THE DUAL CIRCULATION OF THE LUNGS AND THEIR CONNECTIONS L. B u T O W , B.Sc. in P hysiotherapy (Stell.)* SEPTEMBER 1980 P H Y S I O T H E R A P Y 63 SUM M ARY T h e bronchial an d pulm onary circulations are o u t­ lin ed and their relevance to certain pathological condi­ tions is conjectured. INTRO DUCTIO N T h e m ain function of the lung is respiration. H ere, the distribution o f th e blood is ju st as im p o rtan t as the distribution of th e air. I t could be said th a t disturbance iiL th e v en tilatio n /p erfu sio n b alan ce is p ro b ab ly the # % t com m on cause of respiratory failure in patients i M i lung disease (M eyer, 1976). K now ledge ,o f. the bronchial and pulm o n ary circulation, and th eir different reactions to hypoxia, is thus very im p o rtan t in u n d er­ standing the way in w hich certain existing com m unica­ tions betw een the tw o systems com e into play an d the effect th ereo f under certain pathological conditions. A NA TO M Y Bronchial and pulmonary circulation T he lung is supplied by a sm all bronchial and a large pulm onary arterial system. T he bronchial arteries are usually direct branches from the descending ao rta (usually one on th e rig h t and two on the left), b u t there m ay be variations in th eir origin. T hey supply the sm ooth muscle of the bronchi from the carina to th e respiratory bronchioles and also nourish th e connective tissue in this area as well as certain parts of the visceral pleura (Last, 1972). T h e m iddle segm ent of th e oesophagus, a p a rt of the vagus nerve and th e tracheobronchial lym ph nodes are also supplied by th e bronchial arteries. In addition, there are vasa vasorum in close contact with and sup­ plying the pulm onary arteries. T he bronchial arteries divide into num erous branches • em inar presented fo r M .Sc. in Physiotherapy I, •epartm ents o f A natom v and Physiotherapy, U niver­sity of Stellenbosch, 1978. Received 16 July 1980. OPSOM M ING D ie brongiale en pulm onale sirkulasies w ord b eskryf en die m o o n tlike verband ten opsigte van sekere patolo- giese toestande w ord aangedui. before reaching th e respiratory bronchioli — th e so- called b roncho-pulm onary branches. T hese branches divide fu rth e r into lo b u lar and eventually in to bronchial capillaries which join up w ith the pulm o n ary capillaries. T ogether they drain into the pulm onary veins. T h e bronchial capillaries usually supply th e bronchioli although the pulm onary arteries o r even th e pulm onary veins m ay also perform this task (Tobin, 1952). T he bronchial arteries are p a rt of th e systemic circulation and th eir blood pressure (12 0 /8 0 mm Hg) is therefore m ore or less five tim es g reater th an th a t of the pulm o­ nary arteries (2 5 /1 0 m m Hg). T h e bronchial arteries tend to spiral around the bronchioles an d are thus arranged in a dense com m unicating netw ork. T his plexi- form arran g em en t o f th e bronchial arteries is th e reason why the system ic arterial supply to a bro n ch u s is n o t easily cut off (Liebow, 1949). T he bronchial veins also form a netw ork a ro u n d the bronchi. T h e veins which drain th e first p a rt o f the bronchi (extrapulm onary) drain into th e right atriu m via the azygos (right) and hem iazygos (left) veins. T he rest o f th e bronchial veins en ter the pulm o n ary veins on their way to th e left atriu m and th e ir contents m ake up more o r less 2% of th e cardiac o u tp u t (M eyer, 1976). T hese bronchial venous plexi m ay anastom ose w ith the venous (pulm onary) capillaries a ro u n d th e alveoli, or they m ay connect by branches to th e pulm o n ary veins (post-capillary), o r they m ay even anastom ose w ith a d jac en t parts o f th e bronchial venous plexi so th a t the blood m ay drain upw ards into the azygos system (Tobin, 1952). T he pulm onary tru n k w hich gives rise to th e p u l­ m onary arteries originates from the right ventricle and bifurcates, sending one b ran ch to each lung. T h e pul­ m onary arteries have a rectilinear course roughly parallel to tjftfe branches o f th e respiratory tree. T hey are tru e end-arteries, connecting only in th e finer capillary n e t­ works (Liebow, 1949). T h e p u lm onary arterioles and Contents - Inhoud The Dual Circulation o f the Lungs and their con­ nections ......................................................................... 