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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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fig. 33. Blood pressures in the pulmonary artery, pulmonary vein, and aorta following the in- 

 jection of 5 mg of acetylcholine into the pulmonary artery of a human subject during open thora- 

 cotomy. The time of injection is indicated by the upright arrow. (Unpublished observations of A. G. 

 Jameson and A. P. Fishman.) 



lower portion of the pulmonary arterial pressure-flow 

 curve in the rabbit (416), the dog (119, 252, 434), 

 and man (89) resembles that of systemic beds: an 

 increase in pressure is associated with a parabolic 

 increase in flow; the inscribed curve is convex to the 

 pressure axis. The upper portion of the pulmonary 

 plot shows an opposite inflection which does not ap- 

 pear in systemic beds. Quantitatively, the pulmonary 

 and systemic arterial curves differ not only in the level 

 of the arterial pressure but also in the large increments 

 in blood flow evoked by slight increments in pul- 

 monary arterial pressure (at constant left atrial pres- 

 sure). 



Curves depicting the relationship between the 

 driving pressures and flow are difficult to establish 

 for either intact dog or man since it is impractical to 

 increase pulmonary blood flow without simultaneously 

 modifying the behavior of the respiration, the heart, 

 and the systemic circulation. However, Lategola has 

 succeeded in drawing a passive pressure-flow curve 

 for the pulmonary vascular tree of the intact dog, 

 using values obtained in the course of graded occlusion 

 of the pulmonary arterial tree by balloon-tipped 

 catheters (252). This curve appears as the solid line 

 in figure 34. The shape of this curve is generally in- 

 terpreted as showing that: a) as flow increases, re- 

 sistance decreases; and b) beyond a transition phase 

 (AQ of approximately 250 per cent), resistance be- 

 comes constant. Moreover, the length of the gently 

 sloping portion of the curve is regarded as a measure 

 of the maximum calibers, both of the patent vessels 

 and of those available to open in parallel; the start 

 of the steeply ascending portion is thought to occur 

 when the system begins to behave as though it were 

 comprised of rigid tubes (89, 252). It should be noted 

 that while the general shape of the pressure-flow rela- 

 tionship seems beyond cavil, the precise levels of flow 

 at which the tubes appear to become rigid are not as 



100 



200 



300 



fig. 34. Relationship between pulmonary blood flow' and 

 pulmonary arterial pressure in dog and man. Note that the 

 origin represents normal or control levels (not zero levels) of 

 both pressure and flow. The line is redrawn after Lategola 

 (•252) and is based on data obtained during graded occlusion 

 of the pulmonary artery tree in the dog. The shaded area rep- 

 resents corresponding measurements in normal man during 

 balloon occlusion of one pulmonary artery both at rest and 

 during mild exercise (42). The individual points represent 

 observations on human subjects during supine exercise. Open 

 circles: mild exercise (382); solid triangles: graded exercise (149); 

 open triangles: mild exercise after pneumonectomy (89). 



convincingly established (281) and the final slope 

 must be considered in the assessment of constancy of 

 resistance. 



Superimposed on the pressure-flow curve of the 

 dog is a shaded envelope which includes the points 

 obtained during similar occlusion of a pulmonary 

 arterv in man (42); in order to exceed the increments 

 in blood flow obtainable at rest, the human subjects 

 performed mild leg exercise during the occlusion of 

 one pulmonary artery. It may be seen that the en- 

 velope of human points closely follows the horizontal 



