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A small patch of specialised cells in the heart, the sinoatrial node, fires an electrical signal on its own that starts every heartbeat. The nerves do not create the beat; they only speed it up or slow it down.

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The sinoatrial (SA) node is a strip of tissue about 15 millimetres long in the wall of the right atrium, the heart's upper right chamber. The electrical charge of its cells drifts upwards on its own until it reaches a trigger point and fires, again and again, so no outside command is needed. The signal spreads across both atria, pauses briefly at a second cluster, the atrioventricular node, so the upper chambers can empty, and then races through the lower chambers so they contract together. Arthur Keith and Martin Flack described the node in 1907. At rest the heart beats about 60 to 100 times a minute, with the vagus nerve from the brain holding the node below its own faster pace. If the node fails, other parts of the heart's wiring can take over at a slower rate, or doctors fit an artificial pacemaker.

Türkçe oku
Kalpteki sinoatriyal düğüm (sinüs düğümü) denen küçük, özelleşmiş bir hücre kümesi, kendiliğinden elektrik sinyali üreterek her kalp atımını başlatır. Sinirler atımı yaratmaz; yalnızca hızlandırır ya da yavaşlatır.

Sinoatriyal (SA) düğüm yaklaşık 15 milimetre uzunluğunda bir doku şerididir ve kalbin sağ üst odacığının, yani sağ kulakçığın duvarında yer alır. Hücrelerinin elektrik yükü kendiliğinden, yavaş yavaş yükselir; bir eşiğe ulaşınca hücreler bir uyarı çıkarır ve döngü baştan başlar. Bu yüzden düğümün dışarıdan emir almasına gerek yoktur. Sinyal önce iki kulakçığa yayılır. Ardından atriyoventriküler düğüm denen ikinci bir hücre kümesinde kısa bir süre bekler; bu bekleme, kulakçıklara kanlarını boşaltacak zaman tanır. Sonra alt odacıklara, yani karıncıklara hızla yayılır ve onların birlikte kasılmasını sağlar. Düğümü 1907'de Arthur Keith ile Martin Flack tanımladı. Dinlenme hâlinde kalp dakikada yaklaşık 60-100 kez atar; beyinden gelen vagus siniri, düğümü kendi doğal hızının altında tutar. Düğüm görevini yapamazsa kalbin iletim sisteminin başka bölümleri işi daha yavaş bir hızla devralabilir ya da hekimler yapay kalp pili takar.

Sources and further reading

Background reading: Sinoatrial node
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