THE BLOOD PRESSURE 993 



which over-distends it before the next stroke of the pump occurs. 

 So now the outflow may be divided into two parts, one part which is 

 forced out by the immediate effect of the stroke of the pump, and 

 another part which is forced out by the elastic reaction of the tube 

 between the strokes. If the strokes be rapidly repeated before the 

 tube has time to empty itself thoroughly, it will get more and more 

 distended. Greater distension means stronger elastic reaction, and 

 therefore stronger outflow of the fluid between the beats. This dis- 

 tension goes on increasing till the fluid forced out between the strokes 

 by the elastic reaction of the wall of the tube is exactly equal to that 

 entering at each stroke, and the flow thus becomes continuous. 



The same thing occurs in the living body. A man's heart at each 

 beat or contraction forces about 60 c.c. of blood into the already dis- 

 tended aorta. The first effect of this is to distend the aorta still 

 further. The elastic reaction of the walls drives on another portion 

 of blood, which distends the next segment of the arterial wall, and so 

 the wave of distension is transmitted with gradually decreasing force 

 along the arteries. This wave of distension is what we feel on the 

 radial artery, or any exposed artery, as the pulse. After each heart- 

 beat the arteries tend to return to their original size, and drive the 

 blood on through the arterioles (the peripheral resistance) into the 

 capillaries and so into the veins. By the time the blood has reached 

 the veins all trace of the heart -beat has disappeared and the pressure 

 has fallen to a few millimetres of mercury. 



INFLUENCE OF THE CAPACITY OF THE VASCULAR SYSTEM 



ON THE CIRCULATION 



So far we have only considered the influence of changes of pres- 

 sure and resistance in a system of tubes with a head of pressure 

 at one end and a free outflow at the other. In the body, however, the 

 vascular system is a closed circuit of elastic tubes presenting varying 

 resistances to the flow of blood, and of varying distensibility at different 

 parts of their course. In this closed system is inserted a pump, the 

 heart, with the function of driving the blood through the system. 

 Since all the blood-vessels are elastic and distensible the capacity of 

 the system is not fixed, but must vary with the internal pressure to 

 which the vessels are subjected. Moreover the position of the dif- 

 ferent parts of the circulation must have an influence on the capacity 

 of the system, since the dependent vessels will be distended not only 

 by the average pressure of the fluid throughout the system but also 

 by the hydrostatic pressure due to the weight of the column of fluid 

 pressing on them. The elasticity of the tubes is also a varying factor 

 and can be considerably altered by the contraction of the muscular 



63 



