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A MANUAL OF PHYSIOLOGY 



mechanical changes produced in the thorax by the respiratory 

 movements. Of these, the influence of variations in the intra- 

 thoracic pressure on the filling of the heart is of special importance. 

 With deep abdominal breathing the changes of intra-abdominal 



-fTK A 



FIG. 112. RESPIRATORY WAVES IN THE BLOOD-PRESSURE : SIMULTANEOUS 

 TRACINGS OF MOVEMENTS OF RESPIRATION AND OF RADIAL PULSE IN HUMAN 

 SUBJECT (LEWIS). 



In A the respiration was diaphragmatic ; in B, costal. In A the respiratory 

 tracing was taken from the abdominal wall ; in B, from the chest. 



pressure also affect the filling of the heart, an increase of pressure 

 (in inspiration) tending to cause more blood to be squeezed from 

 the abdominal veins towards the chest. The changes of vascular 

 resistance in the lungs, due to the alteration in the calibre of 

 the pulmonary vessels, also contribute, but, for such variations 



of intrathoracic pressure as 

 normally occur, only in a 

 minor degree. The changes 

 in the vascular capacity of 

 the lungs that is, in the 

 amount of blood contained 

 in the pulmonary vessels 

 are of importance especially 

 in delaying or accelerating 

 the alterations of blood-pres- 

 sure in the systemic arteries 

 due to the other factors. 



FIG. 113. 



The upper tracing shows the respiratory 

 movements in a rabbit with rather deep 

 and slow diaphragmatic breathing ; the 



lower tracing is the blood-pressure curve ; 

 I, inspiration ; E, expiration, including the 

 pause. 



The intrathoracic pressure, 

 which, as we have seen, is 

 always less than that of the 



atmosphere, unless during a forced expiration when the free 

 escape of air from the lungs is obstructed, diminishes in 

 inspiration and increases in expiration. The great veins outside 

 the chest, the jugular veins in the neck, for example, are under 



