume is the volume of the relaxed respiratory system, i.e., the volume at which net elastic recoil of he total respiratory system is zero (the elastic recoil of chest wall is equal and opposite to that of lungs). In healthy subjects, this occurs at the end of quiet expiration, i.e., at functional residual capacity. Therefore, in healthy adults at rest FRC is essentially equal to relaxation volume of respiratory system and FRC is usually treated as synonymous with relaxation volume. However, in real sense FRC and relaxation volume are not synonymous terms and, while the two volumes are some times equal, more often they are not, i.e., active or passive mechanisms often operate to make FRC different from relaxation volume. For example, neonates have a more compliant (less stiff) chest wall. The chest wall thus has less elastic recoil to balance that of lung, thus decreasing the relaxation volume of lung. FRC then is often maintained above relaxation volume by active mechanisms like inspiratory muscle activity and slowing of expiration. Another example where FRC and relaxation volume may differ is in obstructive lung disease where FRC is dynamically determined and may be considerably above relaxation volume.