The SA node is sometimes referred to as the heart's "natural pacemaker" because it starts the impulses for the heartbeat. The heart beat (contraction) begins when an electrical impulse from the sinus node (also called SA node) moves through the heart's muscle tissue. Electrical system of the heart Electrical signals control the pump For the heart to function properly, the four chambers must beat in an organized way. The heart pumps blood to the lungs and to all the body's tissues by a sequence of highly organized contractions of the four chambers. Under normal conditions, the valves allow blood to flow in only one direction. Blood flow occurs only when there's a difference in pressure across the valves, which causes them to open. The mitral valve has two flaps the others have three.
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