Respiratory physiology, the essentials 7th ed j west (lippincott, 2004)

r Structure and Function How the Architecture 01the LungSubserves Its Function Blood-Gas Interface Airways and Airflow Blood Vessels and Flow Stability of Alveoli Removal of Inhaled Particles We begin with a short review of the refationships between structure and function in the fungoFirst we look at blood-gas interface where the exchange of the respiratory gases occurs Next we look at how oxygen is brought to the interface through the airways, and then how the blood removes the oxygen from the lung Finally two potential problemsof the lung are briefly addressed: how the alveoli maintaintheir stability, and how the lung is kept clean in a polluted environment The lung is for gas exc nge Its prime fun ct ion is to a llow oxygen to move fro m the air into the venous blood and carbon dioxide [Q move o ut The lun g does other jobs too It metabo lizes some compounds, filters unwanted mater ials from the c ircul ation, an d acts as a reservo ir for blood But its cardin al fun ction is to C X~ change gas, and we shall therefo re beg in a t th e blood -gas interface where the gas excha nge occurs Blood-Gas Interface u ," , ur ,i ,, Oxy gen and carbon dioxide move between air and blood by simple diffusion, that is, from an area of high to low partial pressure, * much as water runs downhill Pick's law of diffusion states that the am oun t of gas that moves acro ss a sheet of t issue is proportional to the area of the sheet but inversely proporti onal to its thickness The blood- gas ba rrier is exceedingly thin (Figure I-I) and h as an area of be tween 50 and 100 square meters It is therefore well suited to its funct ion of gas exchange How is it possible to obtain such a prodigious surface are a for diffusion inside the limited thorac ic cav ity?This is done by wrapping the small blood vessels (cap illaries) around an en ormo us number of small air sacs called alveoli (Figure 1-2) There are abou t 300 mill ion alveo li in the hu man lung, eac h abou t 1/3 mm in diam eter If th ey were spheres," their to tal surface area would be 85 square me ters, but the ir volume only liters By co ntrast , a single sph ere of this vo lume wo uld have an int ern al surface area of on ly 1/100 square mete r Thus, the lung gene rates this large diffusion area by be ing divided in to myriad un its Gas is brough t to one side of the blood-gas interface by airways, an d blood to the othe r side by blood ,'essels Airways and Airflow T he a irways consist of a se ries of bra nching tubes wh ich be co me n arrower, sho rter, and more numerous as th ey penetrat e deeper in to the lun g (Figure I ~J) T he trachea divides in to righ t and left main bronchi, which in turn d ivide in to 10' bar , then segme nta l bronchi This process co n tin ues down to the tenninal bran, chioles, wh ich are the sma llest a irways without alveo li All of these bronch i make up the conducting airways T h eir function is to lead inspired air to th e gas ex , ch an ging regions of the lung (Figure 1,4) Beca use the co nduct ing airways con, tai n no alveo li and therefore take no part in gas exc hange, the y const itute the ana tomic dead space Its volume is about 150 ml The termin al bro nch ioles divide into respiratory bronchioles, whic h h ave oc casional alveoli budding from their walls Finally, we come to the alveolar dU ClS , wh ich are completely lined with alveoli This alv eo lated region of the lung where the gas exc nge occ urs is known as the respiratory zone T h e portion of lun g dis, Figure la r w a t Th e eryth r t lo n), a pla sm a a nd a ~ 1.-_ * The part ial pressure of a gas is found by mu lriplving its concentrat ion t by the to ta l pressure For example, dry air has 20.93% , Its partia l pressure (Po) at sea level (barom etric pressure 760 mm Hg] is 20.93/ 100 X 760 = 159 rom Hg When air is inhaled into the upper airways, it is war med and moistened, and th e wate r vapo r pressure is th en 47 mm Hg, so tha t the tot al dry gas pressure is only 760 - 47 = 713 mm Hg The Po , of inspired air is therefore 20.93/ 100 X 713 = 149 mm Hg A liq uid exposed to a gas un til equ ilibration take s plac e h as th e same parti al pressure as the gas For a more complete description of th e gas laws, see App endix A Th e alveoli are not sphe rical hut polyhedral No r is th e whole of their surface available for diffusion (see Figure 1, 1) These numbers are th erefore only approx imate Structure and Function Figure 1-1 Elect ro n microg raph showing a pulmona ry capill ary (C) in the alv eolar wall Note the extremel y th in blood -gas barri er of abo ut 0.3 u rn in some pl aces The large arrow ind icates the diffusi on pat h fro m alveolar gas to th e interio r of t he eryt hrocyte (ECI and includes t he lay er of surfactant (not shown in th e preparation ), alve olar epitheli um (EP), interst it ium (IN), capill ary endoth elium (EN), and pl asm a Parts of stru ctura l cell s call ed fibroblasts (FBI, baseme nt membrane (8M ), and a nu cleu s of an endot helia l cell are also seen Figure 1-3 Cas awa y, allov 'r-:; ol es t o be s~~ " tal to a te rm u tan ce from " Figure 1-2 Sect ion of lu ng showing m any alveo li and a sma ll bronchio le Th e pu lm onary capil lar ies run in t he w alls of the alv eo li (Figure 1-1) The holes in t he alve olar walls are the pores of Kahn but th e respir liters durine During i drawn in to" tion of the J intercost al of the thorax ~ flow like war Structure and Function Figure 1-3 Cast of the airwa y s of a human lu ng The alveoli hav e been pruned aw ay, allowing t he ducti ng airwa ys from t he trach ea to the termina l bro nchioles to be seen tal to a termi nal bronc hiole forms an ana tomical un it called the acinus The d isranee from the terminal bronchiole to the most distal alveolus is only a few mrn, but the respiratory zone makes up most of the lung, its volume being about 2.5 to liters dur ing rest During inspirati on, th e volume of the th oracic cavity increases and air is drawn into the lung The incr ease in volume is brought about par tly by contraction of th e diaph ragm, wh ich causes it to descend, and partly by the actio n of th e int ercostal muscles, which raise th e ribs, thu s increasing the cross-sectional area of the thorax Inspired air flows down to about the terminal bronchio les by bulk flow, like water th rough a hose Beyond that point, the combined cross-sectiona l Z f- Trachea '"0 c; Bronchi ,./' N ·u'"=> c "0 c 0 '( ~ =;Jb.: I Respiratory { "O ~ bronchioles ~ ~ c 1: J~1 Alveola r ducts ",0~ -j J~ c'" "'5 roN c >_o _ 0~ p,\ ,I ' Term inal bronchioles -, r.':: -'}I'\ Bro nch ioles II ~ f- ~ Alveolar {~ ~~ "'!-{ :'- zone-c-s t; I I I I '">-