The Cardiac Cycle and Myogenic Stimulation of the Heart

The Cardiac Cycle 

Overview:

• The sequence of events during one heartbeat
• At rest mean heart rate is 70bpm
• One heart beat takes approximately 0.8 seconds

Stages of the Cardiac Cycle:

• Ventricular Systole -
  • Cardiac muscle of the ventricle contracts
  • Increases pressure in the ventricles
  • Blood is forced into the arteries
  • Semi-lunar valves are forced open
  • Atrio-ventricular valves are forced closed
• Diastole -
  • Cardiac muscle relaxes
  • Semi-lunar valves close
  • Atria fills with blood
  • Forces the atrio-ventricular valves to open
  • Ventricles fill with blood
• Atrial Systole -
  • Cardiac muscle of the atria contracts
  • Increases the pressure in the atria
  • Blood is forced into the ventricles
  • Atrio-ventricular valves remain open
  • Semi-lunar valves remain closed

Pressure and Volume Changes During the Cardiac Cycle:

• Pressure increases as the cardiac muscle contracts
• Atrio-ventricular valve closes and semi-lunar valves open
• Pressure decreases as the cardiac muscle relaxes
• Semi-lunar valve closes and atrio-ventricular valve opens
• Closing of the atrio-ventricular valves returns pressure and volume to starting point

• Left ventricle has a thicker cardiac wall so, generates…