The Cardiac Conduction System
The pumping action of the heart (heartbeat) is controlled by the heart’s electrical system or the cardiac conduction system. This is a group of specialised cells located in the wall of the heart which send electrical impulses to the cardiac muscle causing it to contract. The cardiac conduction system comprises of the: Sinoatrial (SA) node Atrioventricular (AV) node Bundle of His Bundle branches Purkinje fibres Contractions in the heart begin when electrical impulses are sent from the SA node (also known as the natural pacemaker) which is located in the right atrium. The impulse from the SA node causes the atria to contract, pushing blood through the open valves into the ventricles. The electric signal arrives at the AV node which is located between the two atria. From here it travels through the bundle of His, divides into the left and right bundle branches and through the Purkinje fibres. This causes the ventricles to contract. Both ventricles do not contract at precisely the same time, the left ventricle contracts slightly before the right. When the ventricles contract blood from the right ventricle is pumped through the pulmonary valves and onto the lungs, blood from the left ventricle is pumped through the aortic valves and onto the rest of the body. After contraction the ventricles relax, and wait for the next electric impulse. The atria fill with blood and an impulse from the SA node starts the cycle over again. The electrical impulses caused by the heart’s activity can be observed on a graph called an electrocardiogram (ECG), this is a good way to monitor the heart’s cardiac conduction system and is especially used to detect any abnormalities.