882 PHYSIOLOGY 



much more easily distended than the arteries. In Fig. 373 (p. 871) 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. Hg., 

 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 consider- 

 ably distended under a pressure of 10 mm. 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 accommodate 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 accommodated somewhere, so that if the arterial pressure 

 is as great as before, the venous pressure must be greater. In the same 

 way the withdrawal of a certain amount of fluid may lower the mean 

 systemic pressure, say from 10 to 5 mm. Hg. It is still possible for the 

 pump to maintain an arterial pressure equal to that produced when the 

 mean systemic pressure was 10 mm. Hg, but to produce this effect the 

 relative distribution of blood must be altered and the veins must be more 

 empty than they were previously. The maintenance of a constant arterial 

 pressure with varying amount of fluid in the system can therefore be 



