There are two stages in a heart beat. They are called systole and diastole. For your GCSE biology exam you should be able to label a diagram of the heart, put arrows on the diagram to show how blood flows through the heart, and explain how the muscles and valves force blood through the heart. You should also know that the left hand side of the heart pumps blood from the lungs to the rest of the body and that the right hand side pumps blood from the body back to the lungs.
How the Heart Pumps Blood
Systole This should really be called "ventricular systole", but don't worry about this for GCSE.
When your doctor measures your blood pressure and says that it is 120 over 90 he or she means that the blood pressure in your arteries is 120 when the left ventricle is contracting.
The diagram also shows that the atrium is relaxing. Blood rushes into the left atrium from the pulmonary vein: the pressure of the blood in the pulmonary vein is just enough to stretch the walls of the atrium which are muscular but are much thinner than the walls of the ventricle.
The bicuspid valve prevents blood from going back from the ventricle into the atrium. Some people have a "heart murmur", this is when the bicuspid valve does not close properly and some blood does go backwards. The bicuspid valve is supported by tendons which look rather like the strings of a parachute. These tendons prevent the bicuspid valve from being blown inside out.
Diastole This should really be called "ventricular diastole", but don't worry about this for GCSE.
The diagram also shows you that the entrance to the left atrium from the pulmonary vein is closed: this prevents blood from being forced back the way it came. The muscle of the atrium is just strong enough to force blood into the ventricle and stretch its walls.
Here are the two diagrams side by side to help you see the difference between the two stages:
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