DYNAMICS OF PULMONARY CIRCULATION 



'673 



J^iA*tZ&u 



fig. 6. Joseph Barcroft (1872-1947) (left) and Lawrence J. 

 Henderson (1878-194.;) (right) photographed in September 

 1936. (Courtesy of D. B. Dill.) 



guinea pig, it is somewhat lower ( 1 14), whereas in the 

 pig, horse, cow, and steer it is often considerably 

 higher (112, 199J. 



The pulmonary blood flow has also been measured 

 in various conventional and unconventional labora- 

 tory animals including the goat (10), the horse (446), 

 and the cow (112, 199). Although, in general, the 

 larger species have the larger pulmonary blood flows, 

 there is no consistent interspecies relationship be- 

 tween pulmonary blood flow on the one hand and 

 either body surface area or weight on the other (22). 

 Whether the disparities represent real biological dif- 

 ferences, or the inadequacies of weight and body sur- 

 face area as standards of reference, or artifacts arising 

 from trying experimental situations, remains to be 

 decided. 



EXPERIMENTAL ANIMALS AND TEST PREPARATIONS 



The normal pulmonary circulation has only been 

 studied in a few dogs and humans. Instead, most of the 

 observations have been made on the pulmonary cir- 

 culations of anesthetized animals, of artificial prep- 

 arations, and of patients with heart and lung disease. 



Each of these three categories is a major deviation 

 from normal : the use of anesthetized animals succeeds 

 admirably in excluding the elements of anxiety and 

 cooperation; but its substitutes, instead, blunted 



vasomotor responses and changing levels of metabo- 

 lism, respiration, and circulation (257). Artificial 

 preparations, such as isolated vascular rings or iso- 

 lated lungs, certainly allow remarkable control of 

 mechanical parameters and may uncover influences 

 which are obscured in the intact organism; but, by 

 severing nervous connections, by failing to pass pul- 

 monary blood through other vital organs, and by 

 depending on abnormal perfusates, impaired nutrient 

 circulations, deteriorating heart and lungs, and ab- 

 normal gas exchange, they may introduce not only 

 discernible — but also hidden — artifacts (95, 141, 183). 

 Finally, while the study of patients with heart and 

 lung disease may be revealing to physicians who are 

 attempting to gain insights into the mechanism of 

 heart strain and failure, the results from these "ex- 

 periments of nature" can rarely be used to predict 

 the behavior of the normal pulmonary circulation, 

 since both heart and lung disease tend to exaggerate 

 the influence of mechanical factors and to alter the 

 structure of the pulmonary blood vessels. 



"Species difference" is a standard apology for 

 atypical responses of the pulmonary circulation to 

 diverse stimuli (95). Occasionally, the basis for this 

 excuse is a distinctive morphological characteristic 

 (72, 171, 199). For example, the small muscular pul- 

 monary arteries of the rabbit contain much thicker 

 media than do the corresponding vessels of rat, cat, 

 and man (fig. 7). It is easy to imagine that contraction 

 of such hypertrophied muscle could evoke the 

 "gnarly" distortion of the rabbit's vascular tree which 

 follows the infusion of large quantities of norepineph- 

 rine (61); on the other hand, it is somewhat more 

 difficult to imagine such an intense vascular response 

 in species with muscle-poor precapillary vessels. 



Species difference may reside in physiological as 

 well as in anatomical peculiarities (218). For exam- 

 ple, the rabbit is notoriously vagotonic, whereas the 

 cat, dog, and man are generally regarded as sympa- 

 thotonic; moreover, the pulmonary circulation of the 

 rabbit also appears to be more susceptible to the 

 effects of pharmacological agents, such as histamine, 

 than is the pulmonary circulation of the dog (442). 

 Such anatomical and physiological peculiarities occur 

 throughout the animal kingdom, complicating the 

 transfer of information from one species to another 

 (95. 2 39> 35 2 )- 



FUNCTIONAL ANATOMY 



The dependent position of the pulmonary circula- 

 tion — within the lung and thorax on the one hand, 



