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The respiratory system participates in additional, importantphysiological processes, including acid–base regulation, temperature regulation,immune function, and metabolic functions, among others. In the followingchapters, the basic physiology of ventilation, pulmonary perfusion, and gastransport are covered, as well as the critical role of the respiratory system inacid–base balance.Chapter 13Pulmonary Ventilation and Perfusion and Diffusion ofGasesChapter 14The Mechanics of BreathingChapter 15Oxygen and Carbon Dioxide Transport and Control ofRespirationReview Questions145This page intentionally left blank147CHAPTER13Pulmonary Ventilation andPerfusion and Diffusion of GasesRespiration is a complex process that begins with ventilationof the lungs and diffusion of gases between the lungsand blood.
Meanwhile, the lungs are perfused with blood,which transports gases between lungs and tissues, wherethe biochemical processes involved in cellular respirationoccur.BLOOD FLOW IN THE LUNGSThe general principles of blood flow are discussed in Section3, but the systemic and pulmonary circulations are differentin some important aspects. Although right ventricular outputis the same as left ventricular output (normally about 5 litersper minute [L/min] at rest), pressures in the pulmonary circulation are lower than pressures at equivalent points in thesystemic circulation (Fig.
13.1). To accommodate high flowrate at lower pressure, resistance is also low in the pulmonarycirculation. This low resistance is a result of the dense microcirculation and shorter vessels in the lungs compared with thesystemic circulation.Passive Control of Pulmonary ResistancePulmonary resistance is subject to passive control by pulmonary artery pressure and lung volume. Ordinarily, some pulmonary capillaries are collapsed. When pulmonary artery120/80, mean ⴝ 93Arteries–30PulmonarycirculationRightventricle25/0–10VeinsActive Control of Pulmonary ResistancePulmonary vascular resistance is also subject to active controlby chemical and humoral substances (Fig. 13.2).
Hypoxiaaffects pulmonary vessels differently than it affects systemicvessels. In the systemic circulation, tissue hypoxia results inmetabolic vasodilation of the arterial microcirculation. Incontrast, in the pulmonary system, when alveolar oxygen concentration (PAO2) falls, vasoconstriction of arterioles in thehypoxic region occurs. This is the most important mechanism25/8, mean ⴝ 14ArteriesSystemiccirculationRightatrium–2pressure increases, recruitment and distension of pulmonarycapillaries occurs. In other words, collapsed capillaries arerecruited to the circulation by the higher perfusion pressure,and individual capillaries distend, further reducing resistance.Such recruitment and distension are important in reducingpulmonary vascular resistance during high cardiac output,for example, in exercise.
Lung volume has different effectson extra-alveolar vessels and alveolar capillaries. As lungvolume increases, the associated traction causes distension ofextra-alveolar vessels, reducing resistance within them. Incontrast, with increased lung volume, alveolar vessels arecompressed by the inflation of alveoli.
Thus, as lungs areinflated from a very low volume, resistance first falls due tothe effects on extra-alveolar vessels, but as inflation continues, resistance begins to rise as alveolar vessels arecompressed.–12Leftatrium–5Figure 13.1 Pulmonary and Systemic Circulations The pulmonary circulation exists in series with the systemic circulation, and forthis reason, equal cardiac output and stroke volumes are ejected fromthe right and left ventricles.
Pressures and resistances are lower in thepulmonary circulation than in the systemic circulation.Leftventricle120/0Vascular Pressure inSystemic andPulmonaryCirculations (mm Hg)(Bar above figures = mean)–8VeinsA. Effects of increases in pulmonary blood flow and vascular pressuresArterioleCapillariesNormally some pulmonarycapilliaries are closed andconduct no bloodRecruitment: Morecapillaries open aspulmonary vascularpressure or bloodflow increasesDistention: At high vascularpressures individualcapillaries widen andacquire a larger crosssectional areaB. Effects of lung volumeExtra-alveolarvesselsAlveolusAlveolusAlveolarvesselsAlveolusAlveolusLow lung vol.High lung vol.As lung volume increases, increasing traction on extra-alveolar capillariesproduces distension, and their resistance falls.
Alveolar vessels, in contrast,are compressed by enlarging alveoli, and their resistance increases.C. Effects of chemical and humoral substancesAlveolarhypoxiaConstrictarterioles␣-Adrenergic agonists,thromboxane,norephinephrine,angiotensin,histamine,endothelinVasoconstrictors-Adrenergic agonists,bradykinin,prostacyclin,nitric oxideVasodilatorsFigure 13.2 Pulmonary Vascular Resistance Resistance in the pulmonary circulation is affected by pulmonary artery pressure and bloodflow, lung volume, and chemical and humoral substances. Elevation of pulmonary artery pressure causes distension of vessels and recruitment ofcapillaries that are otherwise collapsed (A), resulting in reduced resistance. An increase in lung volume causes traction and thus distension of extraalveolar vessels, but results in compression of alveolar capillaries (B); the combination of these two effects results in lowest pulmonary vascular resistance at intermediate lung volumes.
Pulmonary vessels also constrict and dilate in response to a number of chemical and humoral mediators (C).for short-term regulation of regional pulmonary blood flow,and results in better perfusion of well-oxygenated portions ofthe lungs.right and left main bronchi, which enter the lungs, furtherdivide, and become smaller in diameter.
