DYNAMICS OF PULMONARY CIRCULATION 



1677 



fig. 9. Electron microphotograph of human lung. The red 

 cells (RBC) are shown passing single hie through a pulmonary 

 capillary (CAP) between adjacent alveoli (ALV). 19,370 X. 

 [Courtesy of Dr. Councilman Morgan.] Insert: Network of cap- 

 illaries in the walls of the sacculi alveolares. 330 X- [From 

 Miller (292).] 



70 m 2 ; at three-quarters of the total lung capacity it 

 increases further (to the order of 90 m 2 ) (422). 



Extr avascular Smooth Muscle 



Pulmonary smooth muscle is contained not only 

 in the vessels but also in the tracheobronchial tree 

 and in the pulmonary tissue. In man, the neatly 

 organized tracheobronchial smooth muscle continues 

 down to the mouths of the alveoli (fig. 10) where it 

 is in a position to influence passively the pressure in 

 the alveoli and, thereby, the caliber of the capillaries 

 in the alveolar walls (8, 196). Although parenchymal 

 smooth muscle is apparently plentiful in the amphi- 

 bian and reptilian lung (220, 236), and in patients 

 with chronic pulmonary disease (265), the quantity 

 and arrangement of this parenchymal smooth muscle 

 in the normal human lung is unknown. Nonetheless, 

 because of its close association with the elastic network 

 of the lung, parenchymal smooth muscle may con- 

 ceivably affect vascular calibers directly by con- 

 tiguity and, indirectly, by changing the pulmonary 

 lung volume and distensibility. Moreover, since the 

 musculo-elastic system of the lung is nourished by the 

 bronchial arteries, the possibility exists that agents 

 which reach the lungs by way of the systemic circu- 

 lation may change pulmonary vascular dimensions 

 through their effects on extravascular, rather than 

 intravascular, smooth muscle. 



alveolar surface is ordinarily used for gas exchange; 

 nor is all of the capillary circumference in contact 

 with alveolar wall (196). The portion of the available 

 capillary surface which is actually used appears to 

 vary with the total lung volume, the degree of capil- 

 lary filling and the size of the alveoli. At a volume 

 corresponding to three-quarters of the total lung 

 capacity, the capillary network occupies 60 per cent 

 of the alveolar surface and the capillary blood volume 

 is of the order of 200 to 250 cm 3 (422). 



Over the years, anatomical measurements of the 

 capillary surface area have provided exceedingly 

 variable results: values have ranged from 50 m- to 

 140 m 2 (132). Some of this discrepancy is undoubtedly 

 attributable to methodological differences (143), to 

 the uncertainties of reconstructing the lung on the 

 basis of small sections, and, particularly, to the failure 

 to specify the lung volume at which the measure- 

 ments were made. The recent measurements by 

 Weibel indicate that at the resting position of the 

 lung, the capillary surface is of the order of 50 to 



Systemic Blood Supply of the Lung 



In the normal human and canine lung, the bron- 

 chial arteries arise from intrathoracic systemic 

 arteries and deliver oxygenated blood to the walls 

 of the tracheobronchial tree, the supporting frame- 

 work of the lungs and the walls of the pulmonary 

 arteries and veins (133). Accordingly, they are 

 nutrient arteries. In contrast to pulmonary arteries 

 of equal caliber, the bronchial arterial walls are 

 thick and their innervation is plentiful. In the normal 

 lung, bronchial venous blood drains largely into the 

 azygous veins but some also enters the pulmonary 

 veins (263, 445). 



The quantity of blood carried to the lungs by the 

 bronchial arteries is difficult to measure precisely; 

 the complexity of the problem may be inferred from 

 the wide variety of experimental approaches which 

 have been attempted in both dog and man (93, 133 

 214, 368). Nonetheless, despite inevitable differences, 

 the results of these diverse trials suggest that the 

 bronchial arterial flow ordinarily constitutes only an 



