Completion of ventricular filling: atrial systole. Although most blood enters the ventricles while the atria are relaxed, the last 20% of filling is accomplished when the atria contract and push blood into the ventricles. (This applies to a normal person at rest. When heart rate increases, as during exercise, atrial contraction plays a greater role in ventricular filling). Atrial systole, or contraction, begins following the wave of depolarization that sweeps across the atria. The pressure increase that accompanies contraction pushes blood into the ventricles.
A small amount of blood is forced backward into the veins because there are no one-way valves to block backward flow, although the openings of the veins do narrow during contraction. This retrograde movement of blood back into the veins may be observed as a pulse in the jugular vein of a normal person who is lying with the head and chest elevated about 30°. (Look in the hollow formed where the sternocleidomastoid muscle runs under the clavicle.) An observable jugular pulse higher on the neck of a person sitting upright is a sign that pressure in the right atrium is higher than normal.