The beating of the heart is due to a system of stimulation inherent to the heart itself and not due to stimulation by the central nervous system. Two sets of nerves from the autonomic system do affect the heart, though, one set tending to accelerate the heart rate and one set tending to slow the heart rate. The left and right sides of the heart beat in unison, both halves being stimulated by a single set of heartbeat initiating mechanisms. The initiating mechanisms are composed of nodal tissue, a type of tissue unique to the heart that has the properties of muscle but can transmit impulses like a nerve. The sino-atrial node or S-A node is located near to where the anterior vena cava empties into the right atrium. The S-A node initiates the heartbeat creating an impulse that causes the walls of both atria of the heart to contract. The second node, the atrio-ventricular node or A-V node, is located in the partition between the two atria. When the wave of contraction initiated by the S-A node reaches the A-V node, it sends an impulse through a branching bundle of nodal tissue fibers, called the bundle of His, to all parts of both ventricles causing the ventricles to contract. The heart rate or number of times per unit time that the heart beats is around 70 times per minute in humans. The pressure of the blood in the heart during heart muscle contraction, called systolic pressure, is around 120 mm Hg. The pressure of the blood in the heart during heart muscle relaxation, called diastolic pressure, is around 80 mm Hg. Overall blood pressure is greatest in the heart and arteries and falls off rapidly in the arterioles reaching its lowest point in the veins just before they
