880 PHYSIOLOGY 



resistance to the outflow, there will be a marked difference from the results 

 obtained when the rigid tube was partially obstructed. Each stroke of 

 the pump forces a certain amount of fluid into the tube. Owing to the 

 peripheral resistance this cannot all escape at once, and so part of the 

 force of the pump is spent in distending the walls of the tube, and part 

 of the fluid that was forced in remains in the tube. The distended elastic 

 tube tends to empty itself and forces out the fluid 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 distension 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 distended 

 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 pressure 

 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. More- 

 over the position of the different 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 



