CARDIAC CYCLE 169 



contents until the pressure exceeds that in the pulmonary artery (and aorta), 

 the semilunar valves open, and the blood is discharged into the arteries. 

 The ventricles ordinarily remain contracted for a brief moment after their 

 contents are emptied. 



The ventricular diastole begins next and with the initial relaxation, and 

 the first slight fall of the intraventricular pressure below that of the aorta, 

 the semilunar valves close and the second sound is heard. The relaxation 

 rapidly proceeds and the intraventricular pressure drops to below atmos- 

 pheric pressure, the auriculo-ventricular valves fall open, the blood that has 

 been accumulating in the auricles flows into the ventricles, and the whole 

 heart is in the state of pause described as the point of beginning. 



The duration of the cardiac cycle varies with the heart rate. With a 

 rate of 75 per minute, the cardiac cycle will take 0.8 of a second. In round 

 numbers the systole of the auricle takes o.i of a second with a diastole of 

 0.7 of a second, 0.6 of which is in the pause or rest period. The ventricle 

 requires about 0.3 of a second for the systole, 0.5 of a second for the dias- 

 tole, with 0.2 to 0.3 of this for the pause. It is evident that the whole heart 

 is at rest at the same instant for from o.i to 0.2 of a second. 



The relations of the cardiac sounds to the systole and the diastole have 

 been graphically recorded by Hiirthle, figure 153, page 158, and by Einthoven 

 and Geluk, figure 154, page 159. The former found that in a heart-beat last- 

 ing 0.76 of a second the interval of time between the beginning of the first 

 and second sounds was 0.25 of a second, and that the sounds occur just at the 

 beginning of the ventricular systole and diastole respectively. 



During the cardiac cycle the ventricles are completely closed from the 

 moment of the beginning of the ventricular systole until the pressure amounts 

 to a little greater than the pressure in the corresponding arteries, which 

 takes about 0.2 of a second. From the opening of the semilunar valves 

 until the closure of those valves, about 0.15 of a second, the ventricular cavity 

 is in open communication with the arteries. There is, during the diastole, 

 a second moment of complete closure of the ventricles, from the time of the 

 closing of the semilunar valves until the ventricular pressure falls below the 

 auricular pressure which permits the auriculo-ventricular valves to open. 



The Force of the Cardiac Action. In estimating the amount of 

 work done by a machine it is usual to express it in terms of work units. A 

 convenient work unit for this purpose is the amount of energy required to 

 lift a unit of weight, i.e., i gram or i kilogram, through a unit of height, i.e., 

 i centimeter or i meter, the work required being i gramcentimeter for small 

 units, and i kilogrammeter for large units, respectively. The average work 

 done by the heart at each contraction can be readily computed by multi- 

 plying the weight of blood expelled by the ventricle by the height through 

 which it would have to be lifted to overcome the resistance to its discharge 

 from the cavities into the arteries. 