63 Ascultation of the C h e s t................................................. 66 The Value of a Home Treatment Programme for Paediatric Patients with Respiratory Disorders ... 70 Shortwave Diathermy (SWD) in the Treatm ent of Unresolved Pneumonia ................................................ 72 The C ritical Incident Technique in Physiotherapy Education ....................................................................... ... 73 Straight Leg Raise as a Treatment Technique ... 74 Book Reviews; Abstracts/Opsom m ings ......................... 81 Classified ........................................................................... ... 85 Training Centres in the Republic .................................... 86 1 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) $4 F I S I O T E R A P I E SEPTEMBER 1980 capillaries are sh o rt and th eir diam eters relatively wide so th a t th ere is a decreased resistance to blo o d flow and this system is thus one of passive low pressure. T h e capillaries form a rich plexus around each alveolus and they supply th e alveoli w ith all th eir needs, excluding oxygen. W hen a bronchus is obstructed, the pulm onary vessels supplying th e poorly ventilated alveoli react to th e hypoxia by constricting and shunting th e b lo o d to other areas. (T he b ro n ch ial arteries dilate w ith hypoxia). E m boli th a t block sm all branches of the pulm onary artery provoke a m arked rise in pulm onary arterial pressure, a lth o u g h this is absent when larger arteries are blocked (G anong, 1973), b u t there is som e co n tro ­ versy ab o u t this. T h ere are usually two p u lm onary veins fro m each lung, carrying oxygenated b lo o d to th e left atriu m (Last, 1972), T h e blo o d is not 100% saturated with oxygen as there is m ixing w ith venous blood from the bronchial veins. T he pulm o n ary capillary pressure is ab o u t 10 m m H g and the oncotic pressure ab o u t 25 m m H g. T here is thus an inw ard - outw ard pressure g radient of a b o u t 15 m m H g w hich keeps the alveoli free of fluid. If the pulm onary cap illary pressure exceeds 25 m m H g (failure of left ventricle), this gradient will shift and oedem a and pulm onary congestion will develop (G anong, 1973). Pleural fluid P leu ral fluid moves into the pleural cavity un d er the influence of th e parietal p leural layer an d is absorbed by the visceral layer. F lu id entering the pleural space depends on th e fol­ lowing pressure g rad ien ts: Parietal pleural layer F actors causing fluid to en ter the p leural space: H ydrostatic pressure fin arterial capillary) = 3 0 c m H 20 + N egative pleu ral pressure = 5 cm H 20 T otal = 35 cm H .O F actors keeping fluid out of the pleu ral space: O ncotic pressure (in capillary) = 34 cm H 20 — O ncotic pressure (in p leural fluid) = 8 cm H 20 T o tal = 26 cm H ,0 T h e force responsible for moving th e fluid into the cavity is thus 35 cm H =0 and the force fo r absorbing the fluid is only 26 cm H-O. T h ere is th u s a resu ltan t force of 9 cm H .O in th e direction of the p leural cavity. Visceral pleural layer T h e fluid absorbed by the visceral layer depends on the follow ing pressure gradients: F actors keeping fluid in pleural space: H ydrostatic pressure (in capillary) = I I cm H»0 + N egative p leural pressure = 5 cm H 20 T otal = 16 cm H=0 F acto rs for absorbing fluid: O ncotic pressure (in capillary) = 34 cm H-jO — O ncotic pressure (in p leural fluid) = 8 cm H 20 T otal = 26 cm H-̂ O T h e total force is thus 10 cm H .O in the d irection of the visceral layer and th e fluid is absorbed. Lung zones T he tran sm u ral pressure o f the pulm onary arteries is the result of th e perivascular pressure and the in travascular pressure. T his resu ltan t force is d ependent on gravity; in the upright position the blood pressure is m axim um at th e lung bases. T h e vessels are therefore d ilated at the bases and n airo w er at the apices. T he capillaries in the apex (Zone I) are thus the first to collapse if the capillary pressure should decrease below th a t of th e alveolar pressure. T his does n o t usually occur un d er norm al conditions, but when there is a fall in arterial pressure (e.g. haem