Sinusnoden

The sinus node, also known as the sinoatrial node or SA node, is a specialized area of cardiac tissue located in the upper wall of the right atrium of the heart. It functions as the natural pacemaker of the heart, initiating the electrical impulses that cause the heart to beat. These impulses originate spontaneously within the sinus node and spread outwards through the atria, causing them to contract.

The sinus node is composed of modified cardiac muscle cells that have the unique ability to generate electrical impulses rhythmically without external stimulation. This inherent electrical activity is called automaticity. The rate at which the sinus node fires determines the heart rate. This rate can be influenced by various factors, including the autonomic nervous system (sympathetic and parasympathetic branches) and circulating hormones.

When the electrical impulse leaves the sinus node, it travels through the atrial walls, causing them to contract and pump blood into the ventricles. The impulse then reaches the atrioventricular node (AV node), where it is briefly delayed, allowing the atria to fully empty before the ventricles contract. From the AV node, the impulse travels down the bundle of His and Purkinje fibers, stimulating the ventricular muscle to contract and pump blood to the rest of the body. The sinus node's consistent and rhythmic electrical generation is crucial for maintaining a regular heartbeat and adequate blood circulation.