The compliance of the lungs can be influenced by various respiratory disorders. Emphysema, an obstructive lung disease commonly found in smokers, is associated with the destruction of alveolar septa and the pulmonary capillary bed, resulting in increased lung compliance. This means that for every 1-cmH2O increase in pressure, the increase in volume is greater compared to a normal lung. On the other hand, pulmonary fibrosis, a restrictive lung disease characterized by increased collagen fiber deposition in the interstitial space, leads to decreased lung compliance. In this condition, for every 1-cmH2O change in pressure, the change in volume is less. These alterations in compliance are clinically significant because a lung with low compliance requires larger changes in pleural pressure to bring about changes in lung volumes, resulting in increased work of breathing. Surface tension, which is caused by water molecules at the air-liquid interface, also affects lung inflation. The volumepressure curves of a saline-filled lung and an air-filled lung can be compared to illustrate the effect of surface tension. Due to higher surface tension forces in air-filled lungs, more pressure is required to fully inflate the lung compared to a saline-filled lung. Surface tension is measured as the attractive force of surface molecules per unit length of material. The law of Laplace describes the relationship between pressure within a sphere (such as an alveolus), wall tension, and radius. Alveoli are lined with surfactant, a predominantly lipid-based substance that serves various physiological roles. These include reducing the work of breathing by decreasing surface tension forces, preventing collapse and sticking of alveoli during exhalation, and stabilizing alveoli that tend to deflate at low surface tension. In the absence of surfactant, maintaining constant surface tension at the air-liquid interface would require higher transalveolar pressure at lower lung volumes compared to higher lung volumes. Surfactant helps stabilize alveolar inflation by allowing surface tension to increase as alveoli become larger. As a result, transalveolar pressure needed to keep an alveolus inflated increases with lung volume and transpulmonary pressure, and decreases as lung volume decreases. With surfactant, surface tension is increased at high lung volume and decreased at low lung volume, allowing the lungs to maintain alveoli at different volumes and preventing gas from emptying out of small alveoli into larger ones.
Thecomplianceofthelungsisaffectedbyseveralrespiratorydisorders.Inemphysema,anobstructivelungdiseasethatusuallyoccursinpeoplewhosmokeandisassociatedwithdestructionofthealveolarseptaandpulmonarycapillarybed,thelungismorecompliant;thatis,forevery1–cmH2Oincreaseinpressure,theincreaseinvolumeisgreaterthanthatinanormallung(see
Fig.21.1 ).Incontrast,inpulmonaryfibrosis,arestrictivelungdiseaseassociatedwithincreasedcollagenfiberdepositionintheinterstitialspace,thelungisnoncompliant;thatis,forevery1–cmH2Ochangeinpressure,thechangeinvolumeisless.Thesechangesincomplianceareofclinicalsignificancebecausealungwithlowcompliancerequireslargerchangesinpleuralpressuretoeffectchangesinlungvolumes;asresult,theworkofbreathingisincreasedforeverybreaththattheindividualtakes.
1 liter = 0.2 0.5 liter = 0.1 0.1 liter = 0.02 = 0.2 5 cm H2O 0.1 liter 0.2 = 0.2 0.1 = 0.2 0.5 liter 0.02 •Fig. 21.9 Relationship Between Compliance and Lung Volume. Imaginelungsinwhicha5–cmH2Ochangeinpressureresultsina1-Lchangeinvolume(situation1).Ifonelungisremoved(situation2),thecompliancedecreases,butwhencorrectedforvolumeofthelung,thereisnochange(specificcompliance).Evenwhentheremaininglungisreducedby90%(situation3),thespecificcomplianceisunchanged.
effect on surface tension. Surface tension is a force caused by water molecules at the airliquid interface that tends to minimize surface area, which makes inflating the lungs more difficult. The effect of surface tension on lung inflation is illustrated by a comparison of the volumepressure curves of a salinefilled lung and of an airfilled lung. Higher pressure is necessary to fully inflate the lung with air than with saline because of the higher surface tension forces in airfilled lungs than in salinefilled lungs. Surface tension is a measure of the attractive force of the surface molecules per unit length of material to which they are attached. The units of surface tension are those of a force applied per unit length. For a sphere (such as an alveolus), the relationship between the pressure within the sphere (Ps) and the tension in the wall is described by the law of Laplace:
Equation 21.7 where T is the wall tension (in dynes per centimeter) and r is the radius of the sphere.
The alveoli are lined with a predominantly lipidbased substance called surfactant. Pulmonary surfactant serves several physiological roles, including (1) reducing the work of breathing by decreasing surface tension forces; (2) preventing collapse and sticking of alveoli on exhalation; and (3) stabilizing alveoli, especially those that tend to deflate at low surface tension. In the absence of surfactant, the surface tension at the airliquid interface would remain constant, and the transalveolar pressure needed to keep it at that volume would be higher when alveolar volumes are lower (
Fig. 21.10A ). Therefore, higher transalveolar pressure would be necessary to produce a given increase in alveolar volume at lower lung volumes than at higher lung volumes. Surfactant stabilizes the inflation of alveoli because it allows the surface tension to increase as the alveoli become larger (see
Fig. 21.10B ). As a result, the transalveolar pressure necessary to keep an alveolus inflated increases as lung volume and transpulmonary pressure increases, and it decreases as lung volume decreases. In the presence of surfactant, surface tension is increased at high lung volume and decreased at low lung volume. The result is that the lungs can maintain alveoli at many different volumes. Otherwise, the gas in small alveoli would empty into large alveoli.