Embedded in the wall of the |heart| are four structures that conduct impulses through the cardiac muscle to cause first the atria then the ~ventricles~ to contract. These structures are the |sinoatrial node| (SA ~node~), the |atrioventricular node| (AV ~node~), the bundle of His, and the Purkinje fibers. On the rear wall of the right atrium is a barely visible knot of tissue known as the sinoatrial, or SA ~node~. This tiny area is the control of the |hearts| pacemaker mechanism. Impulse conduction normally starts in the SA ~node~. It generates a brief electrical impulse of low intensity approximately 72 times every minute in a resting adult. From this point the impulse spreads out over the sheets of tissue which make up the two atria, exciting the |muscle fibers| as it does so. This causes contaction of the two atria and thereby thrusts the ~blood~ into the empty ~ventricles~. The impulse quickly reaches another small specialized knot of tissue known as the atrioventricular, or AV ~node~, located between the atria and the ~ventricles~. This ~node~ delays the impulse for about 0.07 seconds, which is exactly enough time to allow the atria to complete their |contractions|. When the impulses reach the AV ~node~, they are relayed by way of the bundle of His and Purkinje fibers to the ~ventricles~, causing them to contract. The electrical current races across the two ~ventricles~ within 0.06 seconds, causing the squeezing, thrusting motion of these powerful pumping chambers.
The |heart| also has its own built in safety factors. The AV ~node~, in an emergency situation, can take over the functions of the SA ~node~ by becoming the generator of the impulses. It is not quite as efficient, generating only a rate of 40 or 50 beats per minute.
