T1 L9: Cardiac pressure-volume cycle & ion action potentials

    Cards (20)

    • Cerebral Circulation - Special Aspects
      Brain maintains all vital functions, so needs constant flow and pressure.
      Auto-regulation to achieve this.
      Circle of Willis: arteries of brain's inferior surface organised into a circle. redundancy of blood supply. 'back-up system of blood supply'
    • Renal Circulation - Special Aspects
      • Portal system: glomerular capillaries to peritubular capillaries
      • 20-25% or cardiac output
      • makes both ACE and RENIN:
      • endocrine functions
      • controlling blood volume
      • responding to renal blood pressure
    • Skeletal Muscle - Special Aspects
      Adrenergic input causes vasodilatation
      Can use 80% of cardiac output during strenuous exercise
      Major site of peripheral resistance
      Muscle pump: augments venous return
    • Skin Circulation - Special Aspects
      • Role in thermoregulation (perfusion can increase 100x)
      • Arterio-venous anastomoses: direct connection between small arteries and small veins without capillaries. Primary role in thermoregulation
      • Sweat glands: role in thermoregulation, produce sweat (plasma ultrafiltrate)
      • Response to trauma: red reaction, flare, wheal
    • Four stages of the Cardiac cycle / "Pressure-Volume Loop"
      1. Ventricular filling
      2. Isovolumic ventricular contraction
      3. Ejection
      4. Isovolumic ventricular relaxation
    • Valve Sounds: S1 & S2

      S1 'lub': heard during isovolumic contraction (mitral valve closes)
      S2 'dub': heard during isovolumic relaxation (aortic valve closes)
    • Isovolumic contraction and relaxation only occur when both aortic and mitral valves are closed.
    • PV loop for mitral stenosis: decreased preload and afterload
      .
    • PV Loop for Aortic stenosis: increased afterload
      .
    • PV loop for mitral regurgitation: increased preload, decreased afterload

      .
    • PV loop for aortic regurgitation: increased preload
      .
    • Auscultation: valve sounds
      Systolic murmur:
      • fluid leaves ventricle
      • so AV regurgitation or SL valve stenosis
      Diastolic murmur:
      • fluid enters ventricle
      • so AV stenosis or SL regurgitation
    • K+ channels
      • Delayed rectifier K+ Channels: open when membrane depolarises with a delay.
      • Inward rectifier K+ channels: open when Vm goes below -60mV (when cells are at rest). Clamps membrane potential at rest, preventing depolarisation
    • Ventricular myocyte action potential phases:
      0. Depolarisation: Na+ gates open in response to wave of excitation from pacemaker
      1. Transient outward current: tiny amount of K+ leaves cell
      2. Plateau phase: inflow of Ca2+ just about balances outflow of K+, until membrane potential becomes too negative and the Ca2+ channels close
      3. Rapid repolarisation phase: Vm falls as K+ leaves cell
      4. Back to resting potential
    • QRS complex shows:
      ventricular depolarisation
    • T wave shows:
      ventricular repolarisation
    • Action potentials in SA node and AV node
      • spontaneously depolarise at rest: not stable because there is no inward rectifier channels
      • upstroke of action potential due to a brief increase in Ca2+ current - not Na+!
      • Only has 3 phases:
      • Depolarisation (influx of Ca2+)
      • Repolarisation (K+ exiting)
      • Pacemaker (diastolic) potential
    • Pacemaker potential
      Resting membrane potential of myocytes in SA and AV node
      it is in a slope bc lack of inward rectifier channels
      Slope of pacemaker potential determines rate of firing
    • Funny current If
      • makes the SA node channel spontaneously active
      • HCN channel (non-specific cation): opens upon hyperpolarisation and closes upon depolarisation
      • leads to a net inward current; lots of Na+ going in and tiny K+ going out
      • Depolarises cell towards 0mV
      • Ivabradine partially inhibits funny current, which slows heart rate in angina and heart failure
    • Blocking ion channels of cardiac action potentials
      Na+ channel block:
      • decreases conduction velocity: changes the organisation of firing in different regions of the heart
      • does NOT prevent depolarisation or affect heart rate
      • used to treat arrhythmias
      Ca2+ channel block:
      • decreases heart rate
      • decreases contractile force
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