Question

now atria contract forcing the blood to enter into the ventricles ( systole )​

Answer 1

Explanation:

As left atrial pressure increases, the mitral valve opens and blood flows into the left ventricle. Figure 1.4. The cardiac cycle. In the systole phase, blood is forced to flow from the two atria into their respective ventricles as the atrial muscles contract due to the depolarization of the atria

Answer 2

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The atmosphere pressure from outside pushes inside our blood vessels otherwise they could burst.

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