Cardiovascular Physiology

• Myocardium

  • comprises 95% of the heart
  • responsible for pumping action
  • striated, involuntary muscle
  • fibers swirl diagonally around heart in bundles
  • heart 'wrings' blood out of the ventricles
• desmosomes are at cell junctions and help keep the fibers together
• intercalated discs occur between individual muscle cells
• gap junction allow action potential from myocyte to myocyte, leads to involuntary contraction
• myocytes form a functional syncytium

  • all the myocytes work together
• intercalated discs contain gap junctions and desmosomes
• ANS modifies the natal heart rate
• sinoatrial (SA) node spontaneously depolarizes

  • no stable RMP
  • 100 bpm
• the atrioventricular (AV) node is located at the base of the RA and sends electrical impulses to the ventricles
• the atrioventricular (AV) bundle (Bundle of His) is the only point where a signal can pass from atria to ventricles
• Purkinje fibers

  • signal transfer to other side of heart wall
• Fibrous skeleton

  • there is an electrical separation between the atria and ventricles because we dont want them contracting at the same time
  • this is in order for the blood to move through the heart
• autorhythmic cells set the pace of the heart
• if something happens to the SA node, the AV node can take over as the pacemaker
• the SA node is the pacemaker of the heart
• Ventricular Myocyte

  • resting membrane potential about -90mV
• Depolarization of ventricular myocyte

  • rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open
  • Na+ channels inactivate quickly
  • sodium flows in because the cytosol is more negative that the interstitial tissue so the calcium moves in
• Plateau of ventricular myocyte

  • maintained depolarization due to Ca2+ inflow when voltage-gated slow Ca2+ channels open and K+ outflow when some K+ channels open
  • because there is a balance of sodium and potassium, there is a plateau
• strength of heart contraction is influenced by substances that alter movement of Ca2+ through the channels (inotropes)
• epinephrine increases contraction force by enhancing Ca2+ influx

  • positive inotrope
• decreases calcium influx = negative inotrope
• Repolarization of ventricular myocyte

  • due to closure of Ca2+ channels and K+ outflow when additional voltage-gates K+ channels open
• Refractory period: the time interval during which a second cardiac action potential cannot be triggered

  • cardiac refractory period > cardiac contraction period
  • this is important because we dont want a second contraction triggered/ a state of tetany
  • when everything is contracting at once
• Steps of an AP in a ventricular myocyte

  1. Depolarization
  2. Plateau
  3. Repolarization
• Atrial contraction occurs when on an ECG?

  P-Q interval
• ventricular contraction occurs when on an ECG?

  QRST
• Electrocardiogram

  • ECG or EKG
  • composite record of action potentials produced by all the heart muscle fibers
  • detected at surface of the body
  • 3 recognizable waves
  • P, QRS, and T
• P wave: atrial contraction
• QRS complex: ventricular contraction

  • atrial relaxation
• T wave: ventricular repolarization/relaxation
• the beginning of the P wave is when there is an action potential in the SA node

  • signal propagating across LA
  • electrical activity occurs before the physical activity
• structurally the AV node is different from SA as it has smaller contractile fibers
• after the P wave from, atria contract

  • slowing of AP conduction at AV node because it contains smaller fibers; less gap junctions
• after QRS complex appears, ventricles contract

  • contraction continues throughout the S-T segment
• after T waves begins, ventricles relax
• ECG Waves, Systole & Diastole Steps

  1. depolarization of atrial contractile fibers produces P wave
  2. atrial systole (contraction)
  3. depolarization of ventricle contractile fibers produces QRS complex
  4. ventricular systole (contraction)
  5. repolarization of ventricular contractile fibers produces T wave
  6. ventricular diastole (relaxation)
• cardiac cycle = 0.8 sec
• heart rate = 75 bpm
• stroke volume = 70 mL
• how long does atrial systole last for?

  0.1s