<prompt> True about Hyaline membrane disease:FRC above the closing volumeFRC below the closing volumeFRC is equal to closing volumeFRC doesnt depend on closing volume  <response>: B i.e. FRC below the closing volume - Closing volume CV or critical CV is minimal volume of gas necessary to prevent small airway collapse. In young adults it is 10 of vital capacity ie 480 ML and becomes equal to - 40 of VC ie FRC at the age of 65 years. In RDS hyaline membrane disease, the FRC decreases below closing volume FRC CV indicating collapse of some alveolar segments during tidal breathing. - Volume of air in lung at the end of 1 force ful inspiration is TLC; 2 forceful expiration is residual volume RV and ; 3 normal tidal expiration is FRC. RV is 1200mL and FRC is 2200 mL. Therefore normally closing volume is more close to RV than to FRC. Closing Volume CV and Closing Capacity CC CV is volume below which the terminal airway connected to alveoli gets closed and hence the gas cannot come in or out of alveoli. In young subjects, the closing volume is about 10 of vital capacity V C. It increases with age and becomes equal to about 40 of VC i.e. FRC at the age of 65 years. Relatively small amounts early disease in the small airways apparently increases the closing volume. Single Breath N2 Washout Test to measure CV From mid inspiration, the subject takes a deep breath vital capacity single breath of pure 100 02 and then exhales steadily. The changes in N2 concentration of expired air at lips is continuously measured during subsequent full exhalation. 4 phases of N2 conc. can be recognized one after another. Phase I or initial gas exhaled is the gas that filled the pure dead space. It contains no N2 because inhaled gas is 100 02. Phase II: Mixture of dead space alveolar gas is exhaled with increasing N2 conc. The volume of dead space is volume of gas expired from peak inspiration to mid poion of phase II. 3. Phase III: Pure alveolar gas is exhaled with constant N2 conc. Phase III terminates at ie transition from phase III to IV is closing volume CV. The CV is the lung volume above RV residual volume at which the airways in lower dependent pas of lungs begin to close off because of the lesser transmural pressure in these areas. In most normal persons phase III also has a slight positive slope indicating a gradual increase in the propoion of expired gas coming from relatively N2 rich upper poions of lung. Phase IV: At the end of expiration, an abrupt increase in N2 conc. is seen signaling closure of airways at the base of lung. This is caused by preferential emptying of apex, which has relatively high concentration of Nz. The gas in upper lung is richer in N2 than lower dependent poions because the upper lung alveoli are more distended at the sta of inspiration and therefore the N2 in them is less diluted with 100 02 inspired from mid inspiration because during a VC breath of 100 02, apex expands less and therefore previously occupied N2 there is less diluted with 02. Functional Residual Capacity FRC or Relaxation Volume It is volume of air remaining in lungs after normal tidal exhalation. and is determined by the balance bw the elastic recoil pressure generated by lung parenchyma to become smaller inward recoil and the pressure generated by chest wall to become larger outward recoil. At FRC, transmural pressure across respiratory system chest wall lung is zero 0. It is positive at greater negative at smaller volumes. - FRC decreases when the chest wall muscles are weak, eg paralysis or the elastic recoil of lung i.e. tendency of alveoli to collapse is more eg hyaline membrane diseaseQ i.e. lung elastic recoil chest wall muscle force. Disease characterized by low FRC include pulmonary edema, pneumonitis RDSHMD. In hyaline membrane disease, FRC usually decreases below closing volumeQ. When closing capacity exceeds FRC FRC CC, some alveolar segments are collapsed during a poion of tidal breathing. This decreases VP ratio and may cause hypoxia and hypercarbia. And when the CC FRC TV. i.e. C C exceeds FRC plus tidal volume, lung segments will be collapsed during inspiration and expiration of tidal breathing. This fuher exaggerate VP mismatch and lit complete atelectasis. So respiratory therapy is designated to increase the lung volume towards normal FRC. - Positive end expiratory pressure PEEP and continuous positive airway pressure CPAP raise FRC to a level above CVCC in diseases associated with alveolar collapse; So the terminal airways remain open throughout the respiratory cycle. In presence of airway obstruction, FRC increases bo premature airway closure, which traps air in the lung. 