Gr 11 Cardiac Cycle

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The cardiac cycle describes the series of events that take place in the heart over the duration of a single heart beat

Systole

Blood returning to the heart will flow into the atria and ventricles as the pressure in them is lower (due to low volume of blood)
When ventricles are ~70% full, atria will contract (atrial systole), increasing pressure in the atria and forcing blood into ventricles
As ventricles contract, ventricular pressure exceeds atrial pressure and AV valves close to prevent back flow (first heart sound)
With both sets of heart valves closed, pressure rapidly builds in the contracting ventricles (isovolumetric contraction)
When ventricular pressure exceeds blood pressure in the aorta, the aortic valve opens and blood is released into the aorta

Diastole

As blood exits the ventricle and travels down the aorta, ventricular pressure falls
When ventricular pressure drops below aortic pressure, the aortic valve closes to prevent back flow (second heart sound)
When the ventricular pressure drops below the atrial pressure, the AV valve opens and blood can flow from atria to ventricle
Throughout the cycle, aortic pressure remains quite high as muscle and elastic fibres in the artery wall maintain blood pressure

Simulation of Cardiac Cycle

The coordinated contraction of cardiac muscle cells is controlled by specialised autorhythmic cells (‘pace makers’)

Atrial Contraction

Within the wall of the right atrium is a specialised cluster of cardiomyocytes which directs the contraction of heart tissue

This cluster of cells is collectively called the sinoatrial node (SA node or SAN)
The sinoatrial node acts as a primary pacemaker, controlling the rate at which the heart beats (i.e. pace 'making’)
It sends out electrical signals which are propagated throughout the entire atria via gap junctions in the intercalated discs
In response, the cardiac muscle within the atrial walls contract simultaneously (atrial systole)

The atria and ventricles of the heart are separated by a fibrous cardiac skeleton composed of connective tissue

This connective tissue functions to anchor the heart valves in place and cannot conduct electrical signals
The signals from the sinoatrial node must instead be relayed through a second node located within this cardiac skeleton
This second node is called the atrioventricular node (or AV node) and separates atrial and ventricular contractions
The AV node propagates electrical signals more slowly than the SA node, creating a delay in the passing on of the signal

Ventricular Contraction

The delay in time following atrial systole allows for blood to fill the ventricles before the atrioventricular valves close

Ventricular contraction occurs following excitation of the atrioventricular node (located at the atrial and ventricular junction)

The AV node sends signals down the septum via a specialised bundle of cardiomyocytes called the Bundle of His
The Bundle of His innervates Purkinje fibres in the ventricular wall, which causes the cardiac muscle to contract
This sequence of events ensures contractions begin at the apex (bottom), forcing blood up towards the arteries

Each normal heart beat should follow the same sequence of electrical events:

The P wave represents depolarisation of the atria in response to signalling from the sinoatrial node (i.e. atrial contraction)
The QRS complex represents depolarisation of the ventricles (i.e. ventricular contraction), triggered by signals from the AV node
The T wave represents repolarisation of the ventricles (i.e. ventricular relaxation) and the completion of a standard heart beat
Between these periods of electrical activity are intervals allowing for blood flow (PR interval and ST segment)

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