The circulation to the lung is unique in its duality and ability to accommodate large volumes of blood at low pressure. The lung has two separate blood supplies, one •Fig. 20.5 The Airway From the Terminal Bronchiole to the Alveo-lus. Notetheabsenceofalveoliintheterminalbronchiole.A,alveolus;RB,respiratorybronchiole;TB,terminalbronchiole.
Theconductingairwaysareinvolvedinseveralmajorpulmonarydiseasescollectivelyreferredtoasobstructive pulmonary disease, includingasthma,bronchiolitis,chronicbronchitis,andcysticfibrosis.Obstructionofairflowthroughtheairwaysiscommonlycausedbyincreasedamountsofmucus,airwayinflammation,andsmoothmuscleconstriction.Asthma isachronicinflammatorydiseaseofthelargeandsmallairways,mediatedpredominantlybylymphocytesandeosinophils,andisassociatedwithincreasedamountsofmucusintheairwaysandwithreversibleconstrictionoftheairwaysmoothmuscle(bronchospasm).Bronchiolitis isadiseaseofthebronchiolesthatusuallyoccurinyounginfantsandiscausedbyviruses,primarilyrespiratory syncytial virus. Chronic bronchitis, adiseasetypicallyofpeoplewhosmoke,isassociatedwithamarkedincreaseinmucus-secretingcellsintheairwaysandanincreaseinmucusproduction.Cystic fibrosis isanautosomalrecessivegeneticdiseasecausedbymutationsintheCl− ionchannelofthecysticfibrosistransmembraneconductanceregulator(CFTR).MutationsintheCFTR genecauseareductioninchlorideandwatersecretionintothemucusoverlyingtheepitheliacells,whichincreasestheviscosityofmucus.Thissituationresultsinmucusaccumulationandchronicpulmonaryinfections,primarilybyPseudomonas aeruginosa.
Notallimportantobstructivelungdiseasesinvolvetheairwaysdirectly.Emphysema isanirreversible,obstructivelungdiseasecommoninpeoplewhosmoke.Thepathogenesisinvolvesaprogressivedestructionoftheelastictissuesinthelungwithalossofalveolar/capillarystructure.Themechanismsoftissuedestructionareunclearbutmayinvolveproteolyticenzymesandothertoxiccompoundsincigarettesmoke.Emphysemaalsooccursinnonsmokingindividualswiththegeneticdisorderα1-antitrypsin deficiency, causedbytheinabilitytoregulateproteolyticenzymes,particularlyelastase.
for the uptake of O2 and removal of CO2 from the body (pulmonary circulation) and the other to supply O2 to lung tissue (bronchial circulation;
Fig. 20.7