RESPIRATION 535 



III. Interaction of Circulation and Respiration. 



The lungs and heart, being packed tightly together in the 

 air-tight thorax, and both undergoing periodic changes, neces- 

 sarily influence one another. At the same time, the close 

 proximity of the respiratory and cardiac centres in the 

 medulla seems to lead to the activity of one influencing the 

 other. 



A. Influence of Respiration on Circulation. — The circulation 

 is modified in two ways by respiration. First, the pulse, and 

 second, the arterial blood pressure undergo alterations. 



1st. Pulse. — (a) Rate. — If a sphygmographic trace, giving 

 the pulse waves during the course of two or three respirations, 

 be examined, it will be found that during inspiration the heart 

 is acting more rapidly, while during expiration its action is 

 slower. 



If the vagus be cut, these changes are not seen, showing 

 that the inspiratory acceleration is not the result simply of 

 the larger amount of blood Avhich enters the heart during 

 inspiration, but is really due to changes in the cardio-motor 

 centre — the accelerating part of wdiich has its activity in- 

 creased during inspiration, Avhile the inhibitory part is more 

 active during expiration. This is therefore partly a reflex effect 

 from the lung through the vagus, although it may be in part 

 due to the proximity of the centres in the medulla. 



(6) Volume. — Not only is the rate of the pulse altered by 

 respiration, but the waves are smaller during inspiration and 

 larger during expiration. This is simply due to there being 

 more time for diastolic filling when the heart is beating more 

 slowly. 



2nd. Blood Pressure. — A. If, in an anaesthetised animal, 

 tracings 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 influence of respiration on arterial pressure is chiefly a 

 mechanical one, depending on the variations in the pressure in 

 the pericardium, which is decreased during inspiration, allowing 



