3 io TEXT-BOOK OF PHYSIOLOGY. 



tained is attended by a fall in pressure. But before this reaches any 

 considerable extent, the heart again contracts and forces its contained 

 volume of blood into the arteries. 



That this may be accomplished it is essential that the cardiac 

 energy be sufficient not only to drive a portion of the blood through 

 the capillaries into the veins, but to oppose the recoiling arteries, and 

 to distend them to their previous extent, so that the incoming volume 

 of blood may be accommodated. This at once reestablishes the 

 pressure at its former level. The alternate rise and fall of the pres- 

 sure is represented by the oscillations of the mercurial column or by 

 the small elevations and depressions on the kymographic tracing. 



During the contraction of the heart the kinetic energy is trans- 

 formed into potential energy, represented by the tense distended walls 

 of the arteries. With the relaxation of the heart and the closure of 

 the semilunar valves the potential energy of the arteries is again trans- 

 formed into kinetic energy, represented by the moving blood. The 

 artery thus continues the work of the heart during its period of in- 

 activity. The rapidity with which the cardiac contractions succeed 

 each other prevents the pressure from sinking below a certain average 

 level. 



The uniform level of the arterial pressure depends on the fact that 

 though more blood enters the arteries during the systole than escapes 

 into the capillaries and veins, as shown by the rise of the mercurial 

 column in the manometer, this is compensated for by a continued 

 escape during the diastole as shown by the fall of the mercurial col- 

 umn.^XSo long a*s the inflow of blood is equaled by the outflow, there 

 is a balancing of opposing forces and the pressure is maintained at a 

 uniform level.^^ 



Changes in Blood-pressure. It is evident from the preceding 

 statements that the arterial blood-pressure as a whole may be increased 

 by: 



1. An increase in the rate or force of the heart's contraction. 



2. An increase in the peripheral resistance. 



3. An increase in the general volume of the arterial blood. 

 And that it may be decreased by : 



1. A decrease in the rate and force of the heart's contraction. 



2. A decrease in the peripheral resistance. 



3. A decrease in the general volume of blood. 



If when the arterial pressure is in a condition of equilibrium the 

 heart ejects into the arteries in a given period of time an increased 

 quantity of blood as a result of an increased rate of contraction, there 

 will be an accumulation of blood temporarily in the arteries (the 

 peripheral resistance remaining the same), for the pressure is only 

 sufficient to force into the capillaries a given volume. 



The same result could be brought about by an increase in the 



