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Transcript
Control of the Cardiac Cycle Objectives •To describe how heart action is coordinated with reference to the SA node, AV node and Purkyne tissue •Interpret and explain ECG traces with reference to normal and abnormal heart activity • The heart is Myogenic so does not need impulses from the nervous system • This means that the cardiac muscle can generate its own contraction independently from the rest of the body • Contraction occurs in an organised manner and the heart acts as a functional unit Atrial Systole • The cardiac cycle is initiated by a specialised area of the right atrium, near to where the vena cava empties in - the Sinoatrial node (the pacemaker) • SAN sets the rhythm for all other cardiac cells to contract (1/second) • SAN sends out electrical impulses to the rest of the atria • This spreads across the both atrial walls as a wave of depolarisation or excitation • Cardiac muscle in the walls of both atria contract in time with the SAN Controlling the contractions • Collagen fibres at the base of the atria prevent the impulse from passing down to the ventricles • This delay ensures that atrial systole is complete and that the ventricles don’t contract until they are filled with blood Ventricular Systole • At the top of the interventricular septum is the Atrioventricular node (AVN) • The AVN picks up electrical impulses through the atrial wall and then generates its own • Impulses travel as a wave of excitation along specialised conducting tissue in the septum Purkinje (purkyne) fibres • Fibres from the RHS and LHS form a bundle of His • The wave of excitation spreads rapidly to the apex of the ventricles then spreads up and out over the ventricle walls • Ventricles contract from the bottom up Electrical Activity of the Heart Annotate this diagram to show how the cardiac cycle is controlled Electrical Activity of the Heart Electrocardiograms (ECG) • ECGs detect changes in the electrical activity of the heart • Electrodes are placed on the skin over opposite sides of the heart and electrical activity is recorded A normal ECG trace P is the wave of excitation sweeping over the atrial walls T is the relaxation of the ventricle walls QRS complex is the wave of excitation spreading through the ventricular walls. It immediately precedes ventricular systole Electrocardiograms Problems with the cardiac cycle • Arrhythmia – irregular beats • Fibrillation – loss of coordination of contractions • Myocardial Infarction – heart attack where the coronary arteries are blocked by a clot and the cardiac muscle cells do not receive oxygen for respiring fatty acids • Enlarged heart • Poor electrical conduction by the Purkyne tissue Normal and Abnormal ECG traces