252 HUMAN PHYSIOLOGY 



to a maximum and as soon as it exceeds the back 

 pressure in the arteries it forces apart the semilunar 

 valves. A rapid emission of blood follows. When the 

 systole of the ventricle wanes it does so with some 

 suddenness; the internal pressure falls below that in the 

 arteries and the incipient reflux of blood closes the 

 semilunar valves sharply. Twice in the course of the 

 heart cycle the ventricles are closed cavities: first, just 

 after the auricular contraction when their content of 

 blood is greatest, and second, when their own systole 

 ends and their volume is least. 



The amount of blood thrown from either ventricle 

 during its contraction has been estimated all the way 

 from 2 to 6 ounces. The highest figure is based on the 

 assumption that the ventricle is considerably distended 

 and then practically obliterated. This cannot be as- 

 sumed and a medium quantity of 4 ounces seems more 

 probable. Pumping at such a rate, one ventricle 

 would put out about all the blood in the body in the 

 course of 40 beats of the heart. This justifies the state- 

 ment that the average corpuscle is sent from a given 

 point say the left ventricle around the systemic and 

 the pulmonary circuits and back to the place of beginning 

 about twice in a minute. 



At the close of the ventricular systole a moderate 

 reduction of the pressure in the ventricle will lead to the 

 closure of the semilunar valves. The pressure must 

 fall much lower to permit the mitral and tricuspid valves 

 to open. But the relaxation is swift and this event 

 follows within a few hundredths of a second. The 

 ventricle ceases to exert any pressure upon its contents 

 and for a brief interval its walls are actually drawing 

 apart with a slight suction effect. The cause of this 

 outward movement will be apparent when we have 

 considered the mechanical conditions in the chest as we 

 shall do when taking up the subject of respiration. 



While the mitral and tricuspid valves are closed, the 

 blood brought to the heart by the veins must be ac- 



