i6?8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 10. Schematic representation of 

 the disposition of smooth muscle in the 

 terminal portions of the respiratory 

 tree. The cut muscle appears as small 

 nodular structures of various sizes and 

 shapes; it ends at the mouths of the 

 alveoli. [After Baltisberger (8).] 



«r •* 



t ft. J i . rvs To* *" \ x. 





exceedingly small fraction of the cardiac output and 

 that the effect of the bronchial circulation on the 

 behavior of the normal pulmonary circulation is 

 negligible. The evidence for the latter conclusion is 

 of three general types: /) the difficulty encountered 

 by anatomists in finding bronchial-pulmonary ar- 

 terial communications in the normal human or 

 canine lung except by elaborate injection techniques 

 (421); 2) the measurements in the dog during arti- 

 ficial perfusion of the lungs which indicate that the 

 normal bronchial arterial flow is of the order of 1 

 to 2 per cent of the cardiac output (55, 423); and 

 3) the measurements in intact man which indicate 

 that the normal bronchial arterial blood flow is too 

 small to be measured by conventional techniques 

 (139, 152). It should be noted that under some ex- 



ceedingly artificial experimental circumstances, the 

 bronchial circulation in the dog has been found to 

 exert an appreciable hemodynamic effect on the 

 pulmonary circulation (95). However, because of the 

 unusual experimental conditions, these results seem 

 to indicate the ultimate potential of the bronchial 

 circulation rather than its actual performance in life. 

 The systemic blood supply of the lung undergoes a 

 remarkable proliferation in various disorders of the 

 heart and lungs (79, 98, 133, 263) : old vessels enlarge 

 and become tortuous; new vessels appear and join 

 with the old to form bizarre Medusaean patterns. 

 Moreover, in contrast to the normal lung, in which 

 precapillary communications between the two circu- 

 lations are difficult to demonstrate (292, 420), the 

 enlarged precapillary anastomoses between the 



