THE PULSATORY VARIATIONS IN BLOOD PRESSURE 



391 



respiratory movements. It is to be noted that inspiration produces a 

 fall in pressure in the veins and a rise in the arteries, whereas expiration 

 causes an increase in the venous and a fall in the arterial pressure (Fig. 

 205). These changes are generally associated with an alteration in the 

 cardiac rhythm, the heart beating more frequently during inspiration. 

 Moreover, these fluctuations do not begin precisely with the onset of 

 the respiratory movements, but somewhat later, the intervening period 

 being about 0.2 second in duration. It happens, therefore, that the 

 arterial rise is always continued for a brief period of time after the be- 

 ginning of the expiratory motion, while the fall is prolonged right into 

 the succeeding inspiratory phase. 



The Cause of the Respiratory Variations. — After the first breath 

 has been taken, the lungs are held in a continuous state of hyperdisten- 

 tion. The elastic fibers contained in them are put on the stretch and 

 must therefore always attempt to recoil. This enables these organs to 

 exert an elastic pull upon the chest wall as well as upon the contents of 



A?\ 



Fig. 205. — Diagrammatic Representation of tee Respiratory Variations in ARTERiAii 

 {AP) AND Venous Pressure (VP). 

 JE, inspiration; EJ, expiration. It is to be noted that the variations in pressure 

 lag behind the onset of the respiratory movement; this interval {JB) being especially 

 evident in the case of the arterial pressure. 



the thoracic cavity, which is betrayed, on the one hand, by a nega- 

 tivity in the intrapleural pressure ( — 6 to —9 mm. Hg) and, on the 

 other, by the low degrees of pressure existing in the central venous 

 system (--5 to —15 mm. Hg). The blood-vessels situated outside 

 the thorax are exposed to positive pressures, and hence, it cannot 

 surprise us to find that the blood in the intrathoracic vessels is con- 

 stantly exposed to this aspiratbry force. But inasmuch as the ar- 

 teries are relatively resistant and unyielding, they are not so severely 

 affected as the veins. 



It must be granted, therefore, that the negative pressure in^ the 

 thorax favors the venous return. Moreover, as the elastic pull upon 

 the venous trunks is greater during inspiration than during expiration, 

 the inspiratory movement must be the more effective of the two. For 

 this reason, it is only natural to assume that the venous pressure is 

 decreased during inspiration and increased during expiration. It 

 may be inferred that these changes in pressure influence the flow in 



