During the cardiac cycle the pressure in the different chambers varies. When the heart is being filled with blood, it is at relatively low pressure in the heart due to the heart muscle relaxing. The more the heart fills, the higher the pressure in the heart chambers becomes, particularly in the ventricles. When the heart is contracting, the pressure increases further because the heart wall is now pushing against the blood. When the pressure in the ventricles is higher than the blood pressure in the arteries, the blood moves out of the ventricles into the arteries. When the contractions stop and the heart relaxes again, the pressure falls.
The low pressure in the heart during diastole contributes to the filling of the heart with blood from the vena cava. In this way the heart drives blood around the body, it pushes it out during systole, and ”drags” it back during diastole.
Blood pressure in the atria varies much less than in the ventricles. This is because the blood volume and muscle power of the atria is much lower than in the ventricles, and as previously mentioned, it’s because most of the blood flows into the ventricles before the atria contract.