230 HUMAN PHYSIOLOGY 



their diastole. At this time there is therefore a great fl>\v 

 of blood into them, but gradually this becomes less and less, 

 until, when the ventricles dilate, another fall in the auricular 

 pressure takes place and another rush of blood from the gr at 

 veins occurs. Gradually this diminishes, and by the time 

 that the auricles contract the flow from the great veins has 

 stopped. 



The contraction of the mouths of the great veins, which 

 precedes the auricular systole, drives blood from the veins 

 into the auricles, and, as these enter into contraction, no flow 

 from the veins can occur, and no back flow from the auricles 

 is possible (Fig. 114). 



B. From Auricles to Ventricles. As the ventricles dilate, 

 a very low pressure develops in them, and hence a great rush 

 of blood occurs from the auricles. During the passive stage 

 of ventricular diastole, the intra- ventricular pressure becomes 

 nearly the same as the auricular, and the flow diminishes or 

 may stop. When the auricles contract a higher pressure is 

 developed, and a fresh flow of blood occurs into the ventricles. 

 When the ventricles contract the auriculo-ventricular valves 

 are closed, and all flow of blood from the auricles is stopped 

 (Fig. 114). 



C. From Ventricles to Arteries. When the ventricles 

 begin to contract the intra-ventricular pressure is low, while 

 the pressure in the arteries is high and keeps the semilunar 

 valves shut. As ventricular systole goes on the intra-ven- 

 tricular pressure rises until after about 0-03 of a second it 

 becomes higher than the arterial pressure (Latent Period). 

 Immediately the semilunar valves are forced open and a rush 

 of blood occurs from the ventricles (Period of Overflow). 

 This usually lasts less than O2 second. If the ventricles are 

 acting powerfully, and if the pressure in the arteries does not 

 offer a great resistance to the entrance of blood, then the 

 ventricles rapidly empty themselves into the arteries. If the 

 heart, however, is not acting forcibly, or if the arterial pressure 

 offers a great resistance to the entrance of blood, then the 

 outflow is slow and more continued. In the former case we 

 get a trace of the intra-ventricular pressure like Fig. 121, a, 

 p. 252, with a well-marked Period of Residual Contraction, 

 and in the latter case the trace is like Fig. 121, b. It is not 



