How your heart works

The structure of the human heart

The heart is made up of three layers:

• Pericardium – thin outer protective sack
• Myocardium – specialised cells making up the thick muscular wall
• Endocardium – thin inner lining of the heart  

Inside the heart there are four chambers – two on the left side and two on the right.

• The two small upper chambers are called the atria
• The two larger lower chambers are called the ventricles

The left and right sides of the heart are divided by a muscular wall called the septum.

There are four valves in your heart. They act like ‘gates’ that open and close, making sure that your blood travels in one direction through your heart – a bit like a one-way traffic system. They are called the tricuspid valve and the pulmonary valve on the right side of the heart, and the mitral valve and the aortic valve on the left.

Like every other living tissue, the heart itself needs a continuous supply of fresh blood. This blood supply comes from the coronary arteries which branch off from the main artery (the aorta) as it leaves the left ventricle. The coronary arteries spread across the outside of the myocardium, supplying it with blood.

How blood travels around your body

As your heart muscle contracts, it pushes blood through your heart. With each contraction, or heartbeat:

• Your heart pumps blood forward from its left side, through the aorta (the main artery leaving the heart) and into the arteries. Blood from the right side is pumped to your lungs.
• The blood continues through the arteries, which divide off into smaller and smaller branches of microscopic capillaries.Travelling through this network of capillaries, blood reaches every part of your body. 
• The blood then travels back to the heart from the capillaries into the veins. The branches of the veins join to form larger veins, which deliver the blood back to the right side of your heart.

As the heart relaxes in between each heartbeat or contraction, blood from your veins fills the right side of your heart and blood from the lungs fills the left side of your heart.

The two sides of the heart are separate, but they work together. The right side of the heart receives dark, de-oxygenated blood which has circulated around your body. It pumps this to your lungs, where it picks up a fresh supply of oxygen and becomes bright red again.

The cardiovascular system

This movement of blood around the body, pumped by the heart, is called circulation. This system is called the cardiovascular system (or heart and circulatory system).

• It contains about five litres (eight pints) of blood, which your heart is continuously circulating.
• Each day, your heart beats about 100,000 times.
• It pumps about 23,000 litres (5,000 gallons) of blood around your body.

For your heart to keep pumping regularly, it needs an electrical supply. This is provided by a special group of heart cells called the sinus node, which is also known as your heart’s natural pacemaker.

What if something goes wrong?

Some people are born with hearts that have not developed properly before birth  - this is called congenital heart disease. Some heart conditions may develop later in life, including coronary heart disease and atrial fibrillation. Sometimes you can inherit a heart condition from your family.

People who have coronary heart disease are at risk of having  a heart attack. Damage to the heart muscle because of a heart attack can lead to heart failure which will affect you for the rest of your life. Your donations are helping our scientists to mend broken hearts like these. You can also watch our stem cell video to find out more.

Heart failure can also be caused by other conditions. How heart failure affects you will depend on the cause of the heart failure and how much of your heart muscle it affects.