The heart beat is caused by impulses arising from two specialised groups of cells within the heart muscle. The Sino-Atrial (SA) node, situated in the wall of the right atrium initiates the beat, and the Atrioventricular (AV) node which is positioned between the ventricles and continues to distribute the wave of impulses.

The heart beat

There are several distinct stages which form a full heart beat. Cardiac Systole describes the period at which the heart contracts and cardiac diastole describes the period of relaxation, between beats. They can however be further divided into diastole and systole of the atria and ventricles.

The phases of a heart beat can also be divided into sections relating to the shape of the electrical signals produced when viewing the heart beat via an ECG (Electrocardiogram). This traces the electrical activity of the heart. The wave shape produced is called the QRS wave, with each part of the wave being labelled to help describe what is happening at each stage.

  • TP Interval (Ventricular Diastole) - Atria and ventricles are relaxed, blood is flowing into the atria from the veins. As the atrial pressure increases above that of the ventricle, the AV valves open, allowing blood to flow into the ventricle
  • P Wave (Atrial Systole) - The SA node fires and the atria contract causing atrial systole which forces all blood into the ventricles, emptying the atria.
  • QR Interval (End of Ventricular Diastole) - The AV valves remain open as all remaining blood is squeezed into the ventricles. The electrical impulse from the SA node reaches the AV node which spreads the signal throughout the walls of the ventricles via specialised cells called bundles of His and Purkinje fibres. The R peak is the end of ventricular diastole and the start of systole.
  • RS Interval (Ventricular Systole) - As the blood is now all within the ventricles and so pressure is higher here than in the atria, the AV valves close. The ventricles start to contract although pressure is not yet high enough to open the SL valves
  • ST Segment (Ventricular Systole) - Pressure increases until it equals Aortic pressure, when the SL valves open. The blood is ejected into the Aorta (and pulmonary artery) as the ventricles contract. At this time the atria are in diastole and filling with blood returning from the veins.
  • T Wave (Ventricular Diastole) - Ventricles relax, the ventricular pressure is once again less than the aortic pressure and so the SL valves close. The cycle continues.

Cardiac volumes

Cardiac Volumes are the amount of blood moved by the heart at any time:

Stroke Volume (SV): This is the amount of blood pumped from the heart, each time the heart contracts. At rest this is between 70-100ml, during exercise the figure increases to 113-179ml. The fitter an individual is, the higher their stroke volume.

Cardiac Output (CO): The amount of blood ejected from the heart in the space of a minute. This can be calculated by multiplying the stroke volume and the heart rate (beats per minute). This can range from 5,000ml at rest to 35,000ml during maximal exercise.

End Diastolic Volume (EDV): The amount of blood within the left ventricle prior to the contraction (systolic) phase of a heart beat.

End Systolic Volume (ESV): The volume of blood remaining in the ventricle after contraction. The stroke volume is equal to EDV-ESV

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