The cardiac cycle describes the sequence of events that occur with each heartbeat. It consists of two phases - systole and diastole. Systole is the contraction phase where the heart muscles contract and blood is ejected. Diastole is the relaxation phase where the heart muscles relax and refill with blood. The full cardiac cycle takes approximately 0.8 seconds and includes electrical, mechanical, and chemical events that result in the coordinated contraction and relaxation of the heart muscles.

1. Cardiac Cycle
By Dr. Mohammed AL-Mojtaba A.Awad

2. Objectives :
• Definition
• Duration
• Events
+Electrical Events
+Mechanical Events
+Excitation-Contraction Coupling
• Heart Sounds

3. Definition
• A Sequence of coordinated events taking place
in the heart during each BEAT .
• Each Heartbeat consist of Two Major periods
+Systole > a part of cardiac cycle in which
heart Contracts & Blood is Ejected .
+Diastole > a part of cardiac cycle in
which heart Relaxes & filled with Blood .

4. Duration :
• It is 0.8 Sec.
Heart Rate is normally 72 beat/min
minute contain 60 seconds >>> So
60/72 = 0.8 Sec.

5. Atrial Events
Atrial Systole : 0.1 sec
Atrial Diastole : 0.7 sec
Ventricular Events
Ventricular Systole : 0.3 scc
Ventricular Diastole : 0.5 sec

6. Events
• Electrical Events
• Mechanical Events
• Excitation–Contraction Coupling

7. Electrical Events
• We should know that Myocardium is a
Physiological Syncytium because of the Gap
Junctions that connect each Myofibril with it’s
Neighouber .
• So Excitation of Single cell will creates
Electrical Current that pass through all
Myocardium
• SA node is the Pacemaker & the generater of
this Electrical Current .

8. • So SA Node EXCITE the near by Cells & starts
the Electrical Events as FOLLOWS :
+RMP (Resting Membrane Potential) = -
90mV
+Initial Depolarization (Na influx)
+Transient Repolarization (K efflux)
+Plateau Phase > also known as final
Depolarization (Ca influx)
+Final Repolarization (K efflux)

9. • The above events occurs at the Cellular Level ,
but at Chambers Level >>>
+Atrial Depolarization
+Ventricular Depolarization (with Atrial
Repolarization Summated in it)
+Ventricular Repolarization

10. Mechanical Events
• Electrical Currents translated into Mechanical
events as Follows :
+Atrial Systole
+Isovolumetric (Isometric) Ventricular
Contraction.
+Ejection (Rapid , Slow)
+Isovolumetric (Isometric) Ventricular
Relaxation
+Passive Ventricular Filling .

11. Excitation-Contraction Coupling :
• Translation of Electrical Events into
Mechanical Forces (Events)
• Before starting we should know the molecular
Basis of Contraction , Like in Skeletal Muscles
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Myofibril contains the Folowing :
+Plasma Membrane (Sarcolemma)
+Cytoplasm (Sarcoplasm)>> Stores Ca ions
+Many may Mitochondria

12. +Network of Tubules >> Sarcoplasmic
Reticulum (Version of Smooth Endoplasmic
Reticulum)
+Unique system to Cardiac Muscle Cell is
Transverse Tubules (T-tubules) which crosses
the Cytoplasm Transversly .

13. • Sarcolemma contains Contractile Proteins like
those of Skeletal Muscles ( Actin , Myocine ,
Troponin & Tropomyocin )
• Troponins are :
+TnT (tropomyocin Binding Troponin)
+TnC (Calcium Binding Troponin)
+TnI (Inhibitory Troponin)
Those Proteins Acts Togather to produces
Shortening in the Length of Myofibrils >>
Contraction

14. Heart Sounds :
• Sounds That is produced by Mechanical
Activities of the Heart >> Produced by
-Valves Closure
-Blood Flow through Chambers
-Contraction of Cardiac Muscles
>>> LUB-DUP

15. • First Heart Sound
Produced by Closure of AV Valves
• Second Heart Sound
Produced By Closure of Semilunar Valves
• Third Heart Sound
Ventricular Gallop >> Rapid Ventricular
Filling .
• Fourth Heart Sound
Atrial Gallop >> Atrial Contraction against
Stiff Ventricle .

16. THANQ