1706 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



the pressure drop across the pulmonary vascular bed 

 may occur in the capillaries (fig. 36) (373). Con- 

 sistent with the latter view are some physiological 

 observations on the isolated lung (318). While such 

 calculations and observations cannot define the major 

 site of pulmonary vascular resistance under natural 

 conditions, they do emphasize that the evidence 

 favoring the precapillary segments is not on firm 

 footing. 



Under certain circumstances, the capillaries do 

 become the major site of pulmonary vascular re- 

 sistance. The most common circumstance is the 

 artificial increase in alveolar pressure in the course of 

 positive pressure breathing (191, 306). It is also 

 possible to imagine such a role for the capillaries in 

 those natural circumstances in which the left atrial 

 pressure happens to fall below alveolar pressures. 

 Such a condition presumably exists in the apical 

 alveoli of the upright human subject and is exag- 

 gerated when the standing subject takes a deep 

 breath. 



In isolated lungs perfused at physiological levels 

 of blood pressure, the pulmonary capillaries appear to 

 be less distensible than the larger pulmonary vessels 

 (124). However, physiologically meaningful measure 

 ments of the distensibility characteristics of the 

 pulmonary capillaries are difficult to obtain for a 

 variety of technical reasons, including the inacces- 

 sible location of the capillaries, the difficulty of 

 making static measurements under in vivo, dynamic 

 conditions, the difficulty in reproducing the natural 

 capillary pressures in experimental preparations, and 

 the uncertainty concerning the distensibility charac- 

 teristics of the pericapillary tissues because of the 

 propensity of the isolated lung to develop pulmonary 

 edema. 



Time Spent by Blood in Pulmonary Capillaries 



Stephen Hales seems to have been the first (1733) 

 to pay serious attention to the rate at which blood 

 flows through the pulmonary capillaries; coupling 

 direct observation with simple arithmetical calcula- 

 tion, he estimated this rate to be approximately 1.4 

 mm per sec in the frog lung (1 78). Curiously enough, 

 the elaborate techniques of the twentieth century 

 (cine-microphotography with lamp black as a tracer 

 in the exposed lung) have provided similar values for 

 the cat (1 to 2 mm/sec) (414). 



The latter observations on the cat are the only 

 direct observations on the time spent by particles in 

 the pulmonary capillaries (o. 1 sec). All other esti- 



mates represent calculations and assumptions based 

 on measurements of alveolar capillary gas exchange. 

 The most commonly cited values are those of Rough- 

 ton, based on the kinetics of the combination of 

 carbon monoxide and hemoglobin in man. Originally, 

 this approach indicated that, on the average, a unit 

 of blood spends approximately three-quarters of a 

 second in gas exchange in the pulmonary capillary 

 at rest, and somewhat less during exercise (364); 

 subsequent refinements in methodology have sug- 

 gested that the contact time at rest may be a little 

 longer, i.e., of the order of 1 sec (223, 317, 365). 

 Other calculations, based on the analysis of the 

 alveolar-arterial oxygen gradient have yielded lower 

 values: 0.18 sec in the dog (295) and 0.23 to 0.5 sec 

 in resting man (295, 396); however, theoretical con- 

 siderations suggest that this approach tends to under- 

 estimate the time of contact (318). Finally, anatomical 

 considerations have led to the low contact time of 

 0.1 sec (301). At first encounter, this is a discouraging 

 span of values. But, in view of the wide variations in 

 methodology, assumptions, and types of calculations, 

 this range of about 0.1 to 1.0 sec in resting man is 

 surprisingly small and, when duly weighed, the 

 generally accepted value of three-quarters of a sec 

 to 1 sec at rest seems quite reasonable. Obviously 

 lacking are simultaneous measurements of contact 

 time by the physiological methods and by direct 

 observation of the exposed lung in the same animal 



(143)- 



The time spent by a unit of blood in the pulmonary 



capillary depends on various hemodynamic influences. 

 Paramount among these is the relationship between 

 the stroke output of the right heart and the pulmonary 

 capillary blood volume; in resting man these two 

 values are of the same order of magnitude. Another 

 determining influence is the nature of the pulmonary 

 capillary blood flow: pulsatile pulmonary capillary 

 flow causes some red cells to spend less time in the 

 pulmonary capillaries than others. Finally, values 

 based on alveolar-capillary gas exchange and the 

 rate of combination of test gases with hemoglobin 

 may misjudge actual contact times; for example, 

 actual time would be expected to differ from calcu- 

 lated time if, as is customarily done, the calculations 

 assume that the hematocrit of pulmonary capillary 

 blood is identical with that of blood sampled from 

 large systemic vessels (143, 317). 



These theoretical considerations may have practical 

 meaning. Ordinarily, the time spent by each unit of 

 blood in the pulmonary capillary is more than ample 

 for complete oxygenation, both at rest and during 



