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