Lung Volumes and Lung Function Tests MCQs

Transpulmonary pressure is the pressure difference between alveolar pressure and intrapleural pressure. Before the start of inspiration or at the end of expiration it is about +5cm H2O. Positive transpulmonary pressure keeps the alveoli open. Intrapleural pressure is the pressure between two layers of pleura. It is about -5cm H2O before the start of inspiration or at the end of expiration. Alveolar pressure is the pressure within the terminal air spaces. It is the sum of pleural pressure and elastic recoil pressure of the lung. It is atmospheric before the start of inspiration or at the end of expiration. Transthoracic pressure is the pressure difference between alveolar pressure and pressure at the body surface. Ref: Fundamentals of Respiratory Physiology By A S Chakrabarty, Page

Loss of functional residual capacity (FRC) occurs in hyaline membrane disease for the following reasons Deficiency of surfactant Displacement of lung gas volume by edema, vascular congestion Reference: Ganong, 25th edition page no.629

Functional residual capacity is the amount of air remaining in the lungs after a normal tidal expiration It is obtained by adding residual volume + Expiratory reserve volume FRC is also called "Equilibrium volume" or "relaxation volume" because inward elastic recoil of lung balances with outward recoil of chest wall at Functional residual capacity Reference: Ganong, 25th edition page no.629

Respiratory muscles consume oxygen in proportion to the work of breathing. The work of breathing is equal to the product of the change in volume for each breath and the change in pressure necessary to overcome the resistive work of breathing and the elastic work of breathing. Resistive work includes work to overcome tissue as well as airway resistance; thus a decreased airway resistance will decrease the work of breathing and the oxygen consumption of the respiratory muscles. A decreased lung compliance would increase the elastic work of breathing. An increase in respiratory rate or tidal volume increases the work of breathing. Reference: Ganong, pp 657-658. Levitzky, pp 49-51.

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 (Δ V /Δ P ). 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. Reference: Ganong, pp 654-657. Levitzky, pp 20-28.

The functional residual capacity (FRC) occurs at the lung volume at which the outward recoil of the chest wall is equal to the inward recoil of the lungs, which occurs when no respiratory muscles are actively contracting, that is, at the end of a tidal expiration. The FRC is therefore also referred to as the resting expiratory level, or simply, the resting level. Reference: Ganong, pp 665-655. Levitzky, pp 29-31.

Inspiratory reserve volume increases by 10% Expiratory reserve volume decreases by 20% Total lung capacity decreases by 5% Tidal volume increases Reference: Ganong 25th edition page no. 628

Functional residual capacity is the amount of air remaining in the lungs after a normal tidal expiration It is obtained by adding residual volume + Expiratory reserve volume FRC is also called as "Equilibrium volume" or "relaxation volume" because inward elastic of the lung balances with outward recoil of chest wall at Functional residual capacity. Reference: Ganong, 25th edition page no.629