## SA Node Anatomy and Physiology ### Location **Key Point:** The sinoatrial (SA) node is located at the junction of the superior vena cava (SVC) and the right atrium, within the wall of the right atrium at the lateral aspect of the sinoatrial groove. ### Blood Supply **High-Yield:** The SA node receives its arterial supply from the SA nodal artery, which is a branch of the right coronary artery (RCA) in approximately 60% of individuals and from the left circumflex artery in about 40% of cases. The RCA supply is more common. ### Innervation **Clinical Pearl:** The SA node receives both sympathetic and parasympathetic innervation: - **Sympathetic:** Via cardiac accelerator nerves (T1–T4 spinal segments) → increases heart rate - **Parasympathetic:** Via the right vagus nerve → decreases heart rate ### Cellular Composition **Key Point:** The SA node contains specialized pacemaker cells (P cells) that lack a true resting membrane potential and spontaneously depolarize, generating the intrinsic heart rate of 60–100 bpm. ### Histology The SA node is a small, crescent-shaped structure approximately 10–20 mm in length, composed of: - Pacemaker cells (P cells) - Transitional cells - Atrial myocytes [cite:Gray's Anatomy 42e Ch 3] 
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