Ans. a (Compliance of lung). (Ref. Ganong Physiology 23rd/ pg. 654-657; Figure 35-10)Lung compliance is an index of lung distensibility or the ease with which the lungs are expanded; thus, compliance is the inverse of elastic recoil. Compliance is defined as the ratio of change of lung volume to the change in pressure required to inflate the lung (dV/dP). Compliance decreases in patients with pulmonary edema or surfactant deficiency and increases when there is a loss of elastic fibers in the lungs, such as occurs in patients with emphysema and with aging.MECHANICS OF BREATHING - Compliance of the lungs & chest wall# Curve of airway pressure plotted against volume, is relaxation pressure curve of the total respiratory system.# The pressure is zero at a lung volume that corresponds to the volume of gas in the lungs at the end of quiet expiration (relaxation volume, which equals the functional residual capacity).# It is positive at greater volumes and negative at smaller volumes. The change in lung volume per unit change in the airway pressure (DV/DP) is the stretchability (compliance) of the lungs and chest wall.Q# It is normally measured in the pressure range where the relaxation pressure curve is steepest, and the normal value is approximately 0.2 L/cm H2O.# The curve is shifted downward and to the right (compliance is decreased) by pulmonary congestion and interstitial pulmonary fibrosis; pulmonary fibrosis is a progressive disease of unknown cause in which there is stiffening and scarring of the lung.# The curve is shifted upward and to the left (compliance is increased) in emphysema.# It should be noted that compliance is a static measure of lung and chest recoil.# Two major forces contribute to lung compliance:o tissue elastic forces ando surface tension forces.Q