366 RESPIRATION. [CH. xxiv. 



pressure continues to fall, then gradually rises until the end of 

 inspiration, and continues to do so for some little time after 

 expiration has commenced. 



As regards the effect of expiration, the capacity of the chest is 

 diminished, and the intra-thoracic pressure returns to the normal, 

 which is not exactly equal to the atmospheric pressure. The 

 effect of this on the veins is to increase their extra-vascular and 

 so their intra-vascular pressure, and to diminish the flow of 

 blood into the left side of the heart, and with it the general 

 blood-pressure, but this is almost exactly balanced by the neces- 

 sary increase of arterial tension caused by the increase of the 

 extra-vascular pressure of the aorta and large arteries, so that the 

 arterial tension is not much affected during expiration either way. 

 Thus, ordinary expiration doe not produce a distinct obstruction 

 to the circulation, as even when the expiration is at an end the 

 intra-thoracic pressure is less than the extra-thoracic. 



The effect of violent expiratory efforts, however, has a distinct 

 action in obstructing the current of blood through the lungs, as 

 seen in the blueness of the face from congestion in, straining ; 

 this condition being produced by pressure on the small pulmonary 

 vessels. 



We may summarise this mechanical effect of respiration on the 

 blood-pressure therefore, and say that inspiration aids the circula- 

 tion and so increases the arterial tension, and that although expi- 

 ration does not materially aid the circulation, yet under ordinary 

 conditions neither does it obstruct it. Under extraordinary con- 

 ditions, however, as in violent expiration, the circulation is 

 decidedly obstructed. 



We have seen, however, that there is no exact correspondence 

 between the point of highest blood-pressure and the end of 

 inspiration, and AVC must suppose that there are other, mechanical 

 factors, such, for example, as the effect of the abdominal move- 

 ments, both in inspiration and expiration, upon the arteries and 

 veins within the abdomen and of the lower extremities, and the 

 influence of the varying intra-thoracic pressure upon the pulmonary 

 vessels, both of which ought to be taken into consideration. As 

 regards the first of these, the effect during inspiration as the 

 cavity of the abdomen is diminished by the descent of the 

 diaphragm should be two-fold : on the one hand, blood would 

 be sent upwards into the chest by compression of the vena cava 

 inferior; on the other hand, the passage of blood downwards from 

 the chest in the abdominal aorta, and upwards in the veins of the 

 lower extremity, would be to a certain extent obstructed. In 

 ordinary expiration all this would be reversed, but if the abdominal 



