Exam5 Phases of Cardiac Cycle

(beginning @ SA node) If (funny current) channels are open allowing Na+ to go in faster than K+ leaves. This depolarization causes the opening of the voltage-gated Ca++ channels to reach threshold and generate an action potential

Propagation of the action potential is conducted as ionic movement thru the gap junctions of the atrial intercalated discs resulting in depolarization of the atrial membrane (P wave)

Atrial contraction fxns to “top off” the ventricular filling

Simultaneously, depolarization is conducted to the AV node (which hesitates due to a minimal concentration of gap junctions) allowing for ventricular coordination

Depolarization continues thru the AV bundle (bundle of His) where there are more gap junctions and therefore more speed of propagation to

the apex of the heart to the Purkinje fibers where there are the greatest number of gap junctions and therefore the quickest speed of propagation

This disruption of the depolarization wave of the ventricles is seen as the QRS complex which masks the atrial repolarization

Ventricular contraction causes an increased ventricular pressure and a drop in atrial pressure

When the ventricular pressure exceeds the atrial pressure it forces the AV valves to close prompting the FIRST HEART SOUND

All heart valves are closed and the ventricles begin to contract leading to ISOVOLUMETRIC CONTRACTION

Once ventricular pressure surpasses the pressure of the conducting vessels, the SLV open ejecting a stroke volume of blood

causing the voltage-gated Ca++ channels to inactivate resulting in repolarization of the ventricular membranes viewed as ventricular relaxation (T wave)

As ventricular pressure falls below the conducting arteries’ pressure, the SLVs close generating the SECOND HEART SOUND

At this time, 2 events happen simultaneously: a) Aortic rebound occurs exposing the coronary artery forcing perfusion of blood into the heart wall b) Ventricles reach ISOVOLUMETRIC RELAXATION

When ventricular pressure is less than atrial pressure the AV valves open forcing blood flow into the ventricles starting ventricular filling