THE CIRCULATION OF THE BLOOD 351 



VARIATIONS IN THE BLOOD -PRESSURE 



A. In the Arterial Pressure. It is apparent from the preceding state- 

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

 normal, by: 



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



2. An increase hi the peripheral resistance. 



It is further apparent that if the pressure is higher than the normal it 

 may be lowered to the normal by a decrease in the value of either one or 

 the other of these factors. 



It is also apparent that the arterial blood-pressure as a whole may be 

 decreased below the normal by : 



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



2. A decrease in the peripheral resistance. 



It is again further apparent, that if the pressure is lower than the 

 normal it may be raised to the normal by an increase in the value of 

 either one or the other of these factors. 



i. An increase in the rate of the heart, when the arterial pressure is in a 

 condition of equilibrium, will lead to an increase in the amount of blood 

 discharged from the ventricle in a unit of time. As a result there will be 

 an accumulation of blood temporarily in the arteries and a rise of pressure 

 (the peripheral resistance remaining the same), for the reason that the 

 pressure is only sufficient to force into the capillaries a given volume, 

 in the same period of time. As the pressure rises the velocity and the 

 outflow will be increased until equilibrium is restored though at a some- 

 what higher level. A rise of pressure from an increase in the rate of the 

 beat alone has been questioned, for it has apparently been demonstrated 

 that there is a definite relation between the normal rate and the volume 

 discharged from the ventricle, and that when the rate is increased, the 

 volume discharged diminishes and hence the pressure remains normal or 

 even falls below the normal. Nevertheless, the results of experiments, 

 which will be alluded to in a following paragraph, indicate that under 

 physiologic conditions an increase in pressure follows an increase in the 

 rate of the heart. This is especially the case, when the large veins enter- 

 ing the heart are well filled with blood. 



An increase in the force of the heart's contraction, the rate remaining 

 the same, will also increase the pressure, a fact explained by the further 

 fact that not all the blood in the ventricle is discharged during the 

 ordinary systole. It has been calculated that perhaps but two-thirds 

 of the ventricular volume is so discharged. If therefore the force of 

 the contraction is increased a larger volume will be discharged at each 

 systole, which will necessarily lead to an accumulation. With the ac- 

 cumulation there goes an increased distention of the artery and a cor- 

 responding increase of pressure. In a short time, therefore, the increased 

 pressure will force out of the arteries at a higher rate of speed this excess 

 of blood until the outflow again equals the inflow. This restores the 

 equilibrium but establishes the mean pressure at a higher level. 



An increase in the rate and force of the heart-beat always follows 

 an inhibition of or a diminished activity or tonus of the cardio-inhibitor 

 center. This condition may be brought about by the arrival of nerve 