Up to 23 generationsof branching airways are present in the system beginning atthe trachea and leading to the alveoli.FUNCTIONAL ANATOMY OF THE LUNGSAND AIRWAYSConducting Zone of the LungThe human lungs consist of the three lobes of the right lungand two lobes of the left lung (Fig.
13.3). The bronchi, bloodvessels, lymphatic vessels, and nerves leave and enter at thehilum of each lung. The airways consist of the trachea (windpipe), right and left main bronchi, smaller bronchi, and bronchioles (Figs. 13.4 and 13.5). The trachea branches into theAirways from the trachea to the terminal bronchioles constitute the conducting zone of the lungs and are not capable ofgas exchange, which takes place only in the respiratory bronchioles and alveoli.
The conducting zone is also known asanatomical dead space, due to the lack of gas exchange inthis area. In an adult, the anatomical dead space containsapproximately 150 milliliters (mL) of air. The trachea hasPulmonary Ventilation and Perfusion and Diffusion of GasesMedial Surface of Right LungGroove for subclavian arteryMedial Surface of Left LungArea for tracheaand esophagusApexArea for esophagusGroove for brachiocephalic(innominate) veinApexGroove forsubclavian arteryGroove forarch of aortaR. upperlobe bronchusGroove forbrachiocephalic(innominate) veinUpper lobeUpper lobeGroove forsuperior vena cavaArea forthymus andmediastinalfatty tissueOblique fissureLower lobeArea forthymus andmediastinalfatty tissueCut edge of pleuraPulmonaryarteriesUpperlobeBronchialarteriesHilusBronchiHorizontalfissureSuperiorpulmonaryveinsHilusCardiacdepressionCardiacnotch(incisura)InferiorpulmonaryveinsMiddlelobeObliquefissureDiaphragmatic surfaceUpperlobeLymph nodesCardiacdepressionLower lobe149Groove for esophagusObliquefissureGroove fordescendingaortaPulmonary ligamentLower lobeLingulaLower lobePulmonary ligamentGroove for esophagusDiaphragmatic surfaceFigure 13.3 Gross Anatomy of the Lung The three lobes of the right lung and the two lobes of the left lung are depicted in this medial viewof the lungs.
Blood vessels, nerves, right and left main bronchi, and lymphatics enter and exit the lungs at the hilum.cartilaginous rings that extend around three quarters of itscircumference, providing the structural support necessary tohold airways open, but also allowing coughing. Cartilaginousplates are present in the walls of bronchi but are lost at thelevel of the bronchioles.Most of the conducting system is lined by ciliated, pseudostratified columnar epithelial cells, mucous-secretinggoblet cells, and several other cell types, with ciliated cells andgoblet cells constituting the majority of cells lining the largerairways (Fig. 13.6). Mucus prevents desiccation of the epithelium and traps particulate matter in inspired air. Particles arecarried upward (out of the lung) by ciliary action.
This processis referred to as mucociliary transport. The epitheliumbecomes cuboidal epithelium in bronchioles, where ciliatedcells are predominant. Goblet cells diminish and are absent interminal bronchioles. Clara cells in bronchioles secrete substances that line the bronchioles and play a role in the defensesystem of the airways; they also act as stem cells for regeneration of bronchiolar epithelium.The walls of the conducting airways also contain smoothmuscle, which is regulated by the autonomic nervous system.Sympathetic stimulation dilates airways through the effects ofnorepinephrine (or circulating epinephrine) on β2 receptorson smooth muscle cells. Parasympathetic stimulation constricts airways through muscarinic receptor activation byacetylcholine.
Airway dilation is characteristic of the “fightor-flight” response (see Chapter 7, page 87).Respiratory Zone of the LungInspired air passes through the trachea, bronchi, and bronchioles, eventually entering the terminal bronchioles, respiratory bronchioles, and alveoli. The respiratory bronchiolesand alveoli are the respiratory zone of the lungs, where gasexchange occurs. There are an estimated 300 million alveoliin the human lungs, with an exchange area of 50 to 100 m2.This enormous surface area, along with the thinness of theair–blood interface, allows for efficient gas exchange.
Thisinterface, known as the alveolar-capillary membrane, consists150Respiratory PhysiologyConnective tissue sheathThyroid cartilageCartilageCricothyroid ligamentElastic fibersCricoid cartilageGlandSmall arteryLymph vesselsNerveEpitheliumConnective tissuesheath (cut away)IntercartilaginousligamentsAnterior wallTracheal cartilagesCross sectionthrough tracheaMucosa showinglongitudinal folds formedby dense collectionsof elastic fibersPosterior wallTachealis muscleNerveEsophageal muscleSmall arteriesEpitheliumGlandElastic fibers Lymph vesselsEparterialbronchusToupperlobeTomiddlelobeToupperlobeTolingulaR. mainL.
mainbronchus bronchusTolowerlobeTolowerlobeIntrapulmonaryExtrapulmonaryIntrapulmonaryFigure 13.4 Structure of the Trachea and Major Bronchi The trachea divides into the right and leftmain bronchi before entering the lungs. The airways branch up to 23 times, leading to the alveoli. The airwaysfrom trachea to the terminal bronchioles are the conducting zone of the lung. With branching, diameter ofairways diminishes. Collagen is less prevalent in the walls of smaller airways and is lost in the bronchioles.of the capillary endothelium, the alveolar epithelium, and thebasement membrane, upon which these two single-cell layersare formed.
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