HEART 405 



The pressure in the auricles is lowest at the moment of 

 their diastole. At this time there is therefore a srreat flow 

 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 

 great 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 OTeat veins in 

 auricular systole drives blood from the veins into the 

 auricles, and prevents any back-flow from the auricles. 



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 later stagfe 

 of ventricular diastole, the intra-ventricular pressure becomes 

 nearly the same as the intra-auricular, and the flow 

 diminishes or may stop. When the auricles contract, a 

 higher pressure is developed causing a fresh flow of 

 blood into the ventricles. When the ventricles contract 

 the auriculo- ventricular valves are closed, and all flow of 

 blood from the auricles is stopped (fig. 178). 



G. From Ventricles to Arteries. — When the ventricles 

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

 the pressure in the arteries is high, which keeps the semi- 

 lunar valves shut. This is the Latent or Presphygmic 

 Period. As ventricular systole goes on, the intra-ventricular 

 pressure rises, until, after about 0"03 of a second, it becomes 

 higher than the arterial pressure. Immediately the semi- 

 lunar valves are forced open and a rush of blood occurs 

 from the ventricles. This is the Period of Outfloiu, which 

 usually lasts less than 0'2 second. 



(a) If the ventricles are contracting actively, and if 

 the pressure in the arteries does not offer a great resistance 

 to the entrance of the blood, the ventricles rapidly empty 

 themselves into the arteries, and the intra-ventricular pressure 

 varies as shown in fig. 175, h. 



(6) If the heart, however, is not contracting actively, or if 

 the arterial pressure offers a great resistance to the entrance of 

 blood, then the outjioiu is slow and more continued, and in 



