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Mechanical performance of the heart

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Physiology 3120
Tom Stavraky

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Human Physiology Monday, January 18, 2010 “CV VII” Mechanical Performance of the Heart • Factors controlling cardiac output  Heart rate control  Stroke volume control  Preload: amount of blood in ventricle before it contracts  When EDV increases, stroke volume also increases • This occurs because highest crossbridge formation in cardiac muscle occurs when the muscle is maximally stretched; increased force of contraction & increased stroke volume • Frank-Starling law: increased venous retain means increased stroke volume; adaptive function because the heart pumps out just as much as it receives  Frank-Starling curve • A (control) • B (with sympathetic stimulation); at a given EDV, stroke volume increases with sympathetic stimulation • C (with parasympathetic stimulation); at a given EDV, stroke volume decreases with parasympathetic stimulation  The force against which the heart has to pump is the afterload; pressure in aorta is the afterload faced by the LV; pressure in pulmonary artery is the afterload faced by the RV • Increase in afterload causes a brief decrease in stroke volume output; after several beats, stroke volume returns to normal o At the end of systole, there is a small increase in ESV o Increases EDV and thus stretches the ventricle; increases force of contraction (by the Frank-Starling law), which increases stroke volume back to normal • If the afterload
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