During inspiration, the diaphragm and chest wall muscles contract, causing the diaphragm to move downward and the rib cage to move outward and upward. This leads to a decrease in pleural pressure. The negative pleural pressure is transmitted to the lungs, resulting in a decrease in alveolar pressure. When the glottis is open, gas moves into the airways as alveolar pressure drops below atmospheric pressure. As gas flows into the airways and reaches the alveoli, the pressure gradient along the airways decreases. Eventually, flow stops when there is no longer a pressure gradient from atmospheric to alveolar pressure. At the start of inspiration, there is a decrease in pleural pressure due to the contraction of inspiratory muscles, which causes gas to flow from alveolar pressure (higher) to atmospheric pressure (lower). Exhalation is driven by the elastic recoil of the lungs and pleural pressure. The relationship between pressure changes, airflow, and volume during inspiration and exhalation can be seen in Figure 21.2.

In normal individuals during tidal volume breathing, the decrease in alveolar pressure at the start of inspiration is small, ranging from 1 to 3 cm H2O. However, in individuals with airway obstruction, the pressure drop across obstructed airways is larger, resulting in a larger decrease in alveolar pressure. Airflow stops when there is no pressure gradient, which occurs when alveolar pressure and atmospheric pressure are equal (Figure 21.3). The volumes and factors determining them are important in understanding lung mechanics. Transpulmonary pressure at the start of inspiration is positive, greater than the transmitted fall in alveolar pressure, as shown in Equation 21.1. Positive transpulmonary pressure is necessary for lung volume to increase, and lung volume increases as transpulmonary pressure increases (Figure 21.1). During inspiration, the chest wall expands to a larger volume. Due to negative pleural pressure during quiet breathing in relation to atmospheric pressure, the transmural pressure across the chest wall is negative, as depicted in Equation 21.2.

During exhalation, the diaphragm moves higher into the chest, causing pleural pressure to increase (i.e., become less negative). This leads to a positive alveolar pressure and the opening of the glottis. The various lung volumes and capacities can be seen in Figure 21.3, including total lung capacity (TLC), functional residual capacity (FRC), inspiratory capacity (IC), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), residual volume (RV), vital capacity (VC), and tidal volume (VT).
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.