THE CIRCULATION OF THE BLOOD 



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expansion and recoil is termed the pulse pressure. It is defined as the rhyth- 

 mic change in pressure at any given point of the arterial system; and in 

 amount, is the difference between the diastolic and the systolic pressures, at 

 the corresponding points. The volume of blood ejected from the ventricle 

 is frequently termed the pulse volume. 



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

 and recoil movement of the arterial wall, which together constitute the pulse, 

 are illustrated in Fig. 169 in which A B represent the artery subdivided into 

 six equal parts, indicated by the letters a to g. In accordance with this 

 subdivision of the artery the systole of the heart may be also divided into six 

 parts, during the first three of which the heart increases in power, and during 

 the last three of which it decreases in power, gradually falling to zero. The 

 effect on the arterial wall of the discharge of blood from the ventricle is 

 illustrated in the figure. During the first one-sixth of the systole a certain 

 volume of blood is forced into the artery, which at this moment is already 

 full of blood. Of this volume a portion moves forward while another 



FIG. 169. DIAGRAM SHOWING THE DEVELOPMENT OF A PULSE WAVE. (Rottet.) 



portion moves sideways as the arterial wall begins to expand under the 

 pressure of the heart: At the end of the first one-sixth of the systole the 

 condition of the arterial wall may be represented by the lines ib. During 

 the second one-sixth the artery expands still more as the volume of blood 

 increases under the increasing force of the heart, so that at the end of the 

 second period the expansion of the arterial wall is not only greater at the 

 point a but in addition has extended over a greater length of the artery so 

 that the condition of the artery may be represented by the lines 20. Dur- 

 ing the third sixth the same process continues; the incoming volume of blood 

 still further expands the artery at 0, as well as successive portions further on 

 as far as d, so that at the height of the systolic power the condition of the 

 artery may be represented by the lines $d. 



The force of the heart now begins to decline and from this moment on, the 

 elastic force of the artery preponderates and in consequence the arterial wall 

 begins to recoil at the point a. At the end of the fourth sixth of the systole, 

 therefore, the arterial wall at a, has recoiled to 2, while the expansion at a 

 has advanced to 63 where the present force of the heart plus the elastic recoil 

 of the arterial wall at a, are equal to the elastic force of the arterial wall at b. 

 At this moment the condition of the artery may be represented by the lines 

 2, 3, e. During the two remaining sixths of the cardiac systole, the same 

 process continues until, through elastic recoil, the artery has returned to its 

 original condition at a, and the expansion has extended as far as g, while the 

 height of the expansion has advanced to d$ where the force of the systole 

 and the force of the elastic recoil balance each other. At the end of the 

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