DYNAMICS OF PULMONARY CIRCULATION 



■675 



fibers, suggesting an arrangement capable of con- 

 trolling either the pressure of the wall on its contents 

 or the volume of blood contained within the large 

 vessel (198). In general, the structure of the large 

 vessels seems better suited for varying their distensi- 

 bility than their geometry; nonetheless, the possibility 

 exists that constriction of large vessels may also effect 

 pulmonary vascular resistance to blood flow (126). 



In the normal human lung the pulmonary arteries 

 are end-arteries, continuing without branching to the 

 level of the first alveoli in the walls of the respiratory 

 bronchioles (292, 421). Unfortunately, too little is 

 known of the pattern of branching to serve as a re- 

 liable basis for predicting the distribution of resistance 

 along the length of the pulmonary vascular tree 

 (140, 169, 205). The arterial portion of the pulmonary 

 circulation lies adjacent to the bronchial tree; indeed, 

 in the region of the respiratory bronchiole, arterial 

 bifurcations straddle the airway (117). Consequently, 

 the arterial branches are more susceptible to passive 

 distortion by the conducting airways than are the 

 venous branches which are situated at the periphery 

 of the lobule. 



The pulmonary veins are end-veins (421). Their 

 musculo-elastic components are more irregularly 

 dispersed than those of the corresponding pulmonary 

 arteries; their media contain more collagenous fibers. 

 At the entry of the veins into the left atrium, exten- 

 sions of cardiac muscle become incorporated into 

 the venous walls. The suggestion has been made that 

 under certain experimental conditions, these muscular 

 extensions may act as "throttles" (56, 121, 392, 394). 



small muscular pulmonary vessels. From the point 

 of view of vasomotor activity, three concepts are 

 generally held: /) vascular smooth muscle is pre- 

 requisite for active change in caliber; 2) during a 

 change in vasomotor tone, the small, muscle-con- 

 taining vessels are the site of changed resistance; and 

 j) the thicker the media, the more apt is the vessel to 

 constrict, the less apt is it to undergo passive dilation, 

 and the more likely is it to offer appreciable resistance 

 to perfusion (59, 141). 



The anatomical characteristics of the small muscu- 

 lar pulmonary vessels are illustrated in figure 8. The 

 upper half of this figure depicts the structure of ex- 

 ceedingly small (30 n) pulmonary vessels: in neither 

 the pulmonary "arteriole" or venule is smooth 

 muscle discernible; by way of contrast, the coat of 

 smooth muscle in the systemic arterioles is readily 

 apparent. The lower half of this figure contrasts a 

 small pulmonary artery and a small pulmonary 



vein — each about 50 ^ in diameter — with a systemic 

 arteriole of approximately the same size; pulmonary 

 arterioles of this size are to be found at the level of 

 the alveolar ducts and alveoli, buried in pulmonary 

 tissue (118). It may be seen that the pulmonary 

 arteriole contains only a thin rim of smooth muscle; 

 in the corresponding pulmonary venule of 55 /j, no 

 smooth muscle can be recognized; on the other hand, 

 the systemic arteriole contains a thick media. It is 

 difficult to imagine the pulmonary vessels shown in 

 figure 8 as the sites of intense vasoconstriction. 



Somewhat better suited for vasomotor activity 

 are the larger precapillary vessels. These "small 

 muscular arteries" range from 100 to 1000 n in 

 diameter (403), contain well-formed media, and lie 

 adjacent to the respiratory bronchioles. They are 

 usually separated from the pulmonary tissue by 

 perivascular lymph spaces and their muscular coats 

 thin as they proceed peripherally to the vicinity of 

 the alveolar ducts. From these muscular vessels, the 

 pulmonary arterioles generally arise at right angles 

 so that the configuration of muscle at their origins 

 often appears sphincteric (118, 196). 



The corresponding venules of 100 to 1000 y. lie 

 at the periphery of the lobule. And, in contrast to 

 the small muscular arteries, smooth muscle is either 

 poorly organized or absent and the elastic fibers are 

 irregular and indistinct. Consequently, even pul- 

 monary veins up to 1000 /* in diameter seem to be 

 poorly equipped for vasomotor activity. 



capillaries. At the alveolar border, the precapillary 

 vessel subdivides to form a racemosing network of 

 capillary segments sandwiched between adjacent 

 alveolar walls (fig. 9, insert) (292). Whether these 

 capillaries lie free between the alveoli or indent 

 them — a structural distinction relevant to estimates 

 of pericapillary pressure — is uncertain. 



The capillary circulation has certain distinctive 

 features: a) each of these capillary segments is ap- 

 proximately 10 to 14 p in length and 7 to 9 n in 

 diameter (422); b) except in congested lungs the red 

 cells pass through in single file (fig. 9); c) the capillary 

 networks in different parts of the lung differ with 

 respect to the length, caliber, and number of con- 

 stituent vessels (162, 292); d) "pores," presumed on 

 physiological grounds to exist in the pulmonary 

 capillary wall, have not been seen by electron mi- 

 croscopists; e) chemical analyses have failed to settle 

 if the capillary wall is predominantly aqueous or 

 lipoid in nature (375); /) there appear to be neither 

 contractile cells around the capillaries nor smooth 



