378 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



tinuing the initial driving force of the heart even during the diastolic 

 period, so that the blood is forced to escape into the capillaries in a 

 perfectly steady stream and not remittently. Obviously, therefore, 

 the pressure in the arteries is increased during each systole of the heart, 

 and, as the ventricles are emptied rather quickly (0.3 sec.), this rise 

 must develop with a certain abruptness. The diastolic decline, on the 

 other hand, is gradual, because the' peripheral resistance is adjusted 

 in such a way that a very copious escape of arterial blood during 

 this period cannot result. By means of a proper adjustment of this 

 resistance, the arterial system is constantly kept in a condition of 

 overfilling. 



The aforesaid systolic-diastolic variation in the arterial pressure 

 forms the basis of the arterial pulse. Although primarily dependent 

 upon the activity of the heart, its place of origin is really in the root of 

 the aorta, whence the individual fluctuations in pressure are trans- 



liOmm- 



FIQ. 197. THE CARDIAC VARIATIONS IN THE ARTERIAL BLOOD-PRESSURE. 

 S, systolic pressure; D, diastolic pressure; M, average pressure. The systolic- 

 diastolic difference constitutes the pulse-pressure. A, abscissa. 



rnitted throughout the arterial system in the form of successive waves. 

 Thus, it happens constantly that the central portion of this system is in 

 a state of maximum distention, while its more distal segments still 

 retain their diastolic caliber. A moment thereafter, however, these 

 conditions are reversed, the advancing wave causing the peripheral 

 portion to become distended, while the more central portions recoil 

 and bring their elastic power to bear upon the blood within them. The 

 pulse, therefore, is essentially a reproduction of the changes in pressure, 

 modified by the elastic qualities of the arterial wall. 



Each systole of the heart generates a certain amount of energy 

 which is transferred in part to the arterial wall where it is stored as 

 potential energy, to be made use of subsequently during the diastolic 

 period of the organ. As the cardiac energy is transmitted at regular 

 intervals, this elastic recoil of the arteries must also occur at regular 

 intervals. It is betrayed externally by an alternate expansion and 

 shrinkage of the arteries, or "pulse," which is most manifest near 



