1. How The Heart Works

2. Inside the Heart The heart has four hollow chambers inside.
The two upper chambers are called atria. Each is an atrium.
The walls of the heart are made of cardiac muscle.
The wall down the middle is called the septum. It divides the heart into two parts.
The two lower chambers are called ventricles.

3. The are two semilunar valves stop blood flowing back into the heart from the arteries.
There are two valves that stop blood flowing back into atria from the ventricles.
The other valve is called the biscupid or mitral valve.
The final valve is called the tricuspid valve.

4. The Cardiac Muscle The walls of your heart are made of cardiac muscle.
When this contracts the heart gets smaller so blood gets pumped out.
The right side pumps deoxygenated blood to the lungs, at a low pressure.
The left side pumps oxygenated blood all round the body, at a much higher pressure. This needs more force, so the left side has thicker walls.
The left ventricle has the thickest walls, because it works hardest of all.
When it contracts, it pumps blood into the aorta and round the body.

5. How the heart pumps blood
(1) When the heart is relaxed, both sides fill with blood from the veins. (but no blood can flow in from the arteries).
(2) The atria contact. The veins contract where they join atria. So blood from the atria is forced into the ventricles.
(3) Then the ventricles contract. The valves between the ventricles and atria close. So the blood is forced out of the heart, into the arteries.
(4) The heart muscle relaxes again and steps 1 – 3 are repeated.

6. This cycle of events is called the cardiac cycle.
One complete contraction and relaxation is called a heart beat.
Your heart beats around 70 times a minute when you are resting (less if you are fit).
This can rise to 200 beats a minute when you run very fast.
The number of beats per minute is called your heart rate.