What occurs when the atria contract?

When the atria contract, they are actively pushing blood into the ventricles, which is a crucial phase of the cardiac cycle known as atrial systole. This contraction occurs just before the ventricles contract and leads to the filling of the ventricles with blood. The pressure created by the contraction of the atria helps to ensure that the ventricles are adequately filled with blood before they begin their own contraction.

The contraction of the atria is also synchronized with the electrical activity of the heart, specifically the firing of the sinoatrial (SA) node, which sets the pace for the heart rate. While this electrical activity can influence the overall heart rate, the immediate effect of atrial contraction is primarily the movement of blood to the ventricles. Therefore, the most accurate representation of what happens when the atria contract is that blood is squeezed into the ventricles.