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TEXT-BOOK OF PHYSIOLOGY. 



Though all the relations between pressure and velocity in the table are 

 possible, those which are most physiological are probably 5 and 6, for in both 

 instances there is a minimum alteration in pressure, but a maximum altera- 

 tion in blood flow or velocity. The first instance is the condition most 

 favorable for the functional activity of organs, for the reason that the volume 

 of blood which the organ receives in a unit of time is increased without any 

 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 defined 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, 

 a definite volume of blood is again 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; 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. 

 W ith the cessation of the systole and perhaps even before, the recoil of the 

 arterial walls at once occurs, beginning at the root of the aorta and rapidly 

 p assing over the arteries to the capillaries. 



The mode of development as well as the propagation of the expansion 

 an d recoil movement of the arterial wall, which together constitute the pulse, 

 are illustrated in Fig. 168 in which A B represent the arter y subdivided into 



