664 PHYSIOLOGY OF RESPIRATION. 



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. In addition to these mechanical 

 factors there is a physiological element which enters into the 

 problem, namely, the change in heart-rate, which tends to aug- 

 ment the outflow from the heart during 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 

 discharges its efferent impulses into the phrenic nerves it also 

 sends impulses by a sort of overflow into the neighboring cardio- 

 inhibitory center. This latter center is, thereby, partially in- 

 hibited, its tonic effect on the heart is diminished, and the rate of 

 the heart is increased. The variability in this factor in different 

 individuals may help to explain the fact that in some cases the 

 rise in blood-pressure occurs during the phase of inspiration, while 

 in others it coincides rather with the expiration. 



* For th,e older literature, see de Jager, "Journal of Physiology," 7, 130. 



