BLOOD PRESSURE 



355 



either the arterial, capillary or venous pressure, because the blood 

 pressure really presents definite differences in accordance with the 

 three divisions of the vascular system. 



The pressure to which the circulating blood is subjected is the prod- 

 uct of a reaction participated in by four factors; namely by: (a) 

 the energy of the heart, (b) the quantity of the circulating blood, (c) 

 the elasticity of the blood-vessels, and (d} the peripheral resistance. 

 Under ordinary conditions, this pressure displays a certain constancy, 

 and retains a level considerably above zero throughout the circu- 

 latory system with the exception of the central veins. In addition 

 it is to be noted that it is subject to cer- 

 tain minor variations which are de- 

 pendent chiefly upon the action of the 

 heart and the respiratory movements. 

 These details will be brought out more 

 fully by the subsequent discussion. 



The Energy of the Heart. — Each 

 ventricular systole forces a definite 

 quantity of blood into the arteries. 

 Assuming that the other three factors 

 remain unchanged, it may be concluded 

 that the pressure must rise whenever 

 a new amount of blood is added to 

 that already existing in these channels, 

 and that the pressure must fall when- 

 ever the ventricles enter the state of 

 diastole. This relationship implies that 

 the energy of the heart must be pro- 

 portional to the ventricular output and 

 must embrace the following minor fac- 

 tors: 



(a) The volume of the cardiac out- 

 put, 



(6) The frequency with which these 

 discharges are repeated, and 



(c) The force with which the blood is ejected. 

 The first condition is determined by the capaciousness of the cardiac 

 chambers, ol* better, by the power of filling of the heart, the second by 

 the number of the discharges occurring in a given period of time, and 

 the third by the force with which the emptying of the ventricles is 

 effected. Right here it should be emphasize^ that the energy of the 

 heart which, as has just been stated, is only one of the factors upon which 

 blood pressure depends, is subject to fluctuations, because the condi- 

 tions previously cited, do not always act in unison, but may actually 

 counteract each other. Thus, the volume of the different ventricular 

 outputs may be increased owing to a greater filling power or relaxa- 

 bility of the cardiac musculature, without being associated with a rise 



Fig. 186. — Record of Blood- 

 pressure Showing the Cardiac and 

 Respiratory Variations. 



The time registered in seconds, 

 serves as the abscissa. 



