MECHANICS OP THE CIRCULATION IN THE VESSELS in 



observed that the rate of escape of the blood was nearly 50 per cent, 

 greater during the systole than during the pause of the heart. The 

 existence of the dicrotic wave on this tracing was long looked on as the 

 best proof that it was not an artificial phenomenon. 



The wave of increased pres- 

 sure, as it runs along the arterial 

 system, carries with it wherever 

 it arrives an increase of potential 

 energy. But this excess of po- 

 tential energy is continually being 

 worn down, owing to the friction 

 of the vascular bed ; and although 

 in the comparatively large arteries 

 the loss of energy is not great, it 

 rapidly increases as the arteries 

 approach their termination, and 

 begin to break up into the narrow 

 arterioles which feed the capillary 

 network. For not only is the ratio 

 of the total surface to the total 

 cross-section, and therefore the 

 friction, increased with every 

 bifurcation, but the mere change 

 of direction and division of the 

 wave cannot take place without 

 loss of energy. For this reason 

 the fluctuations of blood-pres- 

 sure are greater in the large arteries near the heart than in arteries 

 smaller and more remote. In the wide and much-branched capillary 

 bed the pulse-wave disappears altogether, and the blood-pressure 

 becomes relatively constant or permanent. And it is for some 



purposes convenient to look upon 

 the blood-pressure in the arteries as 

 made up of a permanent element, 

 with pulsatory oscillations super- 

 posed on it. Since no portion of the 

 arterial system is more than partially 

 emptied in the interval between two 

 blood-waves, the minimum or per- 

 manent pressure is always positive 

 i.e., always above that of the atmosphere, the beats of the heart 

 succeeding each other so rapidly that the successive waves over- 

 lap or ' interfere,' and are only separated at their crests. 



If the heart is stopped while a blood-pressure tracing is being 

 taken and we shall see later on how this can be done (p. 157) the 

 minimum line of the tracing goes on falling towards the zero-line. 



Fig. 42. Curves of Blood-Pressure taken 

 with a Spring Manometer from the 

 Carotid Artery of a Dog (Hiirthle). 

 When i was taken the blood-pressure 

 was high; 2 corresponds to a medium, 

 3 to a low, and 4 to a very low, blood- 

 pressure; p is the primary elevation 

 this and the succeeding elevations 

 between p and a are called systolic 

 waves; the systolic waves are followed 

 by a marked elevation d, which corre- 

 sponds to the dicrotic wave. 



V v'W 



Fig- 43- Blood - Pressure Tracing. 

 The horizontal straight line inter- 

 secting the curves is the line of 

 mean pressure. 



