Updated: 12/12/2020

Cardiac Physiology

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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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