3 68 



TEXT-BOOK OF PHYSIOLOGY 



change in pressure; and it is an established fact that within physiological 

 limits it is the volume of blood which an organ receives rather than the pres- 

 sure under which it is received, that determines its activity. In the second 

 instance, on the cessation of activity the velocity is decreased and the normal 

 condition restored without any appreciable change in pressure. 



THE PULSE 



The Arterial Pulse. The pulse may be denned as a periodic expansion 

 and recoil of the walls of the arterial system. The expansion is caused by 

 the discharge from the heart into the arteries of a volume of blood, approxi- 

 mately 80 c.c., during the systole; the recoil is due to the elastic reaction 

 of the arterial walls on the blood, driving it forward, into, and through the 

 capillaries, during the diastole. 



At the close of the cardiac diastole the arterial system is full of blood and 

 considerably distended. During the occurrence of the succeeding systole, 

 an additional volume of blood is discharged into the aorta. The incoming 

 volume of blood is now accommodated by the discharge of a portion of the 

 general blood volume into the capillaries and by the expansion of the arteries 

 both in a transverse and longitudinal direction. The expansion naturally 

 begins at the root of the aorta and at the beginning of the systole. As the 

 blood continues to be discharged from the heart, the expansion increases in 

 extent up to at least the middle of the systole; at the same time adjoining 

 segments of the aorta and its branches expand in quick succession, and 

 by the time the systole is completed the expansion has traveled over the 

 entire arterial system as far as the capillaries, while, owing to the recoil 

 of the arterial walls, it has almost disappeared in the neighborhood of the 

 heart. At this moment the height of the expansion wave lies at some in- 

 definite distance from the heart. With the cessation of the systole a gen- 

 eral recoil at once occurs. The arterial walls continuing to recoil, force 

 the blood into the capillaries, until the arteries return to the condition 

 they were in just before the previous systole. 



This expansion and recoil 'which thus pass from the beginning to the 

 end of the arterial system assumes the form of a wave and therefore is known 

 as the pulse-wave or pulse. Preceding and causing the expansion of the 

 arterial system there is an increase of the general blood-pressure; preceding 

 and leading to the recoil of the arterial system, there is a decrease of the 

 general blood-pressure, both of which facts are shown by the small curves on 

 a blood-pressure tracing, and for this reason the pressure which causes the 



