ARTERIAL PRESSURE. THE PULSE. 245 



steady condition of things with a higher mean pressure in B 

 than before. 



On the other hand 2 if the pump begins to work more 

 slowly while the resistance remains the same, it is clear that 

 the mean pressure in B must fall. If, for example, the 

 pump works only forty times a minute and so sends in that 

 time 180 X 40 = 7200 cub. cent. (240 oz.) into B, which is 

 so stretched that it is squeezing out 10,800 cub. cent. (360 

 oz. ), in that time, it is clear that B will gradually empty 

 itself and its walls become less stretched and the pressure in 

 it fall. As this takes place, however, it will force less liquid 

 in a minute through the small tubes, until at last a pressure is 

 reached at which the squeeze of B only sends out 7200 cub. 

 cent. (340 oz.) in a minute ; and then the fall of pressure will 

 cease and a steady one will be maintained, but lower than 

 before. 



Applying the same reasoning to the vascular system, we 

 see that (the peripheral resistance remaining unaltered), if the 

 heart's force remains the same 1 but its rate increases, arterial 

 pressure will rise to a new level, while a slowing of the heart's 

 beat will bring about a fall of pressure. 



Modifications of Arterial Pressure Dependent on 

 Changes in tha Force of the Heart's Beat. Returning 

 again to Fig. 98 : suppose that, while the rate of the pump 

 remains the same, its power alters so that each time it sends 

 200 cub. cent. (6.6 oz.) instead of 180 (6 oz.) and so in 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 the heart's rate unchanged 

 the stroke of the latter alters, so that at each beat it sends 



