A heartbeat starts with an electrical signal in the wall of the right atrium - called the sino-atrial node (SAN) or pacemaker. This electrical signal sets the rhythm of the heartbeat.
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A wave of electrical activity spreads out from the SAN across both atria causing them to contract.
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A layer of non-conducting collagen fibrous tissue (septum) prevents the wave from being passed directly from atria to inside the ventricles.
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The electrical wave instead passes directly through one region only, the atrioventricular node (AVN), which is located inbetween the atria.
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This AVN is connected to specialised conducting muscle fibres located in the wall in the septum called the bundle of His.
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The AVN after a short delay is responsible for passing the electrical waves down the bundle of His.
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The bundle of His fibres conduct the electrical wave rapidly to the base of the ventricles, where the bundle branches off into the Purkinjie fibres.
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The electrical wave is released from these Purkinjie fibres causing both ventricles to contract quickly at the same time from the apex of the heart upwards.
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