branch of sinuatrial node

例句：

1. 1. The sinuatrial node is the site where the AV node sends signals to the atria.
   心房节是AV节点将信号发送到心房的地点。
2. 2. The sinuatrial node is responsible for regulating heart rate and rhythm.
   心房节负责调节心率和节律。
3. 3. During sleep, the sinuatrial node becomes more active, allowing the heart to beat more slowly.
   在睡眠期间，心房节变得更加活跃，使心脏跳动得更加缓慢。