Atrial systole is the contraction of the heart muscle (myocardia) of the left and right atria. Normally, both atria contract at the same time. The term systole is synonymous with contraction (movement or shortening) of a muscle. Electrical systole is the electrical activity that stimulates the myocardium of the chambers of the heart to make them contract. This is soon followed by Mechanical systole, which is the mechanical contraction of the heart.
As the atria contract, the blood pressure in each atrium increases, forcing additional blood into the ventricles. The additional flow of blood is called atrial kick.
80% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.
Atrial kick is absent if there is loss of normal electrical conduction in the heart, such as during atrial fibrillation, atrial flutter, and complete heart block. Atrial kick is also different in character depending on the condition of the heart, such as stiff heart, which is found in patients with diastolic dysfunction.
Read more about this topic: Cardiac Cycle