RESPIRATION 437 



in consequence more generously filled with blood, will discharge a larger 

 volume into the aorta at each contraction. During expiration the flow of 

 blood through the intra-thoracic vessels will be diminished for the reverse 

 reasons. 



2. On the Arterial Pressure. An examination of a tracing of the arterial 

 pressure will show that it is characterized by small undulations due to the 

 cardiac ^ beat and large undulations due to the respiratory movements, 

 inspiration being accompanied by a rise, expiration by a fall of pressure. 

 These fesults~1i^rea3fl}ra?^^ by the cGBKence in the volume of 

 blood discharged by the left heart into the aorta during the time of the two 

 movements. If a tracing of the respiratory movements and of the blood- 

 pressure be taken simultaneously, it will be found that the rise of pressure 

 does not exactly correspond with inspiration, nor the fall of pressure with 

 expiration; for a certain period after the beginning of an inspiration the 

 pressure continues to fall, and for a certain period after the beginning of an 

 expiration the pressure continues to rise. During the remainder of the 

 period, however, inspiration is attended by a rise, expiration by a fall of 

 pressure. The explanation of these results lies in the fact that at the begin- 

 ning of the inspiration, when the vessels dilate, the blood-flow momentarily 

 slows; the left heart continuing to discharge small volumes into the aorta, 

 the pressure continues to fall. So soon as the left heart begins to be better 



! filled, the pressure at once begins to rise. At the end of an expiration the 

 I flow of blood into the left heart continues and the pressure rises, but with 



the return of the intra-thoracic pressure the vessels diminish in caliber, the 

 1 volume of blood transmitted by them becomes less, the output of the left heart 



declines, and the pressure falls. 



3. On the Flow of Lymph. The fall of the intrathoracic pressure 

 during an inspiratory movement has an accelerating effect on the flow of 

 lymph from the abdominal portion of the thoracic duct into the thoracic 

 portion. The rise of the pressure during an expiratory movement causes a 

 more abundant discharge of lymph from the end of the duct, into the venous 

 blood. The advantages of this mechanism have been alluded to on page 225. 



