The Cardiac Cycle | AQA A Level Biology Revision Notes 2015

Key Concepts

• Cardiac Cycle: Involves the events in one heart beat, including muscle contraction (systole) and relaxation (diastole).
• Continuous Process: Cycles follow one another without gaps, ensuring continuous blood flow.

Volume and Pressure Changes

• Contraction leads to decreased volume and increased pressure in heart chambers.
• Relaxation increases volume and decreases pressure.
• Heart Valves: Open and close due to pressure changes; open when pressure behind is greater, close when pressure in front is greater.

Stages of the Cardiac Cycle

Atrial Systole

• Atria Contract: Decrease in atrial volume, increase in pressure.
• AV Valves Open: Blood moves into ventricles; slight increase in ventricular pressure/volume as ventricles receive blood.

Ventricular Systole

• Ventricles Contract: Volume decreases, pressure increases.
• AV Valves Close: Prevents backflow into atria.
• SL Valves Open: Blood moves into arteries.
• Atria Relax: Begin to refill with blood during ventricular contraction.

Diastole

• Relaxation of Ventricles and Atria.
• SL Valves Close: Due to lower pressure in ventricles compared to arteries.
• Atria Fill: Blood returns via vena cava and pulmonary vein.
• AV Valves Open: Blood flows passively into ventricles; cycle restarts.

Valves During the Cardiac Cycle

Analyzing the Cardiac Cycle

• Pressure Lines on Graph: Represent pressures in left atrium, aorta, and left ventricle.
• Key Points:
  • Point A: Diastole ends, AV valve open.
  • Point B: Start of ventricular systole, AV valve closes.
  • Point C: Ventricular systole, aortic valve opens.
  • Point D: Start of diastole, aortic valve closes.

Worked Example

• Calculate heart rate using cycle time:
  • 0.7 seconds per beat → 85.71 beats/min.

Examiner Tips

• Understanding Graphs: Important for interpreting pressure/volume changes, valve actions, and blood flow in cardiac cycle.

Additional Resources

• Test yourself with additional exam questions on related topics like mass transport in plants and genetic diversity.

Author and Reviewer

• Author: Lra Marie McIvor, expertise in Biology.
• Reviewer: Lucy Kirkham, Head of STEM.

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