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Download The individual action of the heart represents one heartbeat, but the
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The Conductive System and Cardiac Cycle The individual action of the heart represents one heartbeat, but the more technical term used is the cardiac cycle - the mechanical events of one heartbeat. The heart is said to be myogenic – it generates its own electrical impulse to control the mechanical events of the heartbeat. The electrical impulse responsible for stimulating the heart to contract is called the cardiac impulse. The Conduction System The cardiac impulse is initiated from the sino-atrial (SA) node located in the posterior wall of the right atrium. It is often called the pacemaker. The impulse travels through the atria walls causing both the atria to contract. (The ventricles are isolated from the atria and can not be stimulated at this point.) The cardiac impulse reaches and activates the AV node in the right atrium which passes the impulse down into the bundle of His located within the septum of the heart. The AV node actually helps delay the impulse allowing the contraction of the atria to finish before the ventricles begin to contract. The bundle of His splits into left and right branches and spreads the impulse down to the bottom of the heart and then up and around the walls of both ventricles’ via a network of Purkinje fibres, causing both ventricles to contract. (similar to toothpaste being squeezed from the tube – the ventricles are squeezed from the bottom upwards) The ventricles relax and the cycle is repeated with the next cardiac impulse initiated from the SA node. Cardiac Cycle The cardiac cycle represents the mechanical events of one heartbeat. At rest, one complete cycle lasts 0.8 secs and is repeated approximately 72 times a minute. The cardiac cycle consists of two phases that represent the contraction and relaxation of the heart. 1. Diastole – lasting 0.5 secs, represents the relaxation phase. 2. Systole – lasting 0.3 secs, represents the contraction phase. The sequence of events happen like this: DIASTOLE (0.5s) 1. Both atria fill with blood. AV valves closed 2. Atrial blood pressure rises above ventricular pressure 3. Rising blood pressure forces AV valves open and blood passively passes into both ventricles. Semilunar valves closed. SYSTOLE (0.3s) 4. Both atria contract actively forcing the remaining atrial blood into the ventricles. 5. Semilunar valves remain closed. 6. Both Ventricles contract increasing ventricular pressure. 7. Aortic and pulmonary valves forced open. AV valves closed. 8. Blood forced out into: aorta to body tissues/muscles=stroke volume; pulmonary arteries to lungs. (N.B. only 40/50% blood is ejected at rest during ventricular systole (SV). 9. Diastole of the next cardiac cycle begins. Semilunar valves close preventing backflow of blood from aorta and pulmonary arteries.