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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. i 7. Sketch of air sac and its vessels in the lung of a cat. 

 [From VVearn et al. (128).] 



network seen on edge. The smaller capillaries anasto- 

 mosed frequently with each other and appeared to 

 contract and relax. 



In 1933, Daly (38) reviewed pulmonary circulation 

 and briefly mentioned microscopic observations of 

 pulmonary vessels, but these studies were concerned 

 primarily with the response of small vessels to epi- 

 nephrine. There were no papers in which normal 

 vascular patterns were described. 



Wearn et al. (128) introduced a method for the 

 observation of minute vessels in the lung which 

 utilized a quartz rod, for transmitting light, placed 

 inside the chest cavity. The vessels were observed 

 microscopically in the lung tip through a window in 

 the chest wall. Observations were made on both 

 moving and immobilized lungs. Arteriolar vessels 

 showed pulsatile flow and steady flow, a different 

 type of flow often occurring in two arteriolar branches 

 from the same parent arteriole. Arterioles were seen 

 to contract and relax, and reversal of the direction of 

 blood flow was common. The walls of the capillaries 

 were invisible, so that the caliber of the vessel and its 

 course were determined by the column of blood it 

 contained. Capillary vessels were seen to branch and 

 anastomose frequently (see fig. 17). Wearn et al. 

 report spontaneous opening and closing of capillaries, 

 but the fact that the differentiation between arteriole 

 and capillary was based on the number of blood cells 

 which the vessel would accommodate (three or less 

 for a capillary) throws some doubt on the validity of 

 this statement. Also, the suggestion is made later in 

 the report that intermittent flow in capillaries was 



probably due to changes in the flow of arterioles 

 from which the capillaries arose and that no proof of 

 contraction of capillary walls was obtained. The 

 capillary network, depicted by Wearn et al. as covering 

 an alveolus, is very similar to that in many other sites. 



The next detailed report of vascular architecture of 

 the lungs appeared in 1954, when Irwin and his 

 associates (66) published the results of microscopic 

 observations on guinea pigs and rabbits. The tech- 

 nique used was that of transillumination with a 

 quartz rod and oxygen insufflation to prevent re- 

 spiratory movements. These investigators traced 

 pulmonary arterioles to terminal pulmonary arterioles 

 which branched to form capillaries. Blunt termina- 

 tions of pulmonary arterioles, which lie in the septa 

 between alveoli, gave rise to capillaries which then 

 spread over several adjacent alveoli. Pulmonary 

 capillaries were seen to be completely lined cylindrical 

 tubes which branched and anastomosed to form 

 intricate networks over the surfaces of alveoli. The 

 capillary network was often supplied by several 

 terminal arterioles and each network was drained by 

 more than one venule. Arteriovenous shunts were 

 found between a pulmonary arteriole and venule that 

 ran side by side in an alveolar septum. 



Additional observations by Irwin & Burrage (67) 

 were that the diameters of arterioles and venules 

 changed size when measurements of their walls were 

 made over long periods of time, affecting the flow of 

 blood. Alterations in the diameters of the arterioles 

 were more marked than venular changes. Irwin and 

 Burrage report that the walls of the capillaries cover- 

 ing an alveolus were seen to come together to ob- 

 literate their lumina. They suggest that, although the 

 intermittent blood flow in pulmonary capillaries 

 could be due to contraction of either arterioles or 

 venules, the possibility of activity in the capillary walls 

 must be considered. That capillary walls might 

 contain contractile tissue, or that the endothelium 

 lining the cells might swell to block the lumina are the 

 possible means offered by the authors for causing 

 intermittent flow in pulmonarv capillaries. 



A surprisingly small number of investigations of the 

 normal vascular structures and flow of blood in 

 microscopic pulmonarv vessels have been carried out. 

 However, Wearn's, diagrams (128) and the descrip- 

 tions by Irwin et al. (66) of capillary networks covering 

 alveoli indicate that this terminal vascular bed is made 

 up of the same structural components with the same 

 basic form as that of beds in other tissues and organs. 

 Perhaps further investigations will explain the ap- 

 parent closure of "capillary" walls seen by Irwin & 



