658 PHYSIOLOGY OF RESPIRATION. 



pressure rises the amplitude of the pulsations will increase, since the 

 intra-arterial diastolic pressure thereby approaches the extra- 

 arterial pressure. The reverse of course happens with a fall of 

 blood-pressure. Making use of this method it is found* that in 

 some cases in man the blood-pressure falls during inspiration and 

 rises during expiration, while in other cases the reverse relation- 

 ship holds. The cause for this difference is explained below. 



It is generally agreed that the effect of the respiratory 

 movements on the arterial pressure is due mainly to mechanical 

 factors which influence the amount of blood discharged into the 

 aorta. The matter is difficult to analyze successfully,! but the 

 following factors are the ones which have usually been emphasized. 

 At each inspiration the aspiratory action of the thorax upon the 

 venous flow to the right side of the heart is increased, and con- 

 sequently more blood is thrown into the pulmonary circulation 

 and eventually into the left ventricle. This factor would tend to 

 increase the output of the heart during inspiration and thereby 

 raise arterial pressure. On the other hand the blood-capacity 

 of the lungs is increased during inspiration, owing to a stretching 

 of the blood-capillaries during the expansion of the lungs, and this 

 increase in capacity may serve, temporarily at least, to hold back 

 the flow of blood to the left ventricle and thereby cause a fall of 

 pressure during the inspiration. So far as these factors are con- 

 cerned, it is evident that the permanent resultant effect should be 

 in the direction of an increased flow of blood into the aorta and a 

 rise of aortic pressure as a result of inspiration, but the time 

 relations of this rise of pressure may be obscured or reversed by the 

 temporary retarding effect of the increase in the capacity of the 

 capillary bed in the lungs at the beginning of inspiration. There is 

 an additional factor, however, whose influence is more evident as 

 regards the time relation between the rise of aortic pressure and 

 the phase of respiration. This factor is a change in heart-rate 

 caused by the inspiration. 



In some individuals the heart-rate increases very perceptibly 

 during inspiration, the change in rate taking place quite promptly 

 with the beginning of the inspiratory act. In others this phe- 

 nomenon is less marked, or is absent altogether. It has been 

 shown by Fredericq that this change in dogs occurs when the chest 

 is widely opened and the respiratory movements can have no 

 mechanical effect upon the heart. He suggests, therefore, that the 

 accelerated pulse during inspiration is due to an associated activity 

 in the nerve centers of the medulla. When the inspiratory center 



* Erlanger and Festerling, "Journal of Experimental Medicine," 15, 370, 

 1912; also Snyder, "American Journal of Physiology," 36, 430, 1915. 



t For the older literature, see de Jager, "Journal of Physiology," 7, 130. 



