This trapped air results in pulmonary hyperinflation. The premature airway closure increases the volume of air retained in the lungs at the end of expiration this is referred to as air trapping. In obstructive lung disease, factors such as inflammation, bronchoconstriction, excessive mucus, and decreased elastic recoil cause increased airway resistance and lead to earlier small airway closure during expiration. Obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD), asthma, and bronchiectasis, are characterized by airway inflammation, easily collapsible airways, expiratory flow obstruction, and air trapping that results in elevated RV relative to healthy lungs. The air that remains in the lungs after the collapse of all small airways is the residual volume. In sections of small, non-cartilaginous airways, pleural pressure is greater than airway pressure and causes a collapse of the airway (Figure 1B). The pressure inside the airways (Paw) slowly decreases as you move up from the alveoli to the trachea as a result of increased airway resistance. Due to the elastic recoil of the alveoli, the pressure inside of the alveoli remains higher than that of the pleura, and the alveoli remain open. This increases Ppl and Palv above atmospheric pressure. Forcefully exhaling the ERV is an active process requiring the contraction of expiratory muscles in the chest and abdomen. The ERV is the volume of air that can be forcefully exhaled after a normal resting expiration, leaving only the RV in the lungs. VC is composed of the tidal volume, expiratory reserve volume (ERV), and inspiratory reserve volume (IRV). Residual volume can be understood by investigation of breathing that exceeds tidal volumes. Following maximal inspiration, the volume of air that leaves the lungs during a maximal force expiration is known as the vital capacity (VC). Once Palv is greater than atmospheric pressure, air flows out of the lungs. The inspiratory muscles relax, decreasing the size of the chest cavity, and increasing Ppl and Palv. Tidal expiration is a passive process that works in reverse. As a result, air from the relatively high-pressure atmosphere flows into the low-pressure alveoli. Inspiration is an active process requiring the rhythmic contraction of inspiratory muscles that work to expand the chest cavity. The decreased Ppl causes the alveolar pressure (Palv) to decrease 1 cm H2O below atmospheric pressure. On tidal inspiration, inspiratory muscle contraction increases the volume of the chest causing the intrapleural pressure (Ppl) to drop from -5 cm H2O to -8 cm H2O. During tidal breathing, the inspiration and expiration at physiologic rest, the volume of air entering and leaving the lungs is known as the tidal volume (TV). Īlthough breathing mechanics are complex, it is important to remember that air will flow from high-pressure areas to low-pressure areas. Instead, they must be calculated using indirect measurement techniques such as gas dilution or body plethysmography. Calculating the residual volume can give an indication of lung physiology and pathology. The TLC, FRC, and RV are absolute lung volumes and cannot be measured directly with spirometry. FRC is the amount of air remaining in the lungs after a normal, physiologic expiration (Figure 1A). TLC is the total volume of the lungs at maximal inspiration which is about 6 L on average, though true values are dependent on the same factors that affect residual volume. The residual volume is an important component of the total lung capacity (TLC) and the functional residual capacity (FRC). Reference values for residual volume are 1 to 1.2 L, but these values are dependent on factors including age, gender, height, weight, and physical activity levels. In other words, it is the volume of air that cannot be expelled from the lungs, thus causing the alveoli to remain open at all times. The residual volume remains unchanged regardless of the lung volume at which expiration was started. Residual volume (RV) is the volume of air remaining in the lungs after maximum forceful expiration.
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