General Physiology

Electrophysiology: Phases of the Cardiac Cycle

Cardiac Cycle

Phases of the Cardiac Cycle :

The action potential of the heart consists of 5 phases , from 0 to 4


Ι – Phase 4 :    ” resting polarized cells “

– This phase of action potential is associated with diastole of the chamber of the heart.

at that time , the inside is more negative than the outside & the cells are resting & ready to receive an electrical stimulus

Hint :

– Phase Four appears as a flat line ( ___ ) in ECG , which resembles Rest  ” ( _______    :sleeping:   zZzZzZz )  “


ΙΙ- Phase 0 :    ” upstroke ↑ + fast depolarization “

– This phase is due to the opening of the fast Na+ channels causing  Na+ ions to Rush into the cells , increasing the positive charges inside the cells .

– the cells depolarize rapidly and begin to contract

– it is characterized by a sharp, tall upstroke of the action potential on the ECG chart .


ΙΙΙ- Phase 1 :spike + a little bit repolarization

–  inactivation of the fast Na+ channels occurs & the cell begins an early partial repolarization

this early repolariztion is due to the movement of K+ Out of the cell so the cell starts to become less positive in the inside

– contraction is still in process


ΙV – Phase 2” plateau “

– This plateau phase of the cardiac action potential is sustained by a balance between inward movement of Ca2+ and outward movement of K+


V-  Phase 3 :    ” down slope ↓ + Fast repolarization “

– Rapid repolarization occurs to return to the resting polarized phase 4 again .

the Ca2+ channels close so no ca ions can no longer enter to the cells , while the K+ channels are still open so K+ can keep getting out of the cell .

This ensures a net outward current of positive charges , until the membrane is completely polarized ( more -ve inside & more +ve out side )

Hint : repolarization is complete by the end of phase 3 😉


after that , the Phases of the Cardiac Cycle are repeated with each electric shot fired from the Heart pacemakers .


Hint :

It’s more easier to study action potentials when you first memorize the Ion which plays the most important rule in each phase

Na+ mainly in phase 0 ( notice that Na is one letter different that No !  .. & o = 0zero ”   :happy: )

Ca2+ mainly in phase 2 ( notice that both valency of calcium and the phase number = 2 ) 😉

K+ mainly in phase 1 , 3 , 4 ( notice that 1+3=4 😎 )

Play with this animation cool animation  😎 , try to compare between the in&outflux on ions and the chart  :

Hint :

the dark yellow channel is for Na+

the light yellow channel is for Ca+2

the red channels are for K+


Extra information > > > special phases :

*1* – Absolute refractory period;

– it’s a period in which no stimulus -no matter how strong- can cause another depolarization


*2* – Relative refractory period :

– the cell has partially repolarized, so Only a very strong stimulus could cause a depolarization


*3* – Super normal period (SNP) :

it’s a period in which a stimulus weaker than normally required can cause a depolarization

– it is rare and not normal in a healthy heart .


References :

– Essentials of Human Physiology for Pharmacy

– Netter atlas of Physiology

– The free medical dictionary official web site

Phases of the Cardiac Cycle
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Thanks for the info


Good article

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– Kris


Beautiful write-up; thanks 🙏

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