CHANGES IF ARTERIAL PRESSURE. 239 



a minute 12,000 cub. cent. (396 oz.) instead of 10,800 (360 

 oz. ) the quantity which B is stretched enough to squeeze 

 out in that time. Water will in consequence accumulate in 

 B until it becomes stretched enough to squeeze out 12,000 

 -cub. cent. (396 oz.) in a minute, and then a steady pressure 

 at a new and higher level will be maintained. On the 

 other hand if the pump, still beating sixty times a minute, 

 works more feebly so as to send out only 160 cub. cent. (5.6 

 oz.) at each stroke, then B, squeezing out at first more 

 than it receives in a given time, will gradually empty 

 itself until it only presses hard enough upon its contents 

 to force 160 X 60 = 9600 cub. cent. (336 oz.) out in a 

 minute. 



Similarly, if while the resistance in the small arteries 

 and capillaries remains the same and also the heart's rate, 

 the power of the stroke of the latter alters, so that at each 

 beat it sends more blood out than previously, then arterial 

 pressure will rise; while if the heart beats more feebly it 

 Trill fall. 



Modifications of Arterial Pressure by Changes in the 

 Peripheral Resistance. Let the pump c in Fig. 87 still 

 wfrrk otoadily ocnding-1 0,800 cub. cent. (360 oz.) per min- 

 ute into B and the resistance increase, it is clear arterial 

 pressure must rise. For B is only stretched enough to 

 .squeeze out in a minute the above quantity of liquid against 

 the original resistance and cannot at first send out that 

 quantity against the greater. Liquid will consequently ac- 

 cumulate in it until at last it becomes stretched enough to 

 send out 10,800 cub. cent. (360 cubic oz.) in a minute 

 through the small tubes, in spite of the greater resistance 

 to be overcome. A new mean pressure at a higher level 

 will then be established. If on the contrary the resistance 

 diminishes while the pump's work remains the same, then 

 B will at first squeeze out in a minute more than it receives, 

 until finally its elastic pressure is reduced to the point at 

 which its receipts and losses balance, and a new and lower 

 mean pressure will be established in B. 



system increase of the peripheral re- 



_ 

 sistance by narrowing of the' small arteries will TucreiUm ar a 



