BIO 2001
McGilliard
 

THE CARDIAC CYCLE


I.  General Principles.

     A. Contraction of the myocardium generates pressure changes which result in the orderly
          movement of blood.

     B. Blood flows from an area of high pressure to an area of low pressure, unless flow
          is blocked by a valve.

     C. Events on the right and left sides of the heart are the same, but pressures are lower on
          the right.

II.  Definitions.

     A. Systole  =  Period of ventricular contraction.

     B. Diastole  =  Period of ventricular relaxation.

          NOTE:  Normally diastole is longer than systole.
 

III.  Events of the Cardiac Cycle.

   A. Mid-diastole.

          1. Both atria and ventricles are relaxed.

          2. AV valves are open.

          3. Semilunar valves are closed.

          4. Blood is flowing from veins into atria and into ventricles.  Passive filling.

          NOTE:  About 80% of ventricular filling occurs at rest, before the atria contract.

     B. Late diastole.

          1. Wave of depolarization begins at SA node, spreads across both atria.
               P-wave of the ECG.

          2. Atria contract.  This increases the pressure in the atria.

          3. More blood enters ventricles.
          4. End-diastolic volume  =  Volume of blood in each ventricle at the end of diastole.
               NOTE:  This is when the ventricular volume is greatest.

          5. Arterial blood pressure is lowest.  Diastolic blood pressure  =  80 mmHg.

          6. Wave of depolarization reaches AV node.

     C. Systole.

          1. Action potential is conducted through AV node, down bundle of His, across both
               ventricles  --  ventricular depolarization.  QRS-complex of the ECG.

          2. Ventricles contract.  This increases the pressure in the ventricles above the pressure
               in the atria.

          3. AV valves close, preventing backflow into atria  --  first heart sound ("lub").

          4. Semilunar valves open.

          5. About 2/3 of blood in the ventricles is ejected into arteries.
              Stroke volume  =  Volume of blood ejected from each ventricle in a single beat.

          6. Arterial blood pressure rises to its highest point.
              Systolic blood pressure  =  120 mmHg.

     D. Early diastole.

          1. Ventricular repolarization.  T-wave of the ECG.

          2. Ventricles relax.

          3. Ventricular pressure falls below arterial BP.

          4. Semilunar valves close  --  second heart sound ("dub").

          5. AV valves open.

          6. Ventricles begin to fill because ventricular pressure is lower than atrial pressure.
 

     This returns us to mid-diastole, described above.
 

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Updated 3/07/02