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Blood flow to heart during systole & diastole

During systole the heart muscle contracts, it compresses the coronary arteries so blood flow is less to the left ventricle during systole and more during diastole.
Blood flow to the subendocardial portion of Left ventricle occurs only during diastole
Due to lack of blood flow to subendocardial surface of left ventricle during systole this region is prone to ischemic damage and it is the most common site of Myocardial infarction.
Coronary blood flow to the right side of heart is not much affected during systole.
Reason-The Pressure difference between aorta and right ventricle is greater during systole than during diastole so more blood flow to right ventricle occurs during systole.

Effect of Tachycardia on coronary blood flow
When heart rate increases,the period of diastole is shortened therefore coronary blood flow is reduced to heart during tachycardia
Other causes of decreased blood flow to left ventricle
   1-Aortic stenosis
Reason-In aortic stenosis the left ventricle pressure is very high during systole thus, it compresses the coronary arteries more.
  2-When aortic diastolic pressure is low,the coronary blood flow is decreased