orrhage) or a rise in the alveolar pressure (e.g. the use o f positive pressure venti­ lation) it m ay happen. In the m iddle lung regions (Zone II) th e arterial capillary pressure is g reater th an the alveolar pressure and the alveolar pressure is greater th an the venous capillary pressure. T here is thus less like­ lihood o f the arterial capillaries in this area collapsing th an in Zone I. T h e blood flow in this area is d ependent on the arterial-alv eo lar gradient (not the usual arterial- venous gradient). In Zone III (bases) no collapse of cap il­ laries takes place, as the capillary arterial pressure is greater th an th at of th e alveolar pressure and the alveo­ lar pressure is sm aller than th a t o f the venous c a p illa r y pressure. H ere the blood flow is d ep en d en t on arterial-venous gradient (M eyer, 1976 and W est, 1974/. Arterio-arterial and arterio-venous communications I t is generally accepted th a t there is som e overspill o f bronchial capillaries into alveolar (pulm onary) capil­ laries (Last, 1972). H ow ever, it has recently also been proved th at precapillary anastom oses betw een bronchial arteries and pulm onary arteries do exist. T h ere are two types of these anastom oses, nam ely the sh o rt 1 - 2 mm), narrow (5 0 - 100 m icron) type w hich lies peripherally, and th e long (10 - 40 mm), wide (300 - 400 m icron) type w hich are m ore centrally situated. T hese anastom oses have coiled, thick m uscle walls which reduce th e blood pressure from the bronchial to the p u lm onary arterial system (T obin, 1952). D uring deep in sp ira tio n the de- creased pressure in the pulm onary artery will cause m ore blood to flow into it from the b ro n ch ial arteries through the precapillary arterio -arterial shunts (Pum p, 1972). O bstruction o f th e bronchial arteries them selves may lead to necrosis of the extrapulm onary bronchi b u t not of the in trap u lm o n ary bronchi because of th e ir connec­ tions with o th er blood vessels (Tobin, 1952). A p a rt from these arterio-arterial anastom oses, there are also arterio-venous connections betw een th e pulm o­ nary arteries and the pulm onary veins. A natom ically it has been seen th a t there is an arterial loop at tf><* apex o f the acinus and th a t this loop has very l i t / m uscle or elastic tissue in its wall. T his segm ent may be. dilated to form glom us-like protrusions o r to allow the passage o f glass spheres, 200 ft in diam eter and m any tim es the accepted d iam eter o f the capillaries, into th e pulm o n ary veins (Tobin, 1966). In the foetus these arterio-venous shunts m ay play a role in diverting the blood from the pulm onary arteries (and th eir con­ nections w ith the bronchial arteries) into the pulm onary veins. T h e unexpanded alveoli are thus avoided (Tobin, 1952). In norm al, resting subjects w ith no intracardiac shunts, this arterio-venous p u lm onary shunt flow ave­ rages h% of the to tal pulm onary blood flow. W hen using the V alsalva technique on patients, th ere is a rise in oxygen satu ratio n during th e m anoeuvre, and a tall in satu ratio n follow ing release of this technique. I he satu ratio n rise averages 1,2% in norm al people; in patients w ith cardiac o r pulm onary disease this value is even higher, nam ely 3 ,9 - 7 ,5 % . It is supposed th a t this m anoeuvre leads to a fall in tran sm u ral pressure in the pulm onary artery with a decreased blood flow in the pulm onary arterial as well as in th e arterio-venous shunts. The result is th a t m ore blood flows through the capillaries and th a t m ore blood becom es oxygena­ ted which causes the rise in oxygen satu ratio n , lh is does not, however, always apply to p atien ts w ith con­ R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) SEPTEMBER 1980 P H Y S I O T H E R A P Y 65 gestive heart failure in w hom the V alsalva technique might have no effect on tran sm u ral pressure of the pul­ m onary artery (Jose and M ilner, 1959). PATHOLOGICAL CONDITIONS Pulmonary embolism W hen a pulm onary artery and vein of a lobe in a dog’s lung are both occluded, the vascular pressures within th a t lobe rise to abnorm ally high levels (Shedd et a l , 1951). T h e reason for this is th a t the pulm onary artery responds to th e hypoxia by constricting and shunting the