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Transcript
Cardiac Cycle and Heart Sounds Cardiac Cycle • Cardiac Cycle: the events of one complete heart beat, both atria and ventricles contract and then relax • Systole: When the heart is contracting • Diastole: When the heart is relaxing • Since most of the pumping work is done by the ventricles, these terms always refer to the ventricles Cardiac Cycle • We will consider the cardiac cycle in terms of events during 3 period: mid-late diastole, ventricular systole, early diastole Mid-late diastole • At the beginning, the pressure in the heart is low • Blood is flowing passively into and through the atria into the ventricles from the pulmonary and systemic circulation • The semilunar valves are closed and the AV valves are open • Then the atria contract and force the blood remaining in their chambers into the ventricles Ventricular Systole • Ventricular contraction begins and the pressure within the ventricles increases rapidly, closing the AV valves • The semilunar valves are forced open and blood rushes through them out of the ventricles • The atria are relaxed and their chambers are again filling with blood Early Diastole • At the end of systole, the ventricle relax, the semilunar valves snap shut (preventing backflow) and for a moment, the ventricles are completely closed chambers • During early diastole, the AV valves are forced open and the ventricles again, begin to refill rapidly with blood, completing the cycle Heart sounds • The heart sounds are often described by the two syllables “lub” and “ dup” • The first sound “lub” is caused by the closing of the AV valves • The second heart sound “dup” occurs when the semilunar valves close at the end of systole Cardiac Output (CO) • It is the amount of blood pumped out by each side of the heart (actually each ventricle) in 1 minute • It is the product of heart rate (HR) and Stroke volume (SV): CO = HR X SV • Stroke volume is the volume of blood pumped out by a ventricle with each heartbeat: - In general, stroke volume increase as the force of ventricular contraction increase Regulation of Stroke Volume • A healthy heart pumps out about 60% of the blood that enters it • The critical factor controlling stroke volume is how much the cardiac muscle cells are stretched just before they contract: the more they are stretched, the stronger the contraction will be • The important factor stretching the heart muscle is venous return, the amount of blood entering the heart and swell out its ventricles • Anything that increase the volume or speed of venous return also increase stroke volume and force of contraction Regulation of Heart Rate • Although heart contraction does not depend on the nervous system, its rate can be changed temporarily by the autonomic nerves - Sympathetic nervous system: speeds up heart rate - Parasympathetic nervous system: slows down heart rate Regulation of Heart Rate • The heart rate is also modified by various chemicals, hormones and ions - Epinephrine/ thyroxine: increases heart rate - Deficit Potassium ions: cause the heart to beat without strength and abnormal heart rhythms • The average heart rate in female is 72-80 beats per min • The average heart rate in male is 64-72 beats per min