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The heart is composed of special muscle known as cardiac muscle. Heart contractions although automatic, may be influenced when someone is excited or nervous, is very active or via chemicals in the body called hormones. However, the autonomic nervous system is responsible for maintaining a constant background level of activity via the parasympathetic nerves, to the S-A node, via the vagus nerve. There is an area in the heart called the cardiac pacemaker in the upper right atrium wall. Its job is to initiate the atrial contraction by a wave of electrical stimulation. This then reaches the atrioventricular node on the lower right atrium. This will cause a contraction of the ventricles after a delay, then, being conducted through the bundles of His and up the Purkinje fibers. The delay at the AV node is for the atria to send the blood contained within them down to the ventricles, to fill them. This entire process is what is known as the heart rate.
Again it has to deal with purkinje fibers and the seperate nods. but the SA node is known as the pacemaker of the heart.
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