DYNAMICS OF PULMONARY CIRCULATION 



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fa 



r 



h 







fig. 2. Structure of the heart according to Leonardo da Vinci 

 (98). The diagram shows the (nonexistent) pores in the ven- 

 tricular septum, an essential component of the Galenical con- 

 cept of the motion of the blood. 



In recent years, the biological role of the pulmo- 

 nary circulation and lungs has been emphasized 

 (294). For example, the pulmonary vascular endo- 

 thelium seems to contribute enzymes, such as lipo- 

 protein lipases, to the perfusing blood (1). Mast cells, 

 which are abundant in the lungs of many species, are 

 believed to add a wide variety of substances, including 

 heparin, histamine, hyaluronic acid, and serotonin 

 (230). Finally, the walls of the pulmonary blood ves- 

 sels and pulmonary tissue may also neutralize certain 

 endogenous substances (e.g., serotonin) which could 

 exert noxious effects if they gained access to the left 

 heart and systemic circulation (158). 



THE GROWTH OF IDEAS 



The large pulmonary vessels were known to 

 Herophilus of Alexandria in the fourth century, B.C. 

 (228). But not until the time of Harvey (1578-1667) 

 did dispassionate evidence begin to establish the 

 structure and function of the pulmonary circulation 

 (90, 91,1 79). For convenience, the origins and growth 

 of the modern ideas will be sketched under four sepa- 

 rate headings: a) the appreciation and proof that the 

 pulmonary vascular tree constitutes a closed circuit 

 between the right and left hearts, b) the awareness 

 that the lungs are concerned with external respira- 

 tion, <) the systematic analysis of pulmonary hemo- 

 dynamics, and d) the coordinated description of 

 alveolar-capillary gas exchange. While such a presen- 

 tation of the growth of ideas has the advantage of 

 brevity, its lack of historical detail ignores foretellers 



such as Ibn Xafis (291), Servetus (91), and Mayow 

 (144), whose clairvoyance could only be appreciated 

 retrospectively; it also exaggerates the contributions 

 of the "'finishers," whose discoveries crowned the 

 concepts and efforts of others. 



Bridge Between Two Ventricles 



In Harvey's time, Galenical misconception (fig. 2) 

 and philosophic speculation still predominated. Al- 

 though some of Harvey's predecessors and contempo- 

 raries had realized that there were no ventricular 

 pores by which right ventricular blood could bypass 

 the lungs and that the pulmonary artery was too 

 large to serve only as a nutrient vessel, their preoccu- 

 pation with the idea of the vascular system as the 

 generator of essential spirits blinded them to the 

 motion of the blood in the vessels. For Harvey, the 

 idea of the pulmonary circulation as a bridge between 

 the two ventricles was an essential component of his 

 theory of the unidirectional circulation of the blood; 

 he verified his theory by direct experiment and pro- 

 posed "porosities" as the final links between the 

 arteries and veins (195). In 1661, a few years after 

 Harvey's death, Malpighi provided the final proof of 

 pulmonary vascular continuity by visualizing the 

 passage of blood from the pulmonary arteries to the 

 veins by way of the pulmonary capillaries (280). 



Role in External Respiration 



Harvey was concerned solely with the mechanical 

 aspects of the circulation of the blood. His concept 

 did not deal with the prevalent notions about the 

 role of the lungs either as a source of material for the 

 generation of the vital spirit or as a refrigerating 

 device to control the innate heat of the heart (166). 

 Two years after Harvey's death, Lower (fig. 3), using 

 Hooke's new respiration pump, showed that blood 

 became arterialized as it passed from the right to the 

 left side of the heart (172). The idea that ingredients 

 of air, rather than air itself, were the basis of external 

 respiration had to await new developments in chemis- 

 try and in physics. A century was to pass before: a) 

 Black ( 1 788—1867), Lavoisier (1743-1794), and 

 Rutherford (1 753-1814) identified the three respira- 

 tory gases; b) Lavoisier proved that oxygen rather 

 than air was essential for life; and c) Lavoisier and 

 Laplace likened respiration to combustion (90, 91, 

 144). Indeed, not until the mid-nineteenth century 

 was it appreciated that combustion occurred in the 

 tissues rather than in the lungs and that hemoglobin 



