Inside the Heart

The heart is divided into a right and left side by the septum. There are four muscular walled chambers, the upper chambers are the atria and the lower chambers the ventricles. An atrioventricular (AV) valve separates each atrium from its ventricle. A semilunar valve separates each ventricle from its connecting artery.

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Cardiac conduction system

This includes the sinoatrial node, atrioventricular node, Bundle of His and Purkinje fibres, which operate as an electrical signalling system instructing cardiac muscle cells to contract.

Diagram of the cardiac conduction system

The cardiac cycle (each heart beat) is controlled by this special electrical system consisting of specialised muscle cells located in the walls of the heart. The sinoatrial (SA) node initiates the heartbeat and is called the pacemaker, it initiates impulses spontaneously at regular intervals. This firing is independent of nervous input, although the rate of firing can be influenced by nerve impulses from the cardiac centre located in part of the brain stem called the medulla oblongata.

Once the pacemaker discharges, a wave of depolarisation spreads across the atria causing them to contract and push blood into the ventricles. The electrical impulse reaches the atrioventricular (AV) node and after a short delay, which enables the atria to empty completely, the electrical impulse reaches the bundle of His and its branches. The bundle fibres (Purkinje fibres) distribute the electrical impulse throughout the ventricular walls causing them to contract forcing blood into the major arteries.

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