169O HANDBOOK OF PHYSIOLOGY "- CIRCULATION II 



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IPP 



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REST- 21% 2 



fig. 22. Effects of breathing 5' < 

 CO2 and of exercise on the pleural 

 pressures (IPP) and esophageal pres- 

 sures (EP) of a human subject. All 

 pressures are in mm Hg. [After Fishman 

 et al. (132).] 



IPP 





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confusion has arisen from the indiscriminate use of 

 transmural pressures for luminal pressures in the 

 calculation of pulmonary vascular resistance. It 

 should be emphasized that as long as left atrial pres- 

 sure exceeds alveolar pressure, the measurement of 

 the driving pressure across the lung requires only the 

 simultaneous measurements of luminal pulmonary 

 arterial and venous pressures — no matter what the 

 intrathoracic pressure may be. 



PULMONARY BLOOD VOLUME 



The pulmonary vasculature constitutes a distensible 

 reservoir, interposed between the right and left heart. 

 The volume of blood which it contains is of interest 

 on three separate accounts: /) the mechanical be- 

 havior of the lungs; 2) the efficiency of gas exchange; 

 and j) the sustained return of pulmonary venous 

 blood to the left heart. In large part, the volume of 

 blood contained in the lungs at any instant is de- 

 termined passively by the balance between pulmonary 

 inflow, i.e., between the output of the two ventricles; 

 it is also influenced considerably by the ventilation. 

 Whether an element of self-control is also provided by 

 pulmonary vasomotor activity, particularly on the 

 part of the veins (305) or of hypothetical venous 

 sinuses (381), is uncertain. 



fig. 23. Difference between luminal pressures (referred to 

 atmosphere) and transmural pressures (referred to perivascu- 

 lar pressure) along the length of the pulmonary vascular tree. 

 The shaded area represents the luminal pressure. In the capil- 

 laries (PC), which are exposed to alveolar pressure, the luminal 

 and transmural pressures are virtually identical. On the other 

 hand, in the pulmonary artery (PA) and vein (PV), the trans- 

 mural pressure exceeds the luminal pressure by the pleural 

 (perivascular) pressure. 



Measurement of Pulmonary Blood Volume 



For convenience, the methods for measuring pul- 

 monary blood volume may be sorted according to 

 whether they are designed to measure the pulmonary 

 blood volume or a change in pulmonary blood 

 volume. 



In the isolated lung or in thoracotomized animals, 

 the pulmonary blood volume is available for direct 

 mensuration (293, 384); but, because of the surgical 

 manipulations and the drastic experimental condi- 

 tions, the measured volume may differ considerably 



