Cardiac cycle, SAN and Purkyne fibres (WJEC A-level Biology)

This lesson describes the roles of the SAN and Purkyne fibres in the coordination of the three stages of the cardiac cycle. The PowerPoint and accompanying resources are part of the first lesson in a series of 2 lessons that have been designed to cover point [c] in topic 3 of AS unit 2 of the WJEC A-level Biology specification and has a specific focus on the pressure changes that occur in each stage of the cycle

The start of the lesson introduces the cardiac cycle as well as the key term systole, so that students can immediately recognise that the three stages of the cycle are atrial and ventricular systole followed by diastole. Students are challenged on their prior knowledge of the structure of the heart as they have to name and state the function of an atrioventricular and semi-lunar valve from an internal diagram. This leads into the key point that pressure changes in the chambers and the major arteries results in the opening and closing of these sets of valves. Students are given a description of the pressure change that results in the opening of the AV valves and shown where this would be found on the graph detailing the pressure changes of the cardiac cycle. They then have to use this as a guide to write descriptions for the closing of the AV valve and the opening and closing of the semi-lunar valves and to locate these on the graph. By providing the students with this graph, the next part of the lesson can focus on explaining how these changes come about. Students have to use their current and prior knowledge of the chambers and blood vessels to write 4 descriptions that cover the cardiac cycle.

This rest of the lesson focuses on the roles of the SAN and Purkyne fibres as well as the AVN and the bundle of His in the coordination of the heartbeat, continually linking back to the work on the cycle. The SAN is introduced as the natural pacemaker and then time is given to study each step of the conduction of the impulse as it spreads away from the myogenic tissue in a wave of excitation. Moving forwards, students are encouraged to consider why a delay would occur at the AVN and then they will learn that the impulse is conducted along the Bundle of His to the apex so that the contraction of the ventricles can happen from the bottom upwards. The structure of the cardiac muscle cells is discussed and the final task of the lesson challenges the students to describe the conducting tissue, with an emphasis on the use of key terminology.