With the onset of inspiration, the muscles of the diaphragm and chest wall contract, which causes a downward movement of the diaphragm and an outward and upward movement of the rib cage. As a result, pleural pressure decreases during inspiration. This negative pleural pressure is transmitted across the lung tissue and results in a decrease in alveolar pressure. As alveolar pressure decreases below 0 (i.e., from atmospheric pressure to a lower pressure), gas moves into the airways when the glottis is open. As gas flows into the airways to the alveoli, the pressure gradient along the airways decreases, and flow stops when there is no longer a pressure gradient from atmospheric to alveolar pressure. The decrease in pleural pressure at the start of inspiration secondary to inspiratory muscle contraction is and gas again flows from a higher (alveolar) pressure to a lower (atmospheric) pressure. In the alveoli, the driving force for exhalation is the sum of the elastic recoil of the lungs and pleural pressure (see ). This relationship between changes in pressure, changes in airflow, and changes in volume during inspiration and exhalation is displayed in
Fig. 21.2.
During tidal volume breathing in normal individuals, the decrease in alveolar pressure at the start of inspiration is small (1 to 3 cm H2O). It is much larger in individuals with airway obstruction because of the larger pressure drop that occurs across obstructed airways. Airflow stops in the absence of a pressure gradient, which occurs whenever alveolar pressure and atmospheric pressure are equal.
(Fig. 21.3 ) and the factors that determine these volumes are important components of lung mechanics greater than the transmitted fall in alveolar pressure, and, as a result, transpulmonary pressure at the start of inspiration 0.5 is positive (see
Eq. 21.1). Positive transpulmonary pressure 0.4 is necessary to increase lung volume, and lung volume increases with increasing transpulmonary pressure (
Fig.
21.1 ). Similarly, during inspiration, the chest wall expands to a larger volume. Because pleural pressure is negative in 0.1 relation to atmospheric pressure during quiet breathing, 0 the transmural pressure across the chest wall is negative
Eq. 21.2
On exhalation, the diaphragm moves higher into the chest, pleural pressure increases (i.e., becomes less nega tive), alveolar pressure becomes positive, the glottis opens, +0.5 –0.5 •Fig. 21.1 Volume of the Lung as a Function of the Transpulmonary Pressure in Health and Disease. Asthetranspulmonarypressureincreases,lungvolumeincreases.Alsoshownarethechangesinlungvolumeinthepresenceofemphysemaandpulmonaryfibrosis.Notethatforthesamechangeintranspulmonarypressure,inthepresenceofeitherofthesetypesofdiseases,thechangesinlungvolumearedifferent.TLC,totallungcapacity(thetotalvolumeofgasinthelung).
0.3 0.2 •Fig. 21.2 Changes in Alveolar and Pleural Pressure During Quiet Breathing (Tidal Volume). Inspirationisrepresentedtotheleftofthevertical dotted line, andexhalationisrepresentedtotherightofit.Positive(inrelationtoatmospheric)pressuresarerepresentedabovethehorizontal dotted line, andnegativepressuresarerepresentedbelowit.Seetextfordetails.Atpointsofnoairflow(pointsAandC),alveolarpressureis0.FRC,functionalresidualcapacity.
2 1.2 •Fig. 21.3 The Various Lung Volumes and Capacities. ERV,expiratoryreservevolume;FRC,functionalresidualcapacity;FVC,forcedvitalcapacity;IC,inspiratorycapacity;IRV,inspiratoryreservevolume;RV,residualvolume;TLC,totallungcapacity;VC,vitalcapacity;VT,tidalvolume.