102 A MANUAL OF PHYSIOLOGY 



large arteries the loss of energy is not great, it rapidly 

 increases as the arteries approach their termination, and 

 begin to branch. 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 



FIG. 30. BLOOD-PRESSURE TRACING. 

 The horizontal straight line intersecting the curves is the line of mean pressure. 



interval between two blood-waves, the minimum or perma- 

 nent pressure is always positive i.e., always above that of 

 the atmosphere. The only reason for this is that the beats 

 of the heart succeed each other so rapidly that the succes- 

 sive waves overlap 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 

 (? *34) tne minimum line of the tracing goes on falling 

 towards the zero-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 



