THE CARDIAC CYCLE 167 



scription of the events which occur during one complete heart- 

 beat. The period of actual contraction of the heart is termed 

 systole. This is divided into auricular and ventricular systole. 

 The term sphygmic period is applied to that part of ventricular 

 systole during which the blood is actually leaving the ventricles. 

 The period of relaxation and rest of the cardiac muscles is called 

 diastole. The cardiac cycle includes the time of systole and dias- 

 tole of the heart. 



The Events of the Cardiac Cycle. During diastole the blood 

 flows in a steady stream from the great veins through the two 

 auricles into the ventricles, the auriculo-ventricular valves being 

 open. When the ventricles are as full as the weight and the pres- 

 sure of the blood can make them, auricular systole begins. The 

 auriculo-ventricular valves at this instant are floating in the 

 blood which has collected in the ventricles, and are almost in the 

 position of closure, but a narrow chink still remains between 

 them, and through this, auricular systole forces blood under 

 pressure into the ventricle, thus filling the ventricles completely. 

 At the dead stop of auricular systole there are currents of blood 

 reflected back along the sides of the ventricles which strike the 

 under surface of the valves and completely close them. Ven- 

 tricular systole now begins. The closed valves prevent the pass- 

 age of blood back into the auricles, and the entire force of the 

 ventricles is expended in forcing the blood out through the ar- 

 terial openings. Whenever the pressure in the ventricles exceeds 

 that in the arteries, the semilunar valves open and remain open 

 till the force of the ventricle falls below the pressure of blood in 

 the arteries. The time between the closing of the auriculo-ven- 

 tricular valves and the opening of the semilunar valves is called 

 the period of getting up power, or the pre-sphygmic period (Fig. 

 19). 



It is obvious that when the blood is leaving the ventricles the 

 pressure must be less in the arteries than in the heart. Each ven- 

 tricle pours out more blood into its artery than can pass through 

 the capillaries in the same unit of time, and hence the arterial 

 walls are stretched and the blood is put under their elastic ten- 

 sion. At the moment the ventricles exert less pressure than does 



