DYNAMICS OF PULMONARY CIRCULATION 



1671 



During the next seventy years, a wide variety of in- 

 genious experimental preparations and new tech- 

 niques were used to gain information about the remote 

 pulmonary circulation: a) artificial experimental 

 conditions were devised to control some respiratory 

 and circulatory parameters so that others could be 

 measured (40, 1 57) ; b) high-fidelity manometric sys- 

 tems were invented and used to register the details of 

 the pulmonary vascular pressure pulses (183); c) 

 cannulae were placed during open thoracotomy so 

 that pulmonary arteriovenous pressure gradients 

 could be measured in the closed-chest animal imme- 

 diately after operation (225) ; d) angiostomy cannulae 

 were devised so that pulmonary vascular blood pres- 

 sures could be recorded at will in intact, unanesthe- 

 tized animals (183, 187); and e) indirect methods 

 were developed for the estimation of the pulmonary 

 blood flow in intact animals or man (see Chapter 1 7). 



This three-quarters of a century of steady progress 

 underwent sudden acceleration in the logo's. In 1929, 

 Forssmann demonstrated on himself that a catheter 

 could be safely threaded by way of a peripheral vein 

 into the right heart (142); shortly thereafter, Klein 

 measured the pulmonary blood flow by the direct 

 Fick principle in man (90a). By World War II, the 

 stage was set for Cournand, Richards, and their co- 

 workers to begin their systematic studies of the pul- 

 monary circulation in man under natural conditions 

 (92). And, since the ig4o's, right heart catheterization 

 has been used for the sampling of mixed venous blood, 

 for the injection of contrast material and test sub- 

 stances into the pulmonary circulation, and for the 

 recording of blood pressures from the right side of 

 the heart; the technique has also provided access to 

 the venous effluent from special organs and regions 

 of the body and has led to the techniques of left heart 

 catheterization. 



Alveolar-Capillary Gas Exchange 



As indicated previously, the pulmonary circulation 

 is predominantly built for alveolar-capillary gas ex- 

 change. Up to the turn of the present century, the 

 precise nature of alveolar-capillary gas exchange was 

 unclear; particularly uncertain was the mechanism 

 by which oxygen traversed the alveolar-capillary 

 interfaces: some held that oxygen was secreted by the 

 alveoli (177); others maintained that diffusion alone 

 was involved (10). The issue was finally settled in 

 favor of diffusion by August and Marie Krogh (243). 

 These studies by the Kroghs also paved the way for 



measuring the rate of pulmonary capillary blood flow 

 using soluble, inert gases as tracer substances (343). 



To complete the picture of the coordinated circula- 

 tory-respiratory mechanism for external gas exchange 

 (fig. 5), more had to be learned of the physiological 

 behavior and of the physicochemical properties of the 

 blood. To this end, Barcroft (fig. 6) provided pre- 

 cise experimental information concerning the dis- 

 sociation of oxyhemoglobin (9); L. J. Henderson 

 (fig. 6) and his collaborators analyzed blood as a 

 physicochemical system and defined its role in the 

 exchange of the respiratory gases between the 

 atmosphere and the tissues (201, 297). 



The regulation of alveolar-capillary gas exchange 

 came under serious experimental scrutiny in the 

 1940's. In 1946, Euler & Liljestrand (125) proposed 

 that the local concentration of the respiratory gases 

 within the lung — a function of local ventilation- 

 perfusion relationships — might regulate, in turn, the 

 distribution of the pulmonary blood flow; the many 

 experiments subsequently performed by others to test 

 this hypothesis (132) will be considered later in this 

 chapter. Interest in alveolar-capillary gas exchange 

 was also stimulated in the 1 94o's from another direc- 

 tion, i.e., from the practical exigencies of aviation 

 medicine in World War II; from this practical interest 

 developed theoretical models, quantitative formula- 

 tions, and graphic representations which have not 

 only helped to resolve old problems in alveolar- 

 capillary gas exchange but also to point up new ones 

 (267, 327, 345). 



COMPARATIVE PHYSIOLOGY 



There are exceedingly few hemodynamic measure- 

 ments in the nonmammalian vertebrates. In the 

 fishes and sharks, the mean blood pressure in the 

 ventral aorta (to the gills) is of the order of 30 mm Hg; 

 as blood traverses the gills, blood pressure drops to 

 reach a slightly lower level in the dorsal (systemic) 

 aorta (57). 



This arrangement of the circulation in the fishes is 

 unfavorable for the systemic circulation. The hemo- 

 dynamic situation of the systemic circulation begins 

 to improve in the Amphibia and Reptilia in which the 

 pulmonary artery is separate from the aorta and over- 

 rides a functionally single ventricle. Among the 

 Reptilia, pulmonary arterial blood pressures have 

 been measured in the turtle (352) and in the snake 

 (222). In both of these species, the patterns of blood 



