256 PHYSIOLOGY 



pressure in the auricles and veins increases and they are 

 again expanded. 



This is the normal venous pulse. But, if the auriculo-ven- 

 tricular valves are incompetent, when the ventricles contract, 

 blood is forced back into the auricles and veins and a crest 

 develops between the two normal crests. The height of this 

 third crest is a good index of the amount of regurgitation. 



II. Respiratory Variations in Blood Pressure. 



Not only do rhythmic changes in the arterial pressure occur 

 with each beat of the heart, but larger changes are caused 

 by the respirations the rise in pressure in great measure 

 corresponding to the phase of inspiration, the fall in pressure 

 to the phase of expiration. This statement is not quite 

 accurate, as will be seen when considering the influence of 

 respiration on circulation (see page 295). These variations 



I I I 



FlO. 123. Tracing of Arterial Blood Pressure to show large Respiratory Varia- 

 tions, and small Variations due to Heart Beats, a to 6, Inspiration ; 6 to a 1 , 

 Expiration. 



are easily seen in a tracing of the arterial pressure taken with 

 the mercurial manometer (Fig. 123). 



A pulse synchronous with the respirations may also be 

 observed in the great veins at the root of the neck and in 

 the cranium when opened. With each inspiration they tend 

 to collapse, with each expiration they again expand. The 

 reason for this is that during inspiration the pressure inside 

 the thorax becomes low and hence blood is sucked from the 

 veins into the heart, while during expiration the intra-thoracic 

 pressure becomes higher and thus the entrance of blood into 

 the heart is opposed. 



