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Updated: Dec 3 2021

Cardiac Physiology

  • Introduction
    • Primary function of the cardiovascular system is to maintain perfusion
    • The heart serves as a pump
      • the heart contracts and generates pressure to drive blood through the body
  • Cardiac Output
    • Cardiac output (CO) = stroke volume (SV) x heart rate (HR)
      • Fick principle for cardiac output
        • there is a conservation of mass
        • the oxygen consumption must equal the amount of oxygen leaving the lungs in the pulmonary vein minus the amount of oxygen returning to the lungs in the pulmonary artery
        • O2 consumption = CO x [O2]pulmonary vein - CO x [O2]pulmonary artery
    • SV is determined by
      • contractility
        • intrinsic ability of myocardial cells to develop force
        • contractility ∝ end-diastolic volume (preload)
          • Frank-Starling relationship
            • may reduce cardiac output in heart failure with reduced ejection fraction when preload increased
      • preload
      • afterload
      • SV = end diastolic volume (EVD) - end systolic volume (ESV)
    • SV increases with anxiety, exercise, and pregnancy
    • Pulse pressure ∝ SV
      • pulse pressure = systolic pressure - diastolic pressure
    • Distribution of CO
      • liver > kidney > muscles > brain
  • Myocyte Action Potentials
    • Initiated in the sinoatrial (SA) node, the pacemaker of the heart
    • Slowed by the atrioventricular (AV) node, which allows for ventricular filling
    • Action potential causes influx of Ca2+ ions, which triggers the release of Ca2+ from the sarcoplasmic reticulum
      • this allows for the cell to contract
    • Relative conduction velocities
      • bundle of His = Purkinje fibers > atria > ventricles > AV node
  • Myocardial O2 Demand
    • Increased with
      • ↑ contractility
      • ↑ afterload
      • ↑ heart rate
      • ↑ diameter of the ventricle (or wall tension)
  • Maintenance of Mean Arterial Pressure
    • Mean arterial pressure (MAP) is maintained by
      • baroreceptors
      • chemoreceptors
      • renin-angiotensin-aldosterone system
      • anti-diuretic hormone
      • atrial natriuretic peptide
    • MAP = CO x total peripheral resistance (TPR)
    • MAP = 2/3 diastolic pressure + 1/3 systolic pressure
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