THE BLOOD PRESSURE 995 



If the texture of the tubes were uniform, throughout the system 

 it is evident that the rise of pressure in A would approximate very 

 nearly to the fall of pressure in E. In the vascular system the veins 

 are, however, much more easily distended than the arteries. In 

 Fig. 369 (p. 982) is shown the distensibility of corresponding sections of 

 arteries and veins under gradually increasing internal pressures. An 

 artery has a certain capacity even at zero pressure. As the pressure in 

 its interior is increased the artery is distended, and its capacity rises first 

 slowly and then more rapidly, the increment in capacity being greatest 

 between 90 and 110 mm. Hg. The vein, on the other hand, is collapsed 

 when there is no distending force in its interior, so that at zero pressure 

 its capacity is nothing. The slightest rise of pressure, even of 1 mm. Hg. 

 causes a considerable increase in its capacity, and the capacity 

 rises rapidly with increasing pressure up to about 20 mm. Hg. Whereas 

 the artery is most distensible at 100 mm., the vein is at its optimum 

 distensibility at about 10 mm. Hg. If therefore the tubes at E are 

 made of thin-walled rubber tubing they will be considerably dis- 

 tended under a pressure of 10 mui. Hg., which has practically no 

 influence on the thicker-walled arterial tube A. 



A small amount of fluid taken from E would cause very little fall 

 of pressure on this side. A considerable force will be necessary to send 

 this fluid into the more resistant arterial tube, so that on pumping a 

 given amount of fluid from E to A the pressure in E may fall 5 mm., 

 while the pressure in A has to be raised from 50 to 100 mm. Hg. in 

 order to distend the arteries to such an extent that they will accom- 

 modate the fluid taken from E. 



In such a system, when the heart is at rest, the pressure all over the 

 system will be uniform, and in the example we have chosen the mean 

 systemic pressure was 10 mm. Hg. When the heart contracts it takes 

 up fluid from the venous side and piles it up on the arterial side until 

 the pressure on the arterial side is sufficient to cause exactly the same 

 amount of fluid to flow through the peripheral resistance into the veins 

 as is taken by the heart from the veins at each beat. This rise of pressure 

 in the arteries may be many times greater than the fall of pressure in 

 the veins. If more fluid is injected into the system when the heart is at 

 rest the whole system will be more distended and the mean systemic 

 pressure will rise. When the heart contracts it will raise the pressure on 

 the arterial side and lower that on the venous side as before, but it is 

 evident that according to the force of the heart-beat the arterial 

 pressure may be less than, equal to, or greater than the pressure 

 attained before the introduction of fluid. Since, however, the mean 

 systemic pressure is raised, the increased amount of fluid must be accom- 

 modated somewhere, so that if the arterial pressure is as great as 

 before, the venous pressure must be greater. In the same way the 



