Respiratory muscles play a vital role in the process of breathing. During exhalation, additional energy is required, which is not provided by Fig. 22.8A alone. If these respiratory muscles become fatigued due to disease progression or the passage of time, respiratory failure can occur. The work of breathing increases when deeper breaths are taken, as more elastic work is needed to overcome the increase in tidal volume. Similarly, when the respiratory rate increases, the work of breathing also increases, as more flow resistance force needs to be overcome. Normal individuals and those with lung disease adjust their respiratory patterns to minimize the work of breathing. For example, individuals with pulmonary fibrosis (increased elastic work) tend to breathe more shallowly and rapidly, while those with obstructive lung disease (normal elastic work but increased resistive work) tend to breathe more slowly and deeply.

Chronic obstructive pulmonary disease (COPD) is a general term that encompasses diseases such as emphysema and chronic bronchitis. COPD is most commonly seen in individuals who smoke, and both emphysema and chronic bronchitis can coexist in their lungs. In individuals with COPD, especially those with emphysema as a major component, the elastic tissue in the alveolar and capillary walls is progressively destroyed. This results in increased lung compliance and decreased elastic recoil. The decrease in elastic recoil causes the equal pressure point to move towards the alveolus, leading to premature airway closure. This leads to air trapping and increases in RV (residual volume), FRC (functional residual capacity), and TLC (total lung capacity). Additionally, airway resistance is also increased in COPD, further increasing the work of breathing. 

In chronic bronchitis, the accumulation of mucus and airway inflammation causes the equal pressure point to move towards the alveolus, resulting in premature airway closure and increases in RV, FRC, and TLC. Airway resistance and the work of breathing are increased, but lung compliance remains normal.

Restrictive lung diseases like pulmonary fibrosis are characterized by decreased lung compliance. Lung volumes are decreased, but flow rates remain relatively normal. Some of the changes in pulmonary function values in obstructive and restrictive pulmonary diseases are outlined in Table 22.1.

During the third trimester of pregnancy, the enlarged uterus increases intraabdominal pressure and restricts the movement of the diaphragm. This leads to a decrease in FRC and decreased lung compliance in otherwise healthy women.

Individuals tend to adopt a respiratory rate at which the total work of breathing is minimal (indicated by the arrow in Fig. 22.9), especially for those without lung disease.
Fig. 22.8A , is not sufficient, and additional energy is needed for exhalation. With time or disease progression, these respiratory muscles can fatigue and result in respiratory failure. The work of breathing is also increased when deeper breaths are taken (an increase in tidal volume requires more elastic work to overcome) and when the respiratory rate increases (an increase in minute •Fig. 22.8 Mechanicalworkdoneduringarespiratorycycleinanormallung(A),alungwithreducedcompliance(B),andalungwithincreasedairwayresistance(C).Breakdownofthetrapezoidalareasenablesonetoappreciatetheindividualaspectsofthemechanicalworkload,whichincludethefollowing:OABCD,worknecessarytoovercomeelasticresistance;AECF,worknecessarytoovercomenonelasticresistance;AECB,worknecessarytoovercomenonelasticresistanceduringinspiration;ABCF,worknecessarytoovercomenonelasticresistanceduringexhalation(representsstoredelasticenergyfrominspiration).
ventilation requires more flow resistance force to overcome) (
Fig. 22.9 ). Normal individuals and individuals with lung disease adopt respiratory patterns that minimize the work of breathing. For this reason, individuals with pulmonary fibrosis (increased elastic work) breathe more shallowly and rapidly, and those with obstructive lung disease (normal elastic work but increased resistive work) breathe more slowly and deeply.
60 50 40 Total work Elastic work Nonelastic work 20 10 30 10 20 30 40 Mechanical work (kg-m/min) Respiratory rate (no./min)
Chronic obstructive pulmonary disease (COPD)isageneraltermthatincludesdiseasessuchasemphysemaandchronicbronchitis.COPDmostcommonlyoccursinindividualswhosmoke,inwhompathologicchangesinthelungconsistentwithbothemphysemaandchronicbronchitiscancoexist.ForindividualswithCOPDinwhomemphysemaisamajorcomponent,theelastictissueinthealveolarandcapillarywallsisprogressivelydestroyed,whichresultsinincreasedlungcomplianceanddecreasedelasticrecoil.Thedecreaseinelasticrecoilresultsinmovementoftheequalpressurepointtowardthealveolusandprematureairwayclosure.ThisproducesairtrappingandincreasesinRV,FRC,andTLC.Airwayresistanceisalsoincreased.Theseincreasesinlungvolumesincreasetheworkofbreathingbystretchingtherespiratorymusclesanddecreasingtheirefficiency.
Inchronicbronchitis,accumulationofmucusandairwayinflammationcausetheequalpressurepointtomovetowardthealveolus,whichleadstoprematureairwayclosureandincreasesinRV,FRC,andTLC.Airwayresistanceandtheworkofbreathingareincreased,butlungcomplianceisnormal.
Inrestrictivelungdiseasessuchaspulmonaryfibrosis,lungcomplianceisdecreased.Lungvolumesaredecreased,butflowratesarereasonablynormal.Someofthechangesinpulmonaryfunctionvaluesinobstructiveandrestrictivepulmonarydiseasesareshownin
Table22.1
Inthethirdtrimesterofpregnancy,theenlargeduterusincreasesintraabdominalpressureandrestrictsmovementofthediaphragm.TheFRC,asaresult,decreases.Thischangeinlungvolumeresultsindecreasedlungcomplianceandincreasedairwayresistanceinotherwisehealthywomen.
•Fig. 22.9 Effectoftherespiratoryrateontheelastic,nonelastic,andtotalmechanicalworkofbreathingatagivenlevelofalveolarventilation.Individualstendtoadopttherespiratoryrateatwhichthetotalworkofbreathingisminimal(arrow) forthosewithoutlungdisease.
1.