

i or; or TO (W 



EESPIRATION 311 



active during expiration. This is therefore a reflex effect from 

 the lung through the- vagus, and it may be in part due to the 

 proximity of the centres in the medulla. 



But not only is the pulse more rapid during inspiration 

 and slower during expiration, but the waves are smaller 

 during inspiration and larger during expiration. The size of 

 the wave depends much upon the pressure of blood in the 

 arteries, and this change in the pulse thus leads to the fuller 

 consideration of the changes in the arterial pressure due to 

 respiration. 



2nd. Changes in Blood Pressure. If a tracing of the arterial 

 pressure and of the respiratory movements are taken at the 

 same time, it is found that there is a general rise .of pressure 

 during inspiration and a general fall during expiration, but 

 that at the beginning of inspiration the pressure is still falling, 

 and at the beginning of expiration it is still rising. This influ- 

 ence of respiration on arterial pressure is chiefly a mechanical 

 one, depending on the variations in the pressure in the thorax 

 during inspiration and expiration. 



During inspiration the pressure in the thorax falls to below 

 the atmospheric pressure, and thus during this period the 

 heart and great vessels are under a diminished pressure. 

 This diminution in pressure has little influence on the thick- 

 walled ventricles and arteries, but tells markedly on the thin- 

 walled auricles and veins. In these there occurs a diminution 

 in pressure, which, in the case of the vena cava, may fall 

 below an atmospheric pressure, and as a result an increased 

 flow of blood into these vessels from the veins outside the 

 thorax takes place (fig. 133, p. 291). 



But when more blood enters the heart the activity of the 

 organ is increased, and more blood is pumped through it into 

 the arteries, and the pressure in these rises. This explains 

 the great rise in arterial blood pressure during inspiration. 



During expiration the pressure in the thorax rises to above 

 the atmospheric pressure, and thus the pressure on the vessels 

 in the thorax is increased. This tells on the thin-walled 

 veins and auricles ; and thus the flow of blood into them 

 is retarded, and less blood passing into the heart, less is 

 pumped into the arteries, and the arterial pressure falls. 



This, however, does not explain the slight fall of pressure 



