202 HANDBOOK OF PHYSIOLOGY. 



ficient to counteract the increase of arterial tension produced by the ef- 

 fect of inspiration upon the veins of the chest, and the balance of the 

 whole action would be in favor of an increase of arterial tension during 

 the inspiratory period. But if a tracing of the variation be taken at the 

 same time that the respiratory movements are being recorded, it will be 

 found that, although speaking generally, the arterial tension is increased 

 during inspiration, the maximum of arterial tension does not correspond 

 with the acme of inspiration (Fig. 162). 



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

 ished, 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 intra-vascular pressure, and so to diminish 

 the flow of blood into the left side of the heart, and with it the arterial 

 tension, but this is almost exactly balanced by the necessary increase of 



FIG. 162. Comparison of blood-pressure curve with curve of intra-thoracic pressure. (To be 

 read from left to right.) a is the curve of blood-pressure with its respiratory undulations, the slow- 

 er beats on the descent being very marked ; 6 is the curve of intra-thoracic pressure obtained by 

 connecting one limb of a manometer with the pleural cavity. Inspiration begins at i and expiration 

 at e. The intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and 

 then slowly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall 

 becomes more rapid. (M. Foster.) 



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 does 

 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 preventing 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 summarize this mechanical effect of respiration on the blood- 

 pressure therefore, and say that inspiration aids the circulation and so 

 increases the arterial tension, and that although expiration does not 