blood aw ay from th a t area, while the bronchial artery (like any o th er system ic artery) responds by dilating. An extensive opening o f the pre- capillary anastom oses takes place betw een the bronchial and pulm onary arteries. T he high blood pressure of the bronchial arteries is thus transm itted to the pulm onary arteries. T he increased hydrostatic pressure o f the p u l­ m onary arteries is no longer 11 cm H^O, b u t may be ^ V y value up to m ore or less 30 cm H .O , and the fc/sceral pleura, supplied by the pulm onary arteries, no longer has a driving force to absorb the pleural fluid. T h at is why a pulm onary em bolus is usually associated w ith a pleural effusion. Pneumonia T here is a period of hypoxia (anoxia) during pn eu ­ m onia, and th e pulm onary arteries supplying the region react by constricting and shunting the blood to other areas. It is to be expected th a t these p arts w ould thus die as they are deprived o f th eir blood supply. H owever, the dilation of the bronchial arteries an d associated arterio-arterial anastom oses causes the blood to be shunted to the m ore distally situated pulm onary vessels. T h e bronchial capillaries then actually supply the alveoli and parts concerned. I t is thus quite logical to assume th a t the alveoli can n o t be kept dry easily because of the increased pressure in the capillaries surrounding them and so alveolar congestion occurs. W ith the re tu rn of oxygen, the pulm onary arteries begin to dilate again. E ventually the pressure in the pulm onary arteries is back to norm al again and the blood flows norm ally into the pulm onary capillaries surrounding the alveoli. T h e de­ creased hydrostatic pressure provokes absorption from the alveoli and alveolar congestion (consolidation) is relieved. - S i m i l a r changes occur in atelectasis w here th e hypoxia J t e c t s the pulm onary and bronchial circulation. W hen * h e atelectasis is relieved and th e hypoxia reversed, the pulm onary circulation works norm ally again. Pulmonary tuberculosis Because th ere is less blood flow in the apical regions than in the basal regions of th e lung and because there is also a relatively avascular area where the bronchial blood supply ends and before the pulm onary blood supply comm ences, this is th e area where pulm onary the W ° Sb r o n c S ‘° ^ ~ apiCa" y and in Pulmonary emphysema In this condition the calibre of the alveolar capillary stru ctu re 1S h m a f t th a V SUa1, a n d th e n o i™ a l c a p illa r y Kra L r en dlsaPPeared. Newly developed m o se w fth n T b r o n c h i.al a r te r ie s th e r e fo r e a n a sto - ( N a g a is h ^ 1972 ° nM y a r t6 r ’eS th e p r e c a p illa r >' reS ion Bronchiectasis h r J n J h - C!'sease .le a d s to a d e fin ite e n la r g e m e n t of the moses b ltw e e n 'T h “ h thek . f? rm aticm of m ore anasta- moses betw een th e bronchial and pulm onary arteries. T he enlarged bronchial arteries are associated w ith the gran u latio n tissue form ed by chronic infection and w ith the increased m etabolic needs of the hypertrophied m uscle and hyperplastic lym phoid tissue th a t are often observed. New anastom oses also originate in the g ra n u ­ lation tissue. T h e pulm onary arterial blood is shunted away from the diseased portions o f the lung into re la ­ tively intact parenchym a w here the pulm onary blood pressure is presum ably lower. T he anastom oses betw een the bronchial and pulm onary arteries acco u n t in p a rt to r the fact th at there is little o r no d esatu ratio n o f the systemic arterial blood even in severe cases of b ro n ­ chiectasis (Liebow et al., 1949). In severe cases the increased bronchial circulation could eventually becom e 20% o f the cardiac o u tp u t (M eyer, 1976). Liver cirrhosis and portal hypertension In 200 consecutive cases o f cirrhosis, the incidence o f hy d ro th o rax was found to be approxim ately 6% . T h ere are different reasons given fo r this, nam ely hypoalbu- m inaem ia, ascites fluid passing either into p leural space via defects in the tendinous p ortion of the diaphragm or transported via the lym phatic system (Johnson and Loo, 1964). H ow ever, if p o rtal hypertension is present, then, due to collaterals betw een this system and the azygos system, the pressure in the azygos