EFFECT OF RESPIRATION ON THE CIRCULATION 325 



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

 relative effect would be produced as by inspiration. The immediate effect 

 during inspiration of the diminished intra-thoracic pressure upon the pul- 

 monary vessels is to produce an initial dilatation of both artery and veins, 

 and this delays for a moment the passage of blood toward the left side of 

 the heart, resulting in an initial fall in the arterial pressure, but the fall of 

 blood-pressure is immediately followed by a steady rise, since the flow is 

 increased by the initial dilatation of the vessels. The converse is the case 

 with expiration. As, however, the pulmonary veins are more easily di- 

 latable than the pulmonary artery, their greater distensibility increases the 



FIG. 243. 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 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 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.) 



flow of blood as inspiration proceeds, while during expiration, except at its 

 beginning, this property of theirs acts in the opposite direction, and diminishes 

 the flow. Thus, at the beginning of inspiration the diminution of blood 

 pressure, which commenced during expiration, is continued, but after a time 

 the diminution is succeeded by a steady rise. The reverse is the case with 

 expiration, i.e., there is at first a rise and then a fall of blood pressure. 



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

 and the intra-thoracic pressure returns to the normal, which is still slightly 

 below the atmospheric pressure. The effect of this on the veins is to in- 

 crease their extravascular and so their intravascular pressure, and to di- 

 minish the flow of blood into the left side of the heart. This will, of course, 

 react to decrease the general blood-pressure. 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 atmospheric pressure. 

 The effect of violent expiratory efforts, however, does have a distinct action 



