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The Cardiac Cycle •Your heart beats approximately 70 times a minute •The cardiac cycle is the sequence of events which make up one beat •Each cycle lasts about 0.8 s •Systole = contraction; Diastole = relaxation The Sequence of Events in the Cardiac Cycle •Heart filled with blood •Atrial walls contract (atrial systole; 0.1s) •Pressure increases slightly in atria •Blood forced into ventricles through atrio-ventricular valves •Ventricles contract (ventricular systole; 0.3s) •Pressure increases in ventricles and forces atrio-ventricular valves to snap shut •Blood rushes upwards into aorta or pulmonary artery •Ventricles relax (ventricular diastole; 0.4s) •Pressure drops in ventricle; semi-lunar valves in aorta/pulmonary artery shut, preventing back flow •As heart relaxes blood flows in to the atria, and trickles through atrio-ventricular valves and into the ventricles Complete the handout and then copy Figure 9.7 on page 124 Electrocardiograms can be used to detect the waves of excitation flowing through heart muscle SAQ 9.2 p126 How is the heart beat controlled? Cardiac muscle is MYOGENIC: the cells have the capacity to beat on their own. It is important that the cells not able to beat independently of each other The pacemaker of the heart is the SINOATRIAL NODE (SAN) It is a specialised group of muscle that is under nervous control (A2) The SA node send waves of contractions to all parts of the heart Contractions can only pass down a group of conducting fibres called the PURKINJE FIBRES. These pass via the ATRIO-VENTRICULAR NODE (AVN) If the muscle contraction is not coordinated then FIBRILLATION results How is the heart beat controlled?