19.8 The Cardiac Cycle

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8 Terms

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Cardiac cycle

all events in heart from the start of one heart beat to start of the next

  • Includes both systole (contraction) and diastole (relaxation)

Contraction increases pressure; relaxation decreases it

  • Blood moves down its pressure gradient (high to low)

  • Valves ensure that flow is forward (closure prevents backflow)

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ventricular activity

  • Ventricular contraction raises ventricular pressure

    • AV valves pushed closed

    • Semilunar valves pushed open and blood ejected to artery

  • Ventricular relaxation lowers ventricular pressure

    • Semilunar valves close

    • AV valves open

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As the cardiac cycle begins

  • Four chambers at rest

  • Blood returning to both atria

  • Passive filling of ventricles

  • AV valves open

  • Atrial pressure > ventricular pressure

  • Semilunar valves closed

  • Pressure in ventricles < arterial trunk pressure

<ul><li><p>Four chambers at <strong>rest</strong></p></li><li><p><strong>Blood returning </strong>to <strong>both atria</strong></p></li><li><p>Passive <strong>filling of ventricles</strong></p></li><li><p><strong>AV valves open</strong></p></li><li><p>Atrial pressure <strong>&gt; </strong>ventricular pressure</p></li><li><p><strong>Semilunar valves closed</strong></p></li><li><p>Pressure in ventricles <strong>&lt; </strong>arterial trunk pressure</p></li></ul><p></p>
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Atrial contraction and ventricular filling

  • SA node starts atrial excitation

  • Atria contract pushing remaining blood into ventricles

  • Ventricles filled to end-diastolic volume (EDV)

  • Atria relax for remainder of cardiac cycle

<ul><li><p><strong>SA node</strong> starts <strong>atrial excitation</strong></p></li><li><p><strong>Atria contract</strong> pushing <strong>remaining blood into ventricles</strong></p></li><li><p><strong>Ventricles filled </strong>to end-diastolic volume (EDV)</p></li><li><p><strong>Atria relax</strong> for remainder of cardiac cycle</p></li></ul><p></p>
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Isovolumic contraction

  • Purkinje fibers initiate ventricular excitation

  • Ventricles contract, pressure rises, and AV valves are pushed closed

  • Ventricular pressure is still less than arterial trunk pressure, so semilunar valves still closed

<ul><li><p><strong>Purkinje fibers</strong> initiate <strong>ventricular excitation</strong></p></li><li><p><strong>Ventricles contract</strong>, <strong>pressure rises</strong>, and <strong>AV valves</strong> are pushed <strong>closed</strong></p></li><li><p>Ventricular pressure is still <strong>less </strong>than arterial trunk pressure, so<strong> semilunar valves still closed</strong> </p></li></ul><p></p>
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Ventricular ejection

  • Ventricles continue to contract so that ventricular pressure rises above arterial pressure

  • Semilunar valves forced open as blood moves from ventricles to arterial trunks

  • Stroke volume (SV)

    • blood ejected by ventricle

  • End systolic volume (ESV)

    • blood remaining in ventricle after contraction finishes

  • ESV = EDV − SV

<ul><li><p><strong>Ventricles </strong>continue to <strong>contract </strong>so that <strong>ventricular pressure rises above arterial pressure</strong> </p></li><li><p><strong>Semilunar valves forced open</strong> as blood moves from ventricles <strong>to arterial trunks</strong></p></li></ul><p></p><ul><li><p><strong>Stroke volume (SV)</strong></p><ul><li><p><strong>blood ejected </strong>by ventricle</p></li></ul></li><li><p><strong>End systolic volume (ESV)</strong></p><ul><li><p><strong>blood remaining</strong> in ventricle after contraction finishes</p></li></ul></li><li><p><strong>ESV = EDV − SV</strong></p></li></ul><p></p>
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Isovolumic relaxation

  • Ventricles relax and start to expand, lowering pressure

  • Arterial pressure greater than ventricular pressure

  • By sliding back toward ventricles, blood closes semilunar valves

  • AV valves remain closed

  • When all valves are closed, blood neither enters nor leaves and the time is called “isovolumic”

<ul><li><p><strong>Ventricles relax</strong> and start to <strong>expand</strong>, <strong>lowering pressure</strong></p></li><li><p><strong>Arterial pressure greater</strong> than ventricular pressure </p></li><li><p>By sliding back toward ventricles, <strong>blood closes semilunar valves</strong></p></li><li><p><strong>AV valves</strong> remain <strong>closed</strong></p></li><li><p>When<strong> all valves are closed</strong>, <strong>blood neither enters nor leaves</strong> and the time is called “<strong>isovolumic</strong>”</p></li></ul><p></p>
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Ventricular balance

  • Equal amounts of blood are pumped by left and right sides of the heart

  • Left heart pumps blood farther and so must be stronger (thicker)

  • But ejected blood volumes must be the same or edema (swelling) can occur