RESPIRATION. 289 



into the right auricle will, Cfsteris paribus, increase the amount passing 

 through the right ventricle, which also exerts a similar suction action, 

 and through the lungs into the left auricle and ventricle, and thus into 

 the aorta. This all tends to increase the blood-pressure. The effect of 

 the diminished pressure upon the pulmonary vessels will also help 

 toward the same end, i.e., an increased flow through the lungs, so that, 

 as far as the heart and its veins are concerned, inspiration increases the 

 blood-pressure in the arteries. The effect of inspiration upon the aorta 

 and its branches within the thorax would be, however, contrary; for as 

 the pressure outside is diminished the vessels would tend to expand, and 

 thus to diminish the tension of the blood within them, but inasmuch as 

 the large arteries are capable of little expansion beyond their natural 

 calibre, the diminution of the arterial tension caused by this means 



Fig. 217. 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 slower 

 heats on the descent being very marked"; 6 is the curve of intra-thoracic pressure obtained by con- 

 necting one limb of a manometer with the plural 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.) 



would be insufficient to counteract the increase of blood-pressure pro- 

 duced by the effect of inspiration upon the veins of the chest, and the 

 balance of the whole action would be in favor of an increase of blood- 

 pressure during the inspiratory period. But if a blood-pressure tracing 

 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. 217). In fact, at 

 the beginning of inspiration the pressure continues to fall, then gradually 

 rises until the end of inspiration, and continues to do so for some 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 



