THE PULSATORY VARIATIONS IN BLOOD PRESSURE 377 



pendent upon the elastic pull of the lungs upon the soft walls of the 

 central veins, serve as accessory means to augment and to conserve 

 the initial driving force of the heart. 



CHAPTER XXXII 



THE PULSATORY VARIATIONS IN BLOOD PRESSURE 



A. THE CARDIAC VARIATIONS IN ARTERIAL PRESSURE 



The Cause of the Arterial Pulse. — Fluctuations in pressure are 

 encountered in the arteries as well as in the veins; in fact, they are 

 also perceptible at times in the capillaries. They possess a twofold 

 origin, being caused either by the contractions of the heart, or by the 

 movements of respiration. If the former, they are designated as the 

 cardiac, and if the latter, as the respiratory variations in blood pressure. 

 Moreover, as each group of changes makes itself felt in the arteries 

 as well as in the veins, they are again subdivided into the cardiac varia- 

 tions in arterial and venous blood pressure, and into the respiratory 

 variations in arterial and venous pressure. The principal changes 

 due to the activity of the heart, are the so-called arterial pulse and the 

 physiological venous pulse. 



Each ventricular systole adds a certain quantity of blood to that 

 already transferred into the arterial system by the preceding systoles. 

 The arterial pressure increases with each ventricular discharge above 

 that prevailing during the previous diastolic period. Furthermore, 

 owing to the elasticity of the arterial channels, each inrush of blood 

 causes a distention of their walls which is followed by a recoil as soon 

 as the influx has ceased. Obviously, this elastic play serves the pur- 

 pose of lessening the systolic strain upon the cardiac muscle as well as 

 that upon the walls of the blood-vessels, because if the heart were 

 forced to pump into a system of rigid tubes, its contractions would 

 necessarily become labored, owing to the fact that a certain amount of 

 blood would first have to be dislodged from the tubes before a new 

 amount could be accommodated therein. A condition of this kind 

 would occasion a periodic escape of venous blood to counterbalance 

 the quantity of arterial blood forced in, and this intermittent or re- 

 mittent flow would be characterized by very high systolic and very low 

 (iiastolic pressures. 



Contrary to this result, the distensibility of the arterial walls 

 enables this system to accommodate the successive outputs of the heart 

 by simply enlarging its caliber. Moreover, this process insures the 

 least possible expenditure of energy and does not permit of the develop- 

 ment of disturbing fluctuations in pressure and flow. In addition, 

 the subsequent recoil of the arterial walls serves the purpose of con- 



