Define the term cardiac cycle.

It is the full sequence of events of one heartbeat, from the start of one atrial contraction to the start of the next.

What causes the atrioventricular valves to close?

At the start of ventricular systole the ventricular pressure rises above the atrial pressure, and the pressure difference forces the cusps shut.

Explain why the semilunar valves close at the end of ventricular systole.

The ventricles relax and their pressure falls below the arterial pressure, so blood begins to flow backwards and fills the valve cusps, snapping them shut.

OCR A-Level Biology B H422

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The Heart and Monitoring Heart Function

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The Heart and Monitoring Heart Function

The cardiac cycle

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the three stages in order, what opens and closes each set of valves, pressure during diastole, the heart sounds, and calculations from a cycle's timing.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

What are the three stages of the cardiac cycle, in the order in which they occur?

Key term

What happens during atrial systole?

Key term

What causes the semilunar valves to open?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the term cardiac cycle.
Model answer
It is the full sequence of events of one heartbeat, from the start of one atrial contraction to the start of the next.
2
What are the three stages of the cardiac cycle, in the order in which they occur?
Model answer
Atrial systole, then ventricular systole, then diastole.
3
What happens during atrial systole?
Model answer
The atria contract so that atrial pressure rises above ventricular pressure, and the last of the blood is pushed through the open atrioventricular valves into the ventricles.
4
What causes the atrioventricular valves to close?
Model answer
At the start of ventricular systole the ventricular pressure rises above the atrial pressure, and the pressure difference forces the cusps shut.
5
What causes the semilunar valves to open?
Model answer
Ventricular pressure rises above the pressure in the aorta and pulmonary artery, so blood pushes the valves open.
6
Explain why the semilunar valves close at the end of ventricular systole.
Model answer
The ventricles relax and their pressure falls below the arterial pressure, so blood begins to flow backwards and fills the valve cusps, snapping them shut.

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