Describe how heart action is coordinated with reference to the SAN, AVN, and Purkyne Tissue, and explain the importance of the valves within the heart (8 marks)

The SAN (pacemaker) sends a wave of excitation across both atria. This triggers atrial systole, where both atria contract simultaneously. There is a time delay at the AV Node which allows the atria to empty completely and allows time for the ventricles to fill with blood. The AV Node sends a wave of excitation down the Bundles of His which are responsible for conducting waves of electrical activity up finer muscle fibres in the right and left ventricle walls- the Purkyne fibres. This triggers ventricular systole from the apex upwards. Many valves are found within the heart, including the semi-lunar, atrioventricular (AV), and tricuspid. The semi-lunar valves link the ventricles to the pulmonary artery, and the AV valves link the atria to the ventricles, enabling blood to flow between these different chambers. Valves also prevent the back flow of blood. *Key words to gain the relevant marks are underlined.