![]() Taking into consideration the relatively constant P Pl around the lung, each small airway can be subdivided into three segments (Fig. This gradual drop in P aw is secondary to simultaneous increase in the airways resistance towards the trachea. ![]() 3c, P aw decreases from the area next to the alveoli upwards. Both P Pl and P alv rise above P atm however, P alv remains more than P Pl due to the effect of elastic recoil pressure (P el) of the alveolar wall. During forceful expiration, the thoracic cage is compressed to the maximum. Alternatively, expiration below the tidal level is an active process that requires contraction of expiratory muscles. Thoracic cage expands more leading to higher drop in P Pl and P alv compared with tidal inspiration, which explains why more air is delivered to the alveoli compared with tidal inspiration. If inspiration above the tidal limit is required, accessory muscles of inspiration must be activated. This explains why small airways are always opened, even at the end of tidal expiration. During tidal breathing, whether inspiratory or expiratory, intra-airways (P aw) pressure is always more than P Pl. Tidal expiration is therefore a passive process, which needs no further muscle contraction. As a result, air flows outside the alveoli following the pressure gradient, Fig. When inspiratory muscles relax, dimensions of the thoracic cage decrease, P Pl increases from −8 back to −5 cmH 2O and P alv increases one cmH 2O above P atm. The sequence of events reverses during tidal expiration. The drop of P Pl also decreases the airways resistance by dilating the small airways and thus enhancing the air flow further. The rhythmic contraction of inspiratory muscles causes cyclic changes in the dimensions of the thoracic cage and consequently comparable cyclic fluctuation of P Pl.ĭuring tidal inspiration, P Pl drops from −5 to −8 cmH 2O enforcing the intra-alveolar pressure (P alv) to drop one cmH 2O below atmospheric pressure (P atm), Fig. The negative intrapleural pressure (P Pl) is one of the important factors that keep the patency of small airways, which lack cartilaginous support. These two opposing forces lead to a negative pressure within the potential space between the parietal and visceral pleurae. Towards the end of tidal expiration, the lungs tend to recoil inward while the chest wall tends to recoil outwards. ![]() For the details of the procedures, advantages, disadvantages and recommendations for best practice of these techniques, the reader can refer to the reports revised and published by the joint committee of ATS/ERS. However, body plethysmography and dilutional techniques may under-and overestimate lung volumes and capacities, respectively. The procedures used for measurement of RV, FRC and TLC are based on radiological, plethysmographic or dilutional techniques (helium dilution and nitrogen washout methods). RV constitutes part of FRC as well as TLC and, therefore, these capacities are impossible to measure through simple spirometers. Figure 1 gives a schematic summary of the standard lung volumes and capacities. Alternatively, the standard lung capacities are inspiratory (IC), functional residual (FRC), vital (VC) and total lung capacities (TLC). Four standard lung volumes, namely, tidal (TV), inspiratory reserve (IRV), expiratory reserve (ERV), and residual volumes (RV) are described in the literature.
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