system will rise and the pressure in the b ro n ch ial veins th at drain into the azygos system may b e increased to such an ex ten t th a t th e pulm onary-bronchial post-capillary venous anastom oses may open. T his will lead to an increased hydrostatic pressure in the pulm o n ary capilla­ ries w ith decreased absorption of p leural fluid and thus h y d ro th o rax will occur. Heart failure R ight-sided h eart failure is usually secondary to left­ sided failure. W hen the right ventricle begins to fail systemic venous congestion occurs. M ost of th e b ro n ­ chial veins drain into the right atriu m and th e increased pressure in these leads to an increased pressure in the p ulm onary veins of the visceral pleura (through their anastom oses). T h e pleural fluid is n o t absorbed and a p leural effusion occurs. CONCLUSION It is generally accepted th a t there are connections betw een the systemic and pulm onary arterial systems in the lungs. These can be at intercapillary or pre- capillary levels. As regards the precapillary shunts, the m echanism s whereby they open are m ost im p o rtan t in understanding certain lung conditions, e.g. atelectasis, p ulm onary em bolus, pneum onia. T hese mechanisms’ p revent necrosis of the associated lung parenchym a and explain the phenom enon of local p leural effusions. Post- capillary (veno-venous) anastom oses are responsible for the p leural effusions seen in liver cirrhosis and right- sided h eart failure. F inally, it m ust be rem em bered th a t in all obscure cases of hypertrophy and failure o f the rig h t heart, the possibility of widened connections betw een the b ro n ­ chial and pulm onary arteries should be considered (W ood and M illar, 1937/8). References G anong, W. F . (1973): Review o f m edical physiology 6th ed. L ange M edical Publications. Los A ltos C ali­ fornia. ’ Johnston, R . F. and Loo, R. V. (1964). H ep atic hydro­ thorax. A n n . In t. M ed., 61, 3 8 5 -4 0 1 . Jos6, A. D. and M ilner, W. K. (1959). T h e dem onstra­ tion of arteriovenous shunts in norm al hum an sub­ jects and their increase in certain disease states J Clin. Invest., 38, 1915- 1923. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 66 F I S I O T E R A P I E SEPTEMBER 1980 Last, R. J. (1972). A natom y: Regional and applied. 5th Ed. C hurchill L ivingstone. E dinburgh and L ondon. Liebow, A . A., H ales, M . R. and Lindskog, G . E. (1949). E n larg e m en t o f th e b ro n ch ial arteries and their anastom oses w ith th e pulm onary arteries in bronchiectasis. A m . J. P ath., 25, 211. N agaishi, C. (1972). F u n ctio n al anatom y and histology o f the lung. 1st E d. B altim ore an d L ondon, U niversity P a rk Press. M eyer, B. J. (1976). D ie fisiologiese basis van genees- kunde. H .A .U .M . C ape Town. P um p, K. K . (1972). D istribu tion of th e bron chial arteries in the h u m an lungs. C hest, 62, 4 4 7 -4 5 1 . Shedd, D . P ., A lley, R . D . an d L indskog, G . E. (1951). O bservations on th e hem adynam ics o f b ro n ch ial-p u l­ m onary vascular com m unications. J. Thorac. Surg., 22, 537 - 548. T obin, C. E. (1952). T he bronchial arteries and their connections w ith o ther vessels in th e hu m an lung. Surg. G ynec. an d O bst., 95, 741 - 750. T obin, C. E. (1966), A rterio-venous shunts in th e p e ri­ p h eral pulm onary circulation in th e h u m an lung. Thorax, 21, 197 - 204. West, J. B. (1974). R espiratory physiology — th e essen­ tials. Blackwell Scientific P u b licatio n , O xford, L on­ don, E d inburgh, M elbourne. W ood, D . A . and M iller, M . (1937/8). T h e ro le of the dual pulm onary circulation in various pathologic conditions of the lungs. ]. Thorac. Surg., 7, 649 - 670. A cknow ledgem ent P rof. A. D. M alan, D e p a rtm e n t o f A natom y, U niver­ sity o f Stellenbosch. ASCULTATION OF THE CHEST D R. S T E P H E N C. M O R R IS O N ,* M .A ., M.B., B .C hir., M .R .C .P. SU M M A R Y T h e stethoscope a n d its use are described. A s current classification o f breath sounds, voice sou n d s an d a d v e n ­ titious sounds is presented, and its use in diagnosis outlined. F inally, the value o f auscultation to the physio­ therapist is discussed. IN T R O D U C T IO N T h e invention o f the stethoscope by L aennec in 1818 was a m ajo r advance in m edicine; the