Heart Pumps

The sinoatrial node (SAN), located within the wall of the right atrium (RA), normally generates electrical impulses that are carried by special conducting tissue to the atrioventricular node (AVN). Upon reaching the AVN the electrical impulse is sent down conducting tissue that branches into pathways that supply the right and left ventricles. These paths are called the right bundle branch and left bundle branch. The left bundle branch further divides into two sub branches called fascicles. Electrical impulses generated in the SAN cause the right and left atria to contract first. Depolarization (heart muscle contraction caused by electrical stimulation) occurs nearly simultaneously in the right and left ventricles 1-2 tenths of a second after atrial depolarization. The entire sequence of depolarization, from beginning to end (for one heart beat), takes 2-3 tenths of a second. All heart cells, muscle and conducting tissue, are capable of generating electrical impulses that can trigger the heart to beat. Under normal circumstances all parts of the heart conducting system can conduct over 140-200 heart beats per minute. The SAN is known as the "heart's pacemaker" because electrical impulses are normally generated here. At rest the SAN usually produces 60-70 heart beats a minute. It is the SAN that increases its' rate due to stimuli such as exercise, stimulant drugs, or fever.
Should the SAN fail to produce impulses the AVN can take over. The resting rate of the AVN is slower, generating 40-60 heart beats a minute. The AVN and remaining parts of the conducting system are less capable of increasing heart rate due to stimuli previously mentioned than the SAN. Ventricular muscle cells may generate 20-30 heart beats a minute. Heart rates below 35-40 heart beats a minute for a prolonged period usually cause problems due to not enough blood flow to vital organs.