Lesson 5 of 15
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The Cardiac Cycle

The cardiac cycle is composed of systole, diastole, and isovolumic phases (isovolumic contraction and isovolumic relaxation). The simultaneous changes in ventricular, atrial, and aortic pressure occurring during these phases are illustrated in the Wigger’s diagram below.

  • Systole: the ventricular ejection phase, between semilunar valve opening and closure
  • Isovolumic relaxation: the ventricular relaxation phase when pressure is declining without change in volume, between semilunar valve closure and AV valve opening
  • Diastole: the ventricular filling phase, between AV valve opening and closure
  • Isovolumic contraction: the onset of ventricular contraction when pressure is rising without change in volume, between AV valve closure and semilunar valve opening

The components of diastole include:

  1. The rapid filling phase: when the pressure in the ventricles falls below that of the atria.
  2. Diastasis: the middle of diastole when there is minimal flow between the atria and ventricle due to relatively equal pressures.
  3. Atrial contraction: the final filling of the ventricles occurs because of atrial contraction pushing the last of the preload into the ventricle.
Pressure volume loop of the cardiac cycle (Wigger's Diagram)
Wigger’s Diagram from Anatomy & Physiology, Connexions website. http://cnx.org/content/col11496/1.6/, 2013.