IGCSE Biology / Blood and circulation

How the heart beats

Edexcel International GCSE Biology (4BI1), chapter 5: Blood and circulation, specification point 2.65. Free lesson.

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The cardiac cycle is the sequence of events in one heartbeat. Understanding pressure changes explains how the heart’s valves open and close, and how the heartbeat starts without a nerve signal.

The three stages of the cardiac cycle

At rest, one heartbeat takes about 0.8 seconds. The cycle has three stages that repeat, with the end of one cycle leading into the next. Atrial systole: both atria contract, pushing blood through the open atrioventricular valves into the ventricles. Ventricular systole: both ventricles contract. Their pressure rises above the pressure in the atria, closing the atrioventricular valves. When ventricular pressure rises above the pressure in the arteries, the semilunar valves open and blood is pushed into the aorta and pulmonary artery. Diastole: the heart relaxes. Ventricular…

How the heartbeat starts

The signal that starts each heartbeat comes from the heart itself. The pacemaker is in the right atrium, so the heart can beat without a nerve telling it when to contract.

Why the valves open and close

Heart valves respond to pressure differences. A valve opens when the pressure behind it is higher than the pressure in front. If pressure becomes higher on the other side, the valve closes, helping to prevent blood from flowing backwards. The atrioventricular valves are between the atria and ventricles. The semilunar valves are between the ventricles and the arteries.

How to read valve events on a pressure graph

A pressure graph shows time along the bottom and pressure up the side. The left ventricular pressure rises steeply during ventricular systole, reaches a peak, then falls during diastole. Aortic pressure stays high between beats because the artery walls are elastic and maintain pressure. When ventricular pressure rises above aortic pressure, the semilunar valve opens. When it falls below aortic pressure, the valve closes. For the atrioventricular valves, compare atrial and ventricular pressure. The valves open when atrial pressure is higher, and close when ventricular pressure becomes higher.…

Applying pressure changes to the atrioventricular valve

When ventricular pressure becomes higher than atrial pressure, blood is pushed back against the atrioventricular valve. This forces it shut and prevents backflow into the atrium.

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