DYNAMICS OF PULMONARY CIRCULATION 



'7°5 



Size o] Pulmonary Capillary Bed 



Although the extent of the pulmonary capillary 

 bed in a living subject cannot be expressed in absolute 

 units, a change in area can be detected from con- 

 secutive measurements of the pulmonary diffusing 

 capacity. Such measurements, using either oxygen or 

 carbon monoxide as the test gas have shown that: 



a) not all of the available area is in use at rest; and 



b) the capillary area involved in gas exchange in- 

 creases progressively under a variety of circum- 

 stances, e.g., exercise. The major mechanism involved 

 in increasing the area is an increase in transmural 

 pressure (357). The precise way in which this distend- 

 ing pressure is increased varies from circumstance to 

 circumstance. Thus, in some conditions, an increase 

 in capillary blood volume is involved; in others, the 

 perivascular pressures may decrease; at high lung 

 volumes, the capillaries may even be passively- 

 stretched. As expected from theoretical considera- 

 tions, the diffusing capacity is little affected by 

 changes in pulmonary blood flow; only when pul- 

 monary blood flow is severely curtailed does the 

 diffusing capacity decrease (358, 407). Pneumo- 

 nectomy generally (143), but not invariably (89) de- 

 creases the diffusing capacity. 



The maximum diffusing capacity is of interest as 

 a measure of the maximum available pulmonary 

 capillary area. It has been suggested that this max- 

 imal value is reached at a level of blood flow which 

 corresponds to the steep inflection of the flow-pressure 

 curve (fig. 34). However, the experimental support 

 for this hypothesis is inconclusive (348). 



Pulmonary Capillary Blood Volume (Q.,.) 



On the basis of measured differences between the 

 rates of reaction of carbon monoxide with hemoglobin 

 solutions at different oxygen tensions, and the calcu- 

 lation of the average time spent by blood in traversing 

 the pulmonary capillaries, the volume of blood in the 

 pulmonary capillaries of the normal resting subject 

 was originally calculated to be of the order of 60 to 

 75 ml (364, 365); during severe exercise, Qc increased 

 somewhat (to approximately 90 to 100 ml) (223, 

 364). More recent measurements and calculations of 

 the same type have raised the value of the resting 

 Qc to approximately 100 ml, both for the normal 

 subject (15, 244) and for the patient with mitral 

 stenosis (15). At present, there is no way to decide 

 how much of the reported variability is artificial 

 (143, 365) and how much is a consequence of either 



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 _1 0: 



- 200 



TTTTTTT fTTf 



PRE CAPILLARY 



1 T 1 1 1 1 1 1 1 



POSTCAPILLAR Y 



fig. 36. Hypothetical relationship between blood pressure 

 (solid line,) and cross-sectional area (shaded) in the pulmonary 

 vascular bed of the dog. The blood pressure is represented as 

 per cent of initial value. Vascular diameters at key points are 

 indicated by asterisks. According to this schema, the major drop 

 in blood pressure occurs in the region of the pulmonary capil- 

 laries (shaded spike). [Based on Schleier (373).] 



biological differences between subjects or the effects 

 of the breathing maneuvers which are part of the 

 tests (244). A variety of agents and procedures seem 

 to be capable of passively altering Q c (262). 



In the dog, anatomical measurements suggest that 

 the pulmonary capillaries may contain 10 per cent 

 of the total volume of blood between the right ven- 

 tricle and left atrium, i.e., of the order of 1.2 per cent 

 of the total circulating blood volume (169). For the 

 human lung, Weibel found that Q c varies with the 

 lung volume and with the degree of capillary filling. 

 In his preparation, which involved negative (pleural) 

 pressure inflation of the lung and fixation in formalin 

 vapor, Q c was 150 to 200 ml, i.e., approximately 

 twice the volume obtained by physiological measure- 

 ments. Part of the difference between the anatomical 

 and physiological measurements may be the degree of 

 inflation of the lung (422). 



Resistance and Distensibility 



How much of the pulmonary vascular resistance to 

 perfusion lies in the pulmonary capillary bed is a 

 matter of opinion (66). The prevalent notion is that, 

 under ordinary conditions, the resistance function 

 resides in the small, muscular precapillary vessels. 

 On the other hand, calculations of resistance based 

 on anatomical measurements and assumptions have 

 raised the possibility that a large part (up to half) of 



