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fig. 8. Pressure and oxygen tension in the lower extremity of 

 a dog immediately after occlusion of the superficial femoral 

 artery. The pressure in the anterior tibial artery falls, but 

 within 3-4 min rises to a plateau below the initial level which is 

 thereafter maintained. The tissue pOo drops only slightly and 

 temporarily. [From Winblad el al. (192).] 



portion of a right femoral artery were connected by 

 catheter to the left femoral artery, so placed that the 

 catheter lay in the region of re-entry of collaterals and 

 obstructed these, the collateral vessels nevertheless 

 expanded. Schoop & Jahn (157) stress that, for the 

 later development of collaterals, flow is rather more 

 important than differential pressure. 



The appearance of alternative arterial channels to 

 peripheral capillary beds is well exemplified in 

 coarctation of the aorta, where collateral circulation 

 was studied centuries ago. 



Valuable insights into the mechanisms governing 

 the development of collateral circulation have come 

 from the thorough and long-continued observations 

 of peripheral arteriovenous fistulas, both clinical and 

 experimental, by many observers. As early as 1 756 

 and again in 1761 William Hunter published on this 

 subject and noted the appearance of a collateral circu- 

 lation in relation to the fistulas (84, 85). Mont Reid 

 (144) and Emile Holman (77, 79), in particular, con- 

 tributed important information. Holman's major 

 point is that as soon as these fistulas open, there is a 

 fall of pressure in the arterial segment distal to the 

 fistula, as indicated by the cephalad flow of blood 

 within it. The peripheral resistance to onflow of blood 

 in smaller arteries circumventing the fistula is there- 

 fore reduced, whereupon these vessels begin to carry 

 more blood. If the artery distal to the fistula is ligated 

 or even if its lumen is reduced to one-half by means of 

 a constricting aluminum band, the collateral circula- 

 tion is relatively meager. The most telling experiment 



against the idea of "'tissue need," as a determinant of 

 the expansion of the collateral arteries, is that ampu- 

 tation of the extremity does not reduce the extent of 

 the collateral circulation associated with an arterio- 

 venous fistula. It appears that the effect of the fistula 

 in stimulating a collateral circulation is greater than 

 simple ligature of the artery to a limb. 



Deterling el al. (44), however, cautioned that the 

 abundant collateral circulation related to an arterio- 

 venous fistula may be to a great extent spurious, being 

 rather an enormously dilated plexus of veins that 

 does not function as collateral circulation after 

 obliteration of the fistula. If the arteriovenous fistula 

 is excised, the collateral, according to these observers, 

 is usually no greater than that of controls after simple 

 ligature of the artery. They also observed that sym- 

 pathectomy tends to insure a greater collateral. 



A closer definition of conditions at an arteriovenous 

 fistula has been given by Holman & Taylor (79). The 

 development of a large collateral circulation was 

 stated to depend on the existence of a fistula larger 

 than the proximal artery and also on the presence of a 

 widely patent artery distal to the fistula. Flow through 

 the fistula can increase progressively, provided that 

 the proximal artery and vein remain distensible. Sir 

 Thomas Lewis (98) presented evidence, based on 

 calorimetry, that the blood flow to a leg that had been 

 the seat of an arteriovenous fistula for 18 years was 

 actually greater than that of the normal side. 



Because of its double blood supply, the lung offers 

 a useful stage upon which various factors controlling 

 collateral circulation can be analyzed. Expansion of 

 this circulation can occur under various circum- 

 stances of disease. Moreover, it can easily be induced 

 experimentally and measured with considerable 

 accuracy, at various stages, by application of broncho- 

 spirometry and blood gas analysis, or by use of dye 

 distribution techniques. Finally, it is possible to dis- 

 tinguish with confidence certain collaterals as newly 

 formed rather than preformed. 



The relative pressures and flows in the pulmonary 

 and bronchial arteries have been measured and an 

 increase in aortic pressure has been found to augment 

 inflow from the latter (15, 26, 151). 



When the main pulmonary artery to one lung is 

 ligated in the dog, expansion of the collateral circula- 

 tion continues for at least 18 months (20). The mecha- 

 nism is only partly understood, but its major features 

 appear to be as follows: 



Under ordinary circumstances in the normal lung 

 the bronchial and pulmonary arterial circulations 



