THE BLOOD -PRESSURE 881 



factor and can be considerably altered by the contraction of the muscular 

 coats of the vessels, or by pressure on the vessels exerted by the surrounding 

 muscular and elastic structures. 



It will simplify the discussion of the main factors of the circulation 

 in a closed system if, for the present, we neglect the variable factors and 



FIG. 384. Artificial schema to demonstrate the main features of the circulation. 

 The heart is an enema syringe with valves at v and v. The artery is a thick- walled 

 rubber tube. On the venous side is placed a length of wide thin- walled tubing, 

 to represent the large thin- walled distensible veins. The arterioles and capillaries 

 (peripheral resistance) are represented by wide glass tubes packed with sponges. 

 By opening the clamp on the tube D (' splanchnic area arterioles ') the peripheral 

 resistance can be removed, and a free passage of fluid allowed from arterial to 

 venous side. 



see what would take place in such a system of elastic tubes all situated 

 on one horizontal plane. Such a system is represented in the diagram 

 (Fig. 385), and a working model of it in Fig. 384. 



The heart H is interpolated at one part of the 

 circuit, while the free outflow of the fluid from B 

 to D is impeded by the presence of a peripheral 

 resistance at c. Such a system would have a 

 definite capacity at zero internal pressure, but 

 a very much greater amount of fluid might be 

 forced into it under a positive pressure. We will 

 assume that the pressure throughout the system 

 is equal to 10 mm. Hg, i.e. the elastic tubes are 

 all slightly distended. If the heart H now begins 

 to contract it will pump fluid from E into A. 



The pressure in E will fall from 10 mm. to mm., while that in 

 A will rise to a corresponding extent, the resistance at c preventing 

 the free escape of fluid from B to D and so causing the heart to pile up 

 the fluid which it has taken from E into A. 



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, 



