EFFECT OF RESPIRATION ON THE CIRCULATION 307 



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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, an increased 

 flow through the lungs, so that, as far as the mechanical effects on 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 external pressure is dimin- 

 ished, the vessels would tend to expand, and thus to diminish the tension of 

 the blood within them, but, inasmuch as the relative variation in pressure 

 on the large arteries is slight, the diminution of arterial tension caused by 

 this means will be insufficient to counteract the increase of blood pressure 



FIG. 243. Comparison of Blood-pressure Curve with Curve of Intrathoracic Pressure. 

 (To be read from left to right.) a is the curve of blood pressure with its respiratory 

 undulations, the slower beats on the descent being very marked; b 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 intrathoracic 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.) 



produced by the effect of inspiration upon the volume of discharge of 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. When a blood- 

 pressure tracing is 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, figure 243. In 

 fact, at the beginning of inspiration the pressure continues to fall for a brief 

 moment, then gradually rises until the end of inspiration, and continues to 

 do so for a moment after expiration has commenced. For explanation of 

 the influence of heart rate in this variation of blood pressure, associated 

 with the respiratory movement, see page 212. 



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

 muscles are violently contracted, as in extraordinary expiration, the same 



