During mild to moderate exercise, heart rate increase. The change is-

Ans. is 'c' i.e., There is greater decrease in ventricular diastole than ventricular systoleo In a normal person with average heart rate of 75 per min each cardiac cycle lasts about for 0.8 seconds: - Isovolumetric contraction 0.05 sec, rapid ventricular ejection 0.11 sec, slow (reduced) ventricular ejection 0.14 sec (total ejection phase 0.25 sec), protodiastole 0.04 see, isolvoumetric relaxation 0.06 sec. rapid ventricular filling 0.10 sec, diastasis 0.20 sec, and atrial systole 0.10 sec.o So, In a cardiac cycle of 0.8 sec duration, the atrial systole lasts for only about 0.1 sec and the atrial diastole lasts for 0.7 sec.o That means, only during the last phase of ventricular cycle (last rapid filling phase), the atria are contracting (atrial systole). During other six phases, atria are relaxed, i.e., atrial diastole,o However, it should be kept in the mind that above described durations of cardiac cycle are not fixed, the durations of various phases of the cardiac cycle depend on the heart rate,o When the heart rate increases, the durations of all the phases decreases,o However, the duration of diastole decreases much more than duration of systole,o The marked reduction in diastolic time during tachycardia has important clinical implication,o It is during diastole that most of the ventricular filling occurs.o Also, it is during diastole that most of the cardiac muscle, especially the subendocardial portion of the left ventricle, gets adequately perfused by the coronary blood flow.o Hence, at heart rates greater than 180 per minute, there is reduction in ventricular filling (which tends to reduce cardiac output) and cardiac perfusion (which tends to cause myocardial ischemia and infarction).