THE CIRCULATION OF THE BLOOD AND LYMPH 103 



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 ter- 

 mination, and begin to break up into the narrow arterioles which 

 feed the capillary network. For not only is the total surface, 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-pressure 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 superposed on it. Since no portion of the 

 arterial system is more than partially emptied in the interval 

 between two blood-waves, the minimum or permanent 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 overlap or ' in- 

 terfere,' 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. 143) FlG ' S^-BLOOD-PRESSURE TRACING. 

 the minimum line of the tracing ^ he horizontal straight line inter- 

 f ,,. 5 secting the curves is the line of mean 



goes on falling towards the zero- pressure. 



line. When the heart begins '; 



beating again, the pressure-curve rises, not by a continuous 

 ascent, but by successive leaps, each corresponding to a beat of 

 the heart, and each overtopping its predecessor, till the old line 

 of minimum or of mean pressure is again reached. 



The mean arterial blood-pressure is the permanent pressure 

 plus one-half of the average pulsatory oscillation. In a blood- 

 pressure tracing the line of permanent pressure joins all the 

 minima ; the line of maximum pressure joins all the maxima ; 

 the line of mean pressure is drawn between them in such a way 

 that of the area included between it and the blood-pressure 

 curve as much lies above as below it (Fig. 36). As has been 

 said, a tracing taken with a mercury manometer gives approxi- 

 mately the mean blood-pressure. Each beat of the heart is 

 represented on it by a single elevation of variable size, sometimes 

 not amounting to more than one-twentieth of the height of the 

 curve above the line of zero or atmospheric pressure, but some- 

 timesjnuchMarger. The oscillations due to the heart-beat are 



