150 INFLUENCE OF THE RESPIRATORY MOVEMENTS. 



When it is remembered that during inspiration, owing to the dila- 

 tation of the thorax, the arterial blood is retained in larger quantities 

 within the chest-cavity, while the venous blood is more actively drawn 

 into the right auricle by aspiration, it is evident that the tension within 

 the arteries must at first dimmish during inspiration. The expiratory 

 diminution in the size of the thorax, on the other hand, favors the 

 flow of arterial blood into the vascular trunks and dams the venous 

 blood back toward the venae cavae, two factors that tend to heighten 

 the tension in the arterial system. Furthermore, the expiration that 

 immediately precedes an inspiration allows less blood to enter the heart, 

 so that systolic contractions at the beginning of inspiration throw a 

 somewhat smaller quantity of blood into the aorta; the opposite result 

 attends the inspiration that immediately precedes an expiration. 



These variations in tension explain the differences in the size of 

 sphygmographic tracings taken during inspiration and during expira- 

 tion, as seen in Fig. 56, and in Fig. 50, I, III, IV, in which / indicates 

 the inspiratory, and E the expiratory curve. The differences are as 

 follows: (i) the greater tension in the arteries during expiration causes 

 a general heightening of the level of all curves coinciding with expira- 

 tion; (2) during expiration the ascending limb is prolonged because the 

 expiratory movement of the thorax tends to increase the force of the 

 wave produced during expiration; (3) the magnitude of the recoil-ele- 

 vation must be less on account of the increase in pressure during ex- 



FIG. 56. Influence of Respiration on the Sphygmographic Tracing (after Riegel). 



piration; (4) for the same reason the elasticity-elevations are more 

 distinct and approach more nearly the level of the apex of the curve. 

 During the stage of expiration the pulse is somewhat more frequent 

 than during the stage of inspiration. 



This purely mechanical effect of the respiratory movements is modi- 

 fied by the stimulation of the vasomotor center that takes place at the 

 same time. Owing to this nervous influence the arterial pressure 

 which, it is true, is lowest during inspiration begins to rise during 

 inspiration and continues to increase until the end of that phase, reaching 

 its maximum at the beginning of expiration. During the remainder 

 of expiration the blood-pressure falls, and again reaches its lowest level 

 at the beginning of inspiration. These influences leave their imprint 

 upon the sphygmographic curves, which, accordingly, present the signs 

 of increasing or diminishing arterial tension, in accordance with the 

 phases of respiration. There is thus to a certain extent a displace- 

 ment of the pressure- curve to correspond with the respiratory curve. 



The statements of different observers vary with regard to the effect 

 of strong expiratory pressure and of forced inspiration on the shape of 

 the pulse- waves. The simplest way of producing strong expiratory 

 pressure is by means of Valsalva's experiment. During this procedure 

 there is at first an increase in the blood-pressure, with the formation of 

 pulse-waves resembling those produced during ordinary expiration 



