BIO 3520
McGilliard
 

I.  General Principles.

     A.  Contraction of the myocardium generates pressure changes that 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.  Phases of the Cardiac Cycle.
 

     A.  Passive Filling (early diastole).

          1.  Both atria and ventricles are relaxed.

          2.  Ventricular pressure  =  0.

          3.  AV valves are open.

          4.  Semilunar valves are closed.

          5.  Blood is flowing from veins into atria and into ventricles.
               NOTE:  About 80% of ventricular filling occurs during this phase.

     B. Atrial Contraction (late diastole).

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

          2.  Atria contract.  Atrial pressure increases.

          3.  Blood continues to flow into ventricles.

          4.  Ventricular pressure increases slightly.

          5.  Ventricular volume is greatest at the end of diastole.

               End-diastolic volume  =  Volume of blood in each ventricle at the end of diastole.

          6.  Arterial blood pressure is lowest.

               Diastolic blood pressure  =  80 mmHg.

     C.  Isovolumetric Ventricular Contraction (early 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.  Ventricular pressure increases dramatically with no change in ventricular volume.

     D.  Ejection (systole).

          1.  Semilunar valves open as ventricular pressure exceeds arterial blood pressure.

          2.  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.

               Ejection fraction  =  SV/EDV.

          3.  Arterial blood pressure rises to its highest point.

               Systolic blood pressure  =  120 mmHg.

     E.  Isovolumetric Ventricular Relaxation (early diastole).

          1.  Wave of repolarization across ventricles.  T wave of the ECG.

          2.  Ventricles relax.

          3.  Ventricular pressure falls below arterial blood pressure.

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

          5.  Ventricular pressure falls with no change in ventricular volume.
 

     F.  Passive Filling (early diastole).

          1.  AV valves open as ventricular pressure falls below atrial pressure.

          2.  Ventricles begin to fill with blood (see section A above).