Atrium (heart) - Atrial Function

Atrial Function

In human physiology, the atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart during ventricular systole. By being partially empty and distensible, atria prevent the interruption of venous flow to the heart that would occur during ventricular systole if the veins ended at the inlet valves of the heart. In normal physiologic states, the output of the heart is pulsatile, and the venous inflow to the heart is continuous and non-pulsatile. But without functioning atria, venous flow becomes pulsatile, and the overall circulation rate decreases significantly.

Atria have four essential characteristics that cause them to promote continuous venous flow. (1) There are no atrial inlet valves to interrupt blood flow during atrial systole. (2) The atrial systole contractions are incomplete and thus do not contract to the extent that would block flow from the veins through the atria into the ventricles. During atrial systole, blood not only empties from the atria to the ventricles, but blood continues to flow uninterrupted from the veins right through the atria into the ventricles. (3) The atrial contractions must be gentle enough so that the force of contraction does not exert significant back pressure that would impede venous flow. (4) The "let go" of the atria must be timed so that they relax before the start of ventricular contraction, to be able to accept venous flow without interruption.

By preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole, atria allow approximately 75% more cardiac output than would otherwise occur. The fact that atrial contraction is 15% of the amount of the succeeding ventricular ejection has led to a misplaced emphasis on their role in pumping up the ventricles (the so-called "atrial kick"), whereas the key benefit of atria is in preventing circulatory inertia and allowing uninterrupted venous flow to the heart.