Simultaneously, the volume in the box decreases, and the pressure in the box increases. As the subject tries to inspire, the volume in the subject’s lungs increases and the pressure in his or her lungs decreases. After expiring a normal tidal volume, the mouthpiece to the subject’s airway is closed. To measure FRC, the subject sits in a large airtight box called a plethysmograph. Therefore, if volume increases, pressure must decrease, and if volume decreases, pressure must increase. The body plethysmograph employs a variant of Boyle’s law, which states that for gases at constant temperature, gas pressure multiplied by gas volume is constant (P × V = constant). If this measurement is made after a normal tidal volume is expired, the lung volume being calculated is the FRC. The amount of helium that was added to the spirometer and its concentration in the lungs are used to “back-calculate” the lung volume. Because helium is insoluble in blood, after a few breaths the helium concentration in the lungs becomes equal to that in the spirometer, which can be measured. In the helium dilution method, the subject breathes a known amount of helium, which has been added to the spirometer. Two methods are used to measure FRC: helium dilution and the body plethysmograph. Of the lung capacities not measurable by spirometry, the FRC (the volume remaining in the lungs after a normal expiration) is of greatest interest because it is the resting or equilibrium volume of the lungs. Finally, as the terminology suggests, the total lung capacity ( TLC) includes all of the lung volumes: It is the vital capacity plus the residual volume, or 5900 mL (4700 mL + 1200 mL).īecause residual volume cannot be measured by spirometry, lung capacities that include the residual volume also cannot be measured by spirometry (i.e., FRC and TLC). Its value increases with body size, male gender, and physical conditioning and decreases with age. Vital capacity is the volume that can be expired after maximal inspiration. The vital capacity ( VC) is composed of the inspiratory capacity plus the expiratory reserve volume, or approximately 4700 mL (3500 mL + 1200 mL). FRC is the volume remaining in the lungs after a normal tidal volume is expired and can be thought of as the equilibrium volume of the lungs. The functional residual capacity ( FRC) is composed of the expiratory reserve volume (ERV) plus the residual volume, or approximately 2400 mL (1200 mL + 1200 mL). The inspiratory capacity ( IC) is composed of the tidal volume plus the inspiratory reserve volume and is approximately 3500 mL (500 mL + 3000 mL). In addition to these lung volumes, there are several lung capacities each lung capacity includes two or more lung volumes. The volume of gas remaining in the lungs after a maximal forced expiration is the residual volume ( RV) , which is approximately 1200 mL and cannot be measured by spirometry. The additional volume that can be expired below tidal volume is called the expiratory reserve volume, which is approximately 1200 mL. The additional volume that can be inspired above tidal volume is called the inspiratory reserve volume, which is approximately 3000 mL. With this maneuver, additional lung volumes are revealed. Next, the subject is asked to take a maximal inspiration, followed by a maximal expiration. Normal tidal volume is approximately 500 mL and includes the volume of air that fills the alveoli plus the volume of air that fills the airways. Normal, quiet breathing involves inspiration and expiration of a tidal volume ( V T). Residual volume cannot be measured by spirometry.įirst, the subject is asked to breathe quietly. Respiratory exchange quotient (CO2 production/O2 consumption)įigure 5–2 Lung volumes and capacities. Measurements of lung volumes and capacities are made by spirometry. Volume of forced vital capacity expired in 1 second Partial pressure of CO2 in dry inspired air Partial pressure of O2 in dry inspired air Partial pressure of CO2 in arterial blood Table 5–1 Abbreviations and Normal Values Associated with Respiratory Physiology The volume displaced is recorded on calibrated paper ( Fig. Typically, the subject is sitting and breathes into and out of the spirometer, displacing a bell. Static volumes of the lung are measured with a spirometer ( Table 5-1).
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