The cardiac cycle refers to the series of events in the heart from the start of one heartbeat to the next, initiated by the sino-atrial node (SAN) that generates contraction signals. It consists of four main stages: atrial diastole, atrial systole, ventricular systole, and ventricular diastole, each involving the relaxation and contraction of heart chambers to effectively circulate blood. The atrioventricular node (AVN) and Purkinje fibers play essential roles in conducting the excitation waves that coordinate these contractions.

1. THE CARDIAC CYCLE
By: Christabelle Cécile

2. Definition
 Cardiac cycle is the complete cycle of events in the
heart from the beginning of one heart beat to the
beginning of the next
 Cardiac muscle differs from all other muscles of the
body
 It is myogenic
 Heart muscle has its own built in mechanism for
bringing about: contraction & relaxation

3. Structure of heart

4. Mechanism
 Heart muscle cells cannot contract by their own
natural rhythms
 So, cardiac cycle is initiated in a specialised patch
of muscle
 Found in the right atrium called the sino-atrial node
(SAN)
 Located near the opening of venae cavae
 Consists of a small number of cardiac muscle fibres and
few nerve endings
 Stimulus for contraction of the heart is originated in the
SAN

5. Mechanism
 However, there is a band of fibres between the
atria and ventricles which does not conduct the
excitation wave of the SAN
 The only route through is via a patch of conducting
fibres
 Called the atrio-ventricular node (AVN) situated in the
septum

6. Mechanism
 AVN picks up excitation wave as it spreads across
the atria and passes it on to a bunch of conducting
fibres
 called the Purkyne tissue
 This transmits the excitation wave rapidly down to
the base of the septum and then to the walls of the
ventricles
 Thus, causing the cardiac muscle in these walls to
contract. Squeezing blood upwards and into
arteries

7. 4 stages of Cardiac Cycle

8. Stage 1 – Atrial diastole
 Bicuspid and tricuspid valves are closed
 Muscles in the atria relaxes and the pressure
decreases while volume increases inside
 Blood returns to the heart under low pressure in the
veins and enters the two atria
 The venae cavae carry deoxygenated blood to the
right atrium
 The pulmonary veins carry oxygenated blood to the
left atrium

9. Stage 2 – Atrial systole
 It is the time when the heart is filled with blood
 The muscle in the atrial walls contracts with the help
of SAN
 The pressure increases and the atrio-ventricular
valves open so the blood found in atria is forced
into the ventricles
 Blood from atria does not go back into the
pulmonary veins or the venae cavae because of the
semilunar valves which prevent backflow

10. Stage 3 – Ventricular systole
 0.1 second after the contraction of the atria, the
ventricles also contract
 Thick muscular walls of the ventricles squeeze inwards
on the blood, increasing pressure and pushing it out of
the heart
 As pressure in ventricles become greater than that in
atria, the atrio-ventricular valves shut, preventing
backflow to the atria
 Instead, blood rushes upwards into the aorta and the
pulmonary artery, pushing open the semilunar valves

11. Stage 4 – Ventricular diastole
 Here, the muscles relaxes
 As this happens, pressure in ventricles drops
 The high-pressure blood which has just been pushed
into the arteries would flow back into the ventricles
 But, this is prevented by the presence of the
semilunar valves which snap shut as the blood fills
their cusps