The Cardiac Cyle

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The Cardiac Cyle

The Cardiac cycle is activated by an electrical stimulus called the cardiac impulse. This stems from a small mass of tissue found in a region of the right atrium, called the Sino atrial node or pacemaker. The electrical stimulus generated by the Sino-atrial node spreads out into the walls of the atria causing their contraction, in other words atrial systole. A band of fibrous tissue splits the atria from the ventricles, and stops the impulse passing straight on to the ventricles. Here the Atrio-ventricular Node picks up the impulse. From here, it is conducted through the band of fibrous tissue, along a short bundle of specialised conducting fibres called the Bundle of His. The bundle separates within the septum that divides the two ventricles, into left and right branches, which conducts the impulse to the tip of the heart, where the branches turn up along the ventricular myocardium through a finely branching network of Purkinje fibres. Impulses from these cause the walls to contract.

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The Sino-atrial node kicks off systole and this is followed a fraction later by ventricular systole, which because of the nature of the conduction system begins at the apex of the ventricles. Once systole has finished, diastole begins. The transfer of blood through the heart is directed by the heart valves, the opening and closing of which in turn is controlled by the adjustments in blood pressure that happen within the chambers of the heart. Blood flows from an area of high pressure, to an area of low pressure.

Pressure in the atria and ventricles- atrial and ventricular ...

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