instrum ent allow ed diagnosis and assessm ent of cardio-respiratory disorders to be carried o u t w ith precision not previously attainable, an d to this day th e stethoscope rem ains an indispensable p a rt o f th e physician’s arm am entarium . F o r physiotherapists, how ever, the situation is n o t so clear cut. A substantial p ro p o rtio n of physicians and surgeons view w ith deepest suspicion th e sight o f a physiotherapist auscultating a p a tie n t’s chest. In the eyes of these critics it verges on blasphem y fo r th e physio­ therapist to own a stethoscope, or to carry one ab o u t as a m a tte r o f routine. T h e opposite view point is held by m any m em bers o f the physiotherapy profession, believing au scu ltatio n to be very m uch their business, and a valuable source o f info rm atio n p ertin en t to the perform ance o f their w ork. I do n o t propose to enter into th e pros an d cons of this controversy. I consider it beyond dispute, how ever, th a t if the stethoscope is to be used by physiotherapists, it should be used correctly, w ith insight an d understanding. T his article is w ritten to help achieve this end. TH E S T E T H O S C O P E (G reek stethos, the chest; skopeein, to explore). T h ere are th ree com ponents o f the m odem steth o ­ scope. T hey are th e chest piece, th e tubing and the binaural. T h e chest piece may b e o f th e bell type, diaph ragm type, o r a com bination o f these two. T h e two are really variants o f the sam e principle — th a t o f a dam ped diaphragm system . T h e area of skin enclosed by the bell behave as a diaphragm , th e tautness of w hich m ay be varied b y the pressure applied. T h e firm ly applied * R espiratory C linic, G ro o te Schuur H ospital, C ape T o w a R eceived 16 July 1980. O PSO M M 1N G D ie steto sko o p en d ie geb ru ik daarvan w ord beskryf. ’n H uidige klassifikasie van asem halingsklanke, stem - hebbende kla n ke en b yko m en d e klanke w ord voorgestel, en die gebruik daarvan in diagnostiek w ord om skryf. T en slotte w o rd die waarde van ouskultasie vir die fisioterapeut bespreek. bell th erefore subtends an area o f ta u t skin which behaves in a sim ilar way to the diaphragm chest piece; it filters out low frequency sounds, allow ing the higher frequencies to com e through. I t should be rem em bered, though, th a t the volum e of sound is related to the area of the chest piece, so th at th e diaphragm chest piece, being larger, will in general p roduce a higher am plitude of sound than the firmly applied bell. In contrast, the softly applied bell subtends an area of lax skin which will have a m uch low er reso n an t frequency, favouring the transm ission of low frequency sounds. H e a rt sounds, and som e cardiac m urm urs, are in the low er frequency range (2 0 - 1 1 5 cycles/sec), so that cardiologists will usually prefer the softly applied f j , when auscultating th e heart. NL-*" Lung sounds, however, and especially abnorm al lung sounds, are in th e higher frequency range (200 - 2 000 cycles/sec), so th a t use of the diaphragm is generally preferable, although the firmly applied bell could be used. A dvantages of the diaphragm include the higher am plitude o f sound, as alread y m entioned, and the easier application over an uneven or bony chest cage, w here incom plete contact betw een skin and th e rim of the bell w ould result in com plete loss o f sound. T he firm ly applied bell m ay be useful on occasion; for exam ple, in confirm ing the presence o f fine adventitious sounds which can som etim es be generated artificially by m ovem ent o f the diaphragm on the skin surface. T he bell is also useful in children, for w hom the diaphragm m ay be inappropriately large, although p aediatric stethoscopes are available. T h e tubing is o f considerable im portance in the efficiency o f a stethoscope. Sound loss can result from the use of incorrect dim ensions or m aterials. T h e m ate­ rial should be firm, inert, reasonably thick and polished in its in tern al bore fo r m axim um transm ission. Loss of high frequencies can result if the volum e o f the system is too large o r if the diam eter o f the tubing is too fine. A good- com prom ise is T ygon tubing, as used in the R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )