Since the sa node is at the top of the atrial mass, the atria contract from the top down. how does this increase the efficiency of atrial contraction? do the ventricles have a similar arrangement? if so, how does it work?

The cardiac cycle is related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. Atrial systole is the contraction of the heart muscle of the left and right atria. The heart has two areas that initiate impulses, the SA or sinoatrial node, and the AV or atrioventricular node. The heart contains special muscle fibers called Purkinje fibers. Purkinje fibers conduct impulses five times more rapidly than surrounding cells and forms a pathway for conduction of the impulse. This ensures that the heart muscle cells contract in the most efficient pattern. The SA node is located in the wall of the right atrium, near the junction of the atrium and the superior vena cava.

As  bicarbonate ions

Explanation: