DYNAMICS OF PULMONARY CIRCULATION 



l68 3 



nomena, including the bizarre "'butterfly" shadows 

 of pulmonary edema (202) and the maintenance of 

 the virtually normal oxygenation of peripheral arterial 

 blood in patients with atelectasis and pneumonia (85). 



Much more relevant to the performance of the 

 lung in gas exchange is the distribution of the pul- 

 monary capillary blood with respect to alveolar 

 volume, alveolar ventilation, and pulmonary diffusing 

 surfaces. In the normal lung, these parameters are 

 ordinarily quite precisely balanced (131, 145, 427); 

 in disease, the upsets may be quite striking (58). 

 Two approaches are in popular use for relating pul- 

 monary capillary perfusion to alveolar ventilation: 

 the determination of the pattern of change in the 

 alveolar composition of a respiratory gas (284) and 

 of the respiratory exchange ratio (428) during a 

 single expiration; the determination of the rate of 

 increase in the peripheral arterial oxygen saturation 

 (314) and tension (131) during oxygen breathing. 

 It has been pointed out elsewhere that each of these 

 approaches has its own uncertainties (428). 



The comparison of blood flow through different 

 parts of the lungs has generally involved either 

 bronchospirometry or regional sampling of alveolar 

 gas. Bronchospirometry has been particularly fruitful 

 in comparing the perfusion of the two lungs; thus, 

 simultaneous measurements of the uptake of each 

 lung separately have disclosed that ordinarily each 

 lung receives a share of the cardiac output which is 

 proportional both to its gas volume (29) and to its 

 ventilation (203). Accordingly, in man, the right 

 lung receives 55 per cent of the cardiac output (135); 

 this fraction is decreased when the subject turns on 

 his side so that the left lung is down (29). 



Bronchospirometric comparisons of oxygen uptake 

 have also disclosed that gravity rearranges the 

 distribution of the blood flow within each lung: as 

 the human subject stands, the oxygen uptake of the 

 lower lobes increases at the expense of the upper 

 lobes, indicating a preferential distribution of blood 

 flow to the lower lobes; the change in the pattern of 

 the blood flow occurs even though the distribution of 

 ventilation is little altered by the change in posture 

 (286). It should be noted that the use of broncho- 

 spirometry to detect changes in regional blood flow 

 presupposes that all parts of the lungs are breathing 

 the same inspired mixture; when different parts of 

 the lungs are given different inspired gas mixtures to 

 breathe, the procedures and calculations grow much 

 more complicated since all of the variables in the 

 Fick equation — instead of only the oxygen uptake — ■ 

 have to be determined (135, 221, 410). 



Hemodynamic measurements (247) and analyses 

 of alveolar gas have been consistent with the broncho- 

 spirometric measurements. For example, the alveolar 

 gas analyses have shown that: a) the oxygen tension 

 of the upper lobes exceeds that of the lower lobes 

 (285, 330); b) the carbon dioxide tension of the upper 

 lobes is less than that of the lower lobes (284, 330, 

 388), and c) the respiratory exchange ratio of the 

 upper lobes exceeds that of the lower lobes (428). 

 All these observations are consistent with the clinical 

 belief that high oxygen tension in the apices of the 

 lungs, resulting from inadequate perfusion with 

 respect to ventilation, is responsible for the apical 

 localization of pulmonary tuberculosis (344). They 

 also indicate that if intrapulmonary baroreceptor 

 mechanisms for rearranging pulmonary blood flow do 

 exist at the pulmonary bases, they are easily over- 

 whelmed by the mechanical effects of gravity. 



The combination of xenon 133 and external counting 

 was originally used to estimate the distribution of 

 inspired air (32, 234). Subsequently, oxygen 15 (107) 

 and then oxygen'Mabeled carbon dioxide (427) were 

 introduced to relate the distribution of the perfusion 

 to the distribution of the inspired air. In addition to 

 confirming that in the seated normal subject the 

 lower lobes are much better perfused than the upper 

 (8:1) (428), these studies also expressed, in quantita- 

 tive terms, the spectrum of ventilation-perfusion 

 ratios which exist in the lungs of upright normal 

 man, and showed how the ratios gradually convert 

 from high to low values as the base of the lung is 

 approached. Moreover, although these inhomoge- 

 neities have inevitable consequences for the gas 

 tensions in the regional alveoli and capillaries, they 

 were shown to have little significance for the efficiency 

 of the lung in oxygen uptake or carbon dioxide output. 

 Finally, the intrapulmonary distribution of air and 

 blood was demonstrated to become much more 

 uniform when the normal subject assumed the supine 

 position or when mechanical influences, such as 

 anatomical restriction of the lower pulmonary 

 vascular bed by congestion and fibrosis, counteracted 

 the tendency of gravity to direct blood to the lower 

 lobes in the upright position (107). 



Because the normal lung is too inhomogeneous 

 and too complicated to be treated in simple mathe- 

 matical terms, conceptual models of alveolar-capillary 

 gas exchange have been adopted as practical tools 

 for assessing the adequacy of pulmonary capillary 

 perfusion. One particularly useful model has been 

 the homogeneous "ideal" lung, a figurative lung to 

 which actual inhomogeneities can be referred (266, 



