Duplicate gene and its parental copy can subfunctionalize so that each carries out only some of the roles that the ancestral gene performed. 

Ex- a duplicate of the β-hemoglobin locus is expressed in the fetus. 

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Period of relaxation called diastole, during which the heart fills with blood A period of contraction called systole

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The total duration of the cardiac cycle, including systole and diastole, is the reciprocal of the heart rate.

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Atrial systole 

Preceded by the depolarization of the atria

Contraction of the atrium causes an increase in atrial pressure.

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Atrial systole

The P wave is caused by spread of depolarization through the atria and is followed by atrial contraction.

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Atrial systole 

Increase in atrial pressure is reflected back to the veins, it appears on the venous pulse record as the a wave.

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Atrial systole 

ventricle is relaxed, & because the AV valve is open, the ventricle is filling with blood from the atrium, even prior to atrial systole.