As I mentioned earlier, the upper and lower chambers of the heart are electrically insulated from each other by the Atrio-Ventricular Valves (which are made of, like, collagen or something and are not the electrically-reactive myocytes). This means that the Sinoatrial node (SA Node) does not directly stimulate the ventricles to contract.
This sounds like it would be a problem, but it is really a good thing. If the SA Node directly stimulated both the atria and the ventricles, they would contract/pump at the same time. There would be no pause to allow time for each compartment to fill with blood, and cardiac output (Abbreviated as CO, it means the amount of blood the heart pumps out per minute) would be deminished.
The Pathway Through the Atrio-Ventricular Node-
The pathway for electric stimulation to pass between the atria and reach the ventricles is through the Atrio-Ventricular Node (Also called the AV Node). It is a nerve-thingy that picks up the electro-chemical depolarization and carries it to the ventricles.
The AV Node is responsible for the brief delay that occurs between atrial and ventricular contraction. The conduction is carried through the AV Node to the nerve fibers called 'The His Bundle', which then separates into the Left and Right Bundle Branches (Servicing the Left and Right ventricles, respectively) Eventually these branches terminate in the Purkinje Fibers that directly depolarize the myocytes in the ventricles.
(Note: The ion used to transmit the electrical stimulation (aka depolarization) through ventricles system is calcium which, apparently, is slower then Sodium at conducting a depolarization impulse. This becomes esspecially importiant when a person has a calcium imbalance).
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