IQ4 THE CIRCULATION OF THE BLOOD 



Cardiac Cycle. The entire series of occurrences in a single heart- beat 

 is called the cardiac cycle. If the condition of the heart is considered at that 

 moment when its muscular walls are at rest it will be found that the tricuspid 

 and mitral valves are open, that the blood is flowing from the great veins 

 into the auricle and the ventricle, which form a continuous cavity, and 

 that the pressure is about that of the atmosphere, but slowly rising. Now 

 a wave of contraction begins at the sinus node and extends over the auri- 

 cles, which immediately contract and discharge their blood into the ventri- 

 cles, somewhat distending their walls. At this moment the ventricular 

 systole begins, the tricuspid and mitral valves are closed, the flow of blood 

 into the ventricles is checked, and the first heart sound is heard. The con- 

 traction of the ventricles produces a rapidly rising pressure on the enclosed 

 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. 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 of 

 the cycle. 



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

 rate of 75 per minute, the cardiac cycle will take o . 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, o . 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 o . 2 to o . 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 o . 2 of a second. 



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

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

 Geluk, figure 154. The former found that in a heart-beat lasting o. 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 begin- 

 ning 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 o . 15 of a second, the ventricular cavity 

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



