The Cardiac Cycle and Control of Heart Rate

This lesson covers the cardiac cycle (the sequence of events in one complete heartbeat) and the control of heart rate as required by the Edexcel A-Level Biology specification (9BI0). You need to be able to describe and interpret pressure and volume changes during the cardiac cycle, understand the role of the heart's conduction system, and explain how heart rate is regulated.

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The Cardiac Cycle

The cardiac cycle is the sequence of events that takes place during one complete heartbeat. At a resting heart rate of approximately 72 beats per minute, one cardiac cycle lasts about 0.8 seconds.

The cycle has three main phases:

1. Atrial Systole (Atrial Contraction) — ~0.1 s

• The atria contract simultaneously.
• Blood is forced from the atria through the open AV valves (tricuspid and bicuspid) into the ventricles.
• The ventricles are relaxed (in diastole) and passively filling with blood — atrial contraction "tops them up" with the last ~20% of blood.
• The semilunar valves are closed (pressure in the arteries exceeds pressure in the ventricles).

2. Ventricular Systole (Ventricular Contraction) — ~0.3 s