The Normal Cardiac Cycle

General Concepts

• systole - period of chamber contraction
• diastole - period of chamber relaxation
• cardiac cycle - all events of systole and diastole during one heart flow cycle
  

Events of Cardiac Cycle

• mid-to-late diastole: ventricles filled
• pressure: LOW in chambers; HIGH in aorta/pulmonary trunk
• aortic/pulmonary semilunar valves CLOSED
• blood flows from vena cavas/pulmonary vein INTO atria
• blood flows through AV valves INTO ventricles (70%)
• atrial systole propels more blood > ventricles (30%)
• atrial diastole returns through end of cycle
  

• ventricular systole: blood ejected from heart
• filled ventricles begin to contract, AV valves CLOSE
• isovolumetric contraction phase - ventricles CLOSED
• contraction of closed ventricles increases pressure
• ventricular ejection phase - blood forced out
• semilunar valves open, blood -> aorta & pulmonary trunk
  

• isovolumetric relaxation: early diastole
  
• ventricles relax, ventricular pressure becomes LOW
• semilunar valves close, aorta & pulmonary trunk backflow
• dicrotic notch - brief increase in aortic pressure
  

• TOTAL CARDIAC CYCLE TIME = 0.8 second (normal 70 beats/minute)
  

• atrial systole (contraction) = 0.1 second
• ventricular systole (contraction) = 0.3 second
• quiescent period (relaxation) = 0.4 second
  

Category: Physiology Notes