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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 29. Continuous records of pulmonary arterial pressure 

 (P) and flow (Q) from a closed-chest, unanesthetized dog. 

 Blood pressure was recorded through a polyvinyl tube (encased 

 in nylon) inserted through the wall of the main pulmonary 

 artery about 1 cm distal to pulmonic valve. Blood flow was re- 

 corded by an electromagnetic flow meter (modified Kolin 

 type) placed proximal to the bifurcation of the main pulmo- 

 nary artery. Tubing and wires placed surgically and led to out- 

 side between scapulae. For calibration of flow meter, snares 

 around right and left pulmonary arteries were tightened to 

 arrest pulmonary arterial flow. (Courtesy of L. Fisher and 

 D. E. Gregg.) 



assumption that it simulated the geometry and dis- 

 tensibility of the pulmonary circulation in vivo. 

 Within the last few years, investigators have begun to 

 take a more realistic view of the pulmonary circula- 

 tion, recognizing that hemodynamic events within it 

 vary from instant to instant and that phasic differ- 

 ences between pressure and flow (fig. 29) are 

 important; in order to treat these phasic events, they 

 have resorted to a model based on electrical a-c 

 theory (71, 177, 438). However, for the moment, this 

 approach is handicapped by the technical difficulties 

 of recording pulsatile pulmonary blood flow, espe- 

 cially in living systems (153, 237). 



Distensibility and Capacity: 

 Pressure- 1 olume Relationships 



Because of the manner in which the pulmonary 

 circulation is incorporated into the lung, the term 

 "pulmonary vascular distensibility" is a composite 

 one : it connotes not only the elastic properties of the 

 vascular walls but also the tone of their smooth 

 muscle, the perivascular air pressures, the effects of 

 hidden forces such as alveolar surface tension (78, 

 312), the presence of excessive interstitial fluid (239), 

 and the mechanical distortions of adjacent pulmonary 



tissues (146). As in the systemic circulation, the dis- 

 tensibility characteristics are customarily expressed 

 as the change in vascular volume per unit change in 

 transmural pressure. However, in contrast to the 

 systemic circulation, the small precapillary vessels 

 are thin-walled and easily distensible, thereby con- 

 tributing to the pressure-volume characteristics of the 

 pulmonary arterial tree (350). This participation of 

 the pulmonary '"resistance" vessels in the "capaci- 

 tance" function of the pulmonary circulation is of 

 hemodynamic significance: for example, without 

 pulmonary arteriolar sphincters, a larger fraction of 

 the right ventricular stroke volume is apt to escape 

 from the pulmonary arterial tree during and just 

 after each systole than from the systemic arterial tree 

 (240); also, during bradycardia the pulmonary ar- 

 terial pressure may fall to the level of pulmonary 

 venous pressures (187). 



The distensibility characteristics of the pulmonary 

 circulation, and of its individual segments, have been 

 determined under a wide variety of experimental 

 conditions, using greatly different types of prepara- 

 tions. These studies have led to certain generaliza- 

 tions: a) the pressure-volume characteristics of the 

 entire vascular tree (fig. 30) and of the large pul- 

 monary vessels resemble those of a large systemic 



vein (148, 211, 



290); b) as in other distensible 



structures, the blood pressure at any volume is higher 

 when the system is being filled than when it is being 

 emptied ("hysteresis," "delayed compliance," "stress- 



50-- 



fig. 30. Pressure-volume relationship of the pulmonary 

 vascular bed in the dog. To construct this curve, blood was 

 withdrawn at 10-sec intervals after initially elevating pressure 

 in the system to approximately 60 mm Hg. [After Sarnoff & 

 Berglund (371 ).] 



