The heart's own pacemaker.

An impulse starts at the SAN, pauses at the AVN, runs down the AV bundle and Purkinje fibres; SAN rate sets heart rate, ANS speeds or slows it, and cutting the AV bundle leaves the ventricles on their own slower rhythm.

Count QRS complexes over 10 s to read 72 beats/min, then use only the ANS control to bring the rate above 100.

Pacemaker in the left atrium; heart rhythm started by the brain (it is myogenic)

• The human circulatory (blood vascular) system has a muscular chambered heart, a closed network of branching blood vessels, and blood, the fluid that circulates.
• The heart develops from mesoderm and sits in the thoracic cavity between the two lungs, tilted slightly to the left. It is about the size of a clenched fist and is enclosed by a double-walled membranous bag, the pericardium, containing pericardial fluid.
• It has four chambers: two smaller upper atria and two larger lower ventricles. A thin muscular inter-atrial septum separates the atria; a thick-walled inter-ventricular septum separates the ventricles; a thick fibrous atrio-ventricular septum separates each atrium from the ventricle below it, with an opening connecting the two on each side.
• The right atrio-ventricular opening is guarded by the tricuspid valve (three muscular flaps or cusps); the left by the bicuspid or mitral valve. The openings of the right and left ventricles into the pulmonary artery and aorta carry semilunar valves.
• The valves let blood flow only one way, atria to ventricles and ventricles to the pulmonary artery or aorta, and prevent backflow.
• The whole heart is cardiac muscle, and the ventricle walls are much thicker than the atrial walls.
• Nodal tissue, a specialised cardiac musculature, is spread through the heart. The sino-atrial node (SAN) lies in the right upper corner of the right atrium; the atrio-ventricular node (AVN) lies in the lower left corner of the right atrium, near the atrio-ventricular septum.
• From the AVN, the atrio-ventricular bundle (AV bundle) passes through the atrio-ventricular septa, emerges on top of the inter-ventricular septum and divides into right and left bundles, which give rise to fine Purkinje fibres through the ventricular muscle of each side.
• Nodal musculature is autoexcitable: it produces action potentials without any outside stimulus. The SAN produces the most, 70–75 per minute, so it starts and maintains the heart's rhythm and is called the pacemaker.
• The heart normally beats 70–75 times a minute, averaging 72 beats per minute.

Teaching this idea? Put the simulation on your own page or LMS with this code.