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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



meet predominantly, if not exclusively, .is capillaries 

 in the region of the alveolar ducts (121). The distal 

 ramifications of the pulmonary artery supply the 

 alveoli; the bronchial arterial distribution is to the 

 substance of the bronchi, vessels, and interstitial tis- 

 sue. There are at least two forces that tend to prevent 

 the onflow of blood within the terminal bronchial 

 arterial radicals to the alveoli. The first of these is the 

 frictional resistance offered by the narrow vessels; 

 the second is the counterpressure transmitted from the 

 terminal branches of the pulmonary artery. With 

 interruption of the major pulmonary artery the 

 second of these forces is abrogated, whereupon further 

 onflow of blood takes place in the bronchial vessels. 

 As the flow increases in these vessels they become 

 larger, by mechanisms still largely mysterious. 

 Thus there is increasing access to the low resistance 

 capillary bed of the lung. The course of the blood flow 

 is from the aorta through the bronchial arteries to the 

 pulmonary capillaries, pulmonary veins, and left 

 atrium. This represents a left-to-left recirculation of 

 blood. The flow increases from an initial value of less 

 than 25 ml per m- per min to volumes in excess of 

 1 liter per m 2 per min in a period of approximately 

 18 months. In many respects this shunt is analogous 

 to an arteriovenous fistula between the aorta and left 

 atrium (109). As in the case of peripheral arterio- 

 venous fistula, increase in flow is continuous for very 

 long periods. Furthermore, the flow is many times 

 in excess of the "need" of the tissue, evidence that it is 

 induced primarily by the mechanical effect of the 

 low peripheral resistance introduced into the systemic 

 arterial circuit. Collateral flows of considerable 

 magnitude have also been found in the lungs of 

 patients in whom pulmonary arteries had been 

 ligated because of operative misadventure (52, 56). 



Great anatomical enlargement of the bronchial 

 arteries occurs in such congenital lesions of the heart 

 and great vessels as result in a low pulmonary arterial 

 pressure, i.e., tricuspid or pulmonic atresia, or stenosis 

 and tetralogy of Fallot. The collateral circulation 

 tends to be greatest where there is a right to left 

 shunt with polycythemia and a consequently greater 

 tendency to further obstruction of pulmonary ar- 

 teries by thrombosis. Ring and his co-workers (19) 

 were pioneers in attempting measurements of the 

 collateral blood flow in these conditions, although 

 their methods were subject to certain errors. In a 

 series of 38 patients with tetralogy they found col- 

 lateral flows to exceed 500 ml per m'- per min in 

 26 patients, 1 liter per m- per min in 16, and 1.5 liter 

 per m- per min in 8. 



The problem of the genesis of the collateral circu- 

 lation is perhaps simpler on the venous side. The 

 major pulmonary veins and the true bronchial veins 

 that drain into the azygos venous system and right 

 side of the heart are always connected by short pre- 

 capillary stems (199). With occlusion of a major pul- 

 monary vein near the hilum the pressure within it 

 increases, and with this the vis a tergo forcing blood 

 into the connected bronchial venous stems becomes 

 greater. Consequently, blood flow in the stems in- 

 creases and again they become remarkably enlarged 

 (86, 87). Here flow becomes greater not because 

 peripheral resistance is reduced but because pressure 

 at the source is increased. The moment of force again 

 is mechanical, however. Numerous analogues exist in 

 the systemic venous circulation. 



neural factors. There is evidence that the nervous 

 system can exert a control both on the immediate and 

 even late development of collateral circulation. At 

 least in part this is related to its influence on the 

 musculature of vessels. 



As early as 1876 Latschenberger & Deahna (94) 

 observed that sectioning of the major nerves of the 

 thigh in experimental animals prevented collapse of 

 the segment of femoral artery distal to the point of 

 clamping beneath Poupart's ligament; thus the blood 

 pressure did not drop when the artery was opened, as 

 it usually did in the presence of intact nerves. Stefani 

 ( 1 70) found that denervation of an extremity in the 

 salamander was associated with gangrene upon liga- 

 ture of the axillary artery, while neither the ligature 

 itself nor denervation alone produced this untoward 

 result. Nothnagel (129), however, observed that a 

 much greater collateral circulation developed in a 

 period of 8 weeks in an extremity of the rabbit after 

 ligation of the femoral artery when the sciatic and 

 crural nerves had also been transected than when the 

 nerves were intact. He also noted that the more 

 expanded vessels were structurally altered in that 

 they contained more muscle (fig. 9). 



That it was the influence of the sympathetic 

 nervous system specifically that accounted for this 

 observation was suggested by Ferris & Harvey (50), 

 who recorded a sudden rise in the temperature of the 

 dog's extremity some 4 hours after ligature of the 

 femoral artery. That this was the result of a reflex 

 vasodilatation was previously inferred from Halsted's 

 (69) observation of a remarkable increase in the tem- 

 perature of the upper extremity of a man following a 

 resection of a large subclavian aneurysm and ligature 

 of the left subclavian and axillarv arteries. Halsted 



