DYNAMICS OF PULMONARY CIRCULATION 



l68l 



portions of the human pulmonary vascular tree. In 

 keeping with the anatomical evidence for a pre- 

 dominant distribution of nerves to the larger pul- 

 monary vessels, they found that the large pulmonary 

 vessels (of the dog and cow) contain larger quantities 

 of norepinephrine than do the small pulmonary 

 vessels. The greater concentration of nerves in the 

 region of the large pulmonary vessels is consistent 

 with the notion that the pulmonary vascular bed is 

 better innervated for tensing its large vessels than for 

 shrinking the caliber of its small ones (403). However, 

 this attractive idea, which is based on anatomical 

 observations, is inconclusive on several accounts: 

 a) the display of an abundant innervation provides 

 no measure of either the number or the nature of the 

 impulses which the nerves transmit; b) consecutive 

 muscular segments of a single pulmonary vascular 

 unit may be differently affected by a stimulus (95) ; 

 and c) because of its mixed embryological origin in 

 endoderm and mesoderm, pulmonary vascular in- 

 nervation may possess subtle, and as yet undisclosed, 

 features. 



PULMONARY BLOOD FLOW 



In subjects with a normal heart and circulation, 

 the pulmonary blood flow, the pulmonary capillary 

 blood flow and the output of each ventricle (the 

 cardiac output) represent virtually identical quan- 

 tities. In previous chapters of this book, the cardiac 

 output is considered with respect to its measurement 

 (Chapter 17) and control (Chapters 15 and 16); the 

 present section will confine itself to resting measure- 

 ments of pulmonary blood flow, leaving for subse- 

 quent sections the pulmonary capillary blood flow 

 and the behavior of the cardiac output during 

 exercise. 



Normal Values 



For the sake of comparison, cardiac output in man 

 is generally expressed per square meter of body sur- 

 face area (cardiac index): in one representative 

 study, the average cardiac index of a group of basal, 

 postprandial, supine human adults was 3.12 liters/ 

 min/m 2 (sd ±0.40); the corresponding oxygen uptake 

 of this group was 138 ml min/m 2 sd ±14 (87). 

 Unfortunately, even in adults, body surface area is 

 not an ideal standard of reference; it becomes even 

 less reliable when subjects of different age, sex, and 

 body build are compared, since the •'normal" 



values have been derived from a select portion of the 

 adult population. In the unanesthetized dog, the 

 cardiac output per minute is of the order of 150 

 ml per kg (12). It should be emphasized that there 

 are exceedingly few such measurements on the un- 

 anesthetized dog and the values which do exist are 

 far from consistent (303). 



Excitement (discomfort or anxiety) may artificially 

 increase the "basal" cardiac output. This fact has 

 been illustrated by measurements on the unanesthe- 

 tized dog prior to, and following, treadmill exercise: 

 the resting cardiac output, while awaiting the start 

 of treadmill exercise, was higher than the resting 

 cardiac output after the exercise was finished (12). 

 Excitement may continue to operate during the test 

 periods. Fortunately, there are objective criteria 

 which can be used to detect the existence of disturb- 

 ing emotional influences; these include tachycardia, 

 a high respiratory exchange ratio of the expired gas, 

 a high oxygen uptake, and a high pH of systemic 

 arterial blood (136). Transient episodes of emotional 

 stress are apt to introduce appreciable errors into 

 steady-state measurements of flow, particularly by 

 the Fick principle (412, 439); on the other hand, 

 sustained excitement will artificially increase the 

 cardiac output. In the latter instance, the normality 

 of the cardiac output can be appraised by comparison 

 with the simultaneously measured oxygen uptake 

 (fig. 12). Ordinarily, the arteriovenous difference 

 for oxygen is of little help in such an appraisal since 

 its variations at rest approximate the limits of analytic 

 error (e.g., average of 38.4 sd ±6.3 ml per liter 

 (334))- 



Uneven Pulmonary Blood Flow 



The pattern of distribution of the right ventricular 

 output throughout the lung has been examined in 

 several different ways: a) direct inspection of the 

 pulmonary blood vessels; b) fractional, or continuous, 

 analysis of the alveolar component of expired air; 

 c) bronchospirometry or regional sampling of alveolar 

 air; d) external scintillation counting following the 

 breathing of radioactive gases; and e) the use of 

 conceptual models to explain actual respiratory 

 gas exchange. 



Direct inspection of the lung for the determination 

 of the pattern of the pulmonary blood flow has been 

 practiced for at least 90 years (54)- Three types of 

 observations have been made: /) the examination of 

 the surface of the exposed lung in the living animal 

 (21, 105, 419), 2) the postmortem examination of the 



