Circulation of Body Fluids 



543 



cessive chambers. The sequence of contraction is essentially similar in both 

 the two-sided (4- or 3-chambered) and one-sided (2-chambered) hearts. 

 During ventricular contraction, while the aortal (semilunar) valves are open, 

 the aorta fills and the heart empties; ventricular systole (contraction) stops 



12 3 456 7 ^ 



Fig. 197. Correlated events of the cardiac cycle. Venous pulse, ele^)cardiogram, and 

 heart sounds in man; aortic pressure, auricular pressure, ventricular pressure, and ven- 

 tricular volume adapted from the dog. 1, Closure of the AV valves and beginning of 

 ventricular contraction; 2, opening of aortic valves; 2-3, maximal ejection phase; 3-4, 

 reduced ejection phase; 4-5, protodiastolic phase with closure of aortic valves at 5; 5-6, 

 isometric relaxation, opening of A-V valves at 6; 6-7, rapid filling; 7 to auricular systole 

 is phase of diastasis. Time marks at 0.1 sec. on venous pulse curve apply to lower three 

 curves. From Fulton."^ 



when ventricular pressure falls below that of the aorta, then the aortal valves 

 close (Fig. 197). During ventricular contraction the auricular pressure is 

 below that in the ventricle and the auriculoventricular valves remain closed; 

 as the auricles fill their pressure gradually rises while the ventricular pres- 

 sure is falling, the auriculoventricular valves then open, blood enters the 



