Summary The heart and the cardiac cycle

THE HEART AND THE CARDIAC CYCLE


The heart is myogenic- It initiates its own contractions.


In the wall of the right atrium there are specialized fibres called the
sinoatrial Node (SA node). These are the pacemaker cells. SA node
starts a wave of electrical stimulation and causes the atria to contract at
roughly the same time.


The ventricles don't start contracting until the atria have finished as there
is a layer of tissue which is unable to conduct the wave of excitation in
between. The wave reaches the atrioventricular Node between the two
atria which passes the wave to the ventricles via the bundle of His.


The bundle of His branches into purkinje fibers which carry the wave
upwards. This causes ventricles contract up was to push the blood out of
the heart.


3 STAGES OF THE CARDIAC CYCLE


1. Atrial systole- atria contract, atrioventricular (AV) valves open, blood
flows to ventricles
2. Ventricular systole- contraction of ventricles, AV valves close and
semi- lunar valves open so blood leaves the heart via arteries
3. Cardiac diastole- atria and ventricles relax, elastic recoil of the heart
lowers pressure in the heart chambers and blood is drawn from
arteries and veins so semi- lunar valves close to prevent backflow.