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Biology 367.docx - As the cardiac impulse travels from the...

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As the cardiac impulse travels from the S‒A node through the atria,the wave of contraction forces blood through the valves andinto theventricles. (In the atria, the impulse is transmitted from cell tocell acrossthe intercalated discs) . It is a "wave" of contraction becauseparts of theatria farthest from the S-A node contract later than do theparts closest tothe node. The parts of the atria that contract first are also thefirst to relax,thus causing a wavelike motion upon contraction. Once theimpulsereaches the A-V node, it is delayed (by a fraction of a second)before it ispassed on to the ventricles. This transmission delay allowstime for theatria to pump the blood into the ventricles before ventricularcontractionbegins. Unlike the atria, the ventricles contract as a unit. This
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atria farthest, cardiac impulse travels

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