Understanding how your heart functions

Your heart is roughly the size of a fist and sits in the middle of your chest, slightly to the left. It’s the muscle at the centre of your circulation system, pumping blood around your body as your heart beats. This blood sends oxygen and nutrients to all parts of your body, and carries away unwanted carbon dioxide and waste products.

Structure of your heart

Your heart is made up of three layers of tissue:

  • epicardium
  • myocardium
  • endocardium

These layers are surrounded by the pericardium, a thin outer lining protecting your heart.

There are four chambers that make up the heart – two on the left side and two on the right.

The two small upper chambers are the atria. The two larger lower chambers are the ventricles. These left and right sides of the heart are separated by a wall of muscle called the septum.

Circulatory system

Your heart pumps blood around the body all the time - about five litres (eight pints) of it - and this is called circulation. Your heart, blood and blood vessels together make up your cardiovascular system (or heart and circulatory system).

The right side of the heart receives blood that is low in oxygen because most has been used up by the brain and body. It pumps this to your lungs, where it picks up a fresh supply of oxygen. The blood then returns to the left side of the heart, ready to be pumped back out to the brain and the rest of your body.

Blood vessels

Your blood is pumped around your body through a network of blood vessels:

  • arteries - they carry oxygen-rich blood from your heart to all parts of your body, getting smaller as they get further away from the heart
  • capillaries - they connect the smallest arteries to the smallest veins, and help exchange water, oxygen, carbon dioxide and other nutrients and waste substances between the blood and the tissues around them
  • veins - they carry blood, lacking in oxygen, back towards your heart, and get bigger as they get nearer your heart

Blood vessels are able to widen or narrow depending on how much blood each part of your body requires. This action is partly controlled by hormones.


Your heart has four valves. They act like gates, keeping the blood moving in the right direction:

  • aortic valve - on the left side
  • mitral valve - on the left side
  • pulmonary valve - on the right side
  • tricuspid valve - on the right side

Electrical system 

For your heart to keep pumping regularly, it needs electrical signals which are sent to the heart muscle telling it when to contract and relax.

The electrical signal starts in the right atrium where your heart’s natural pacemaker - the sino–atrial node - is situated. This signal crosses the atria, making them contract. Blood is pumped through the valves into the ventricles.

Where the atria meet the ventricles, there is an area of special cells - called the atrio-ventricular node - which pass the electrical signals throughout your heart muscle by a system of electrical pathways, known as the conducting system.

The muscles of the ventricles then contract, and blood is pumped through the pulmonary and aortic valves into the main arteries.

The heart’s natural ‘pacemaker’ - the sino-atrial node - produces another electrical signal, and the cycle starts again.

Blood pressure

This is the measurement of the pressure within the arteries. It plays a vital role in the way your heart delivers fresh blood to all your blood vessels. For blood to travel throughout your body quickly enough, it has to be under pressure. This is created by the relationship between three things:

  • your heart’s pumping action
  • the size and stretchiness of your blood vessels
  • the thickness of the blood itself

One heartbeat is a single cycle in which your heart contracts and relaxes to pump blood. At rest, the normal heart beats approximately 60 to 100 times every minute, and it increases when you exercise.

To ensure an adequate blood supply around your body, the four chambers of your heart have to pump regularly and in the right sequence.

There are two phases to your heart’s pumping cycle:

  • systole - this is when your heart contracts, pushing blood out of the chambers
  • diastole - this is the period between contractions when the muscle of your heart (myocardium) relaxes and the chambers fill with blood

How does a healthy heart work? (https://www.youtube.com/watch?v=ep4cQrYFL0w)

Watch this British Heart Foundation clip on how a healthy heart works.

Read more from Chest Heart & Stroke Scotland on how the heart works.

What can go wrong?


Some people are born with a heart that has not developed properly in the womb before birth - this is called congenital heart disease.

Sometimes you can inherit a heart condition from your family.

Cardiovascular system

Problems with your heart and circulation system include:

  • heart attack
  • angina
  • stroke

Heart disease can happen when your coronary arteries become narrowed by a gradual build-up of fatty material - called atheroma.

If your coronary arteries are narrowed or blocked, the blood supply to your heart will be impaired. This is the most common form of heart disease, known as coronary heart disease (sometimes called coronary artery disease or ischaemic heart disease).

Eventually, your arteries may become so narrow they can’t deliver enough blood to your heart. This can cause angina - a pain or discomfort in your chest, arm, neck, stomach or jaw.

If the fatty material breaks off or ruptures, a blood clot will form, which can cause heart attack (or stroke, if the artery affected is carrying blood to your brain).

Electrical system

Normally your heart will beat between 60 to 100 times per minute. This regular rhythmic beating is dependent upon electrical signals being conducted throughout your heart.

If the electrical signals within your heart are interrupted, your heart can beat too quickly (tachycardia), too slowly (bradycardia) and/or in an irregular way. This is called an arrhythmia - see Chest Heart & Stroke Scotland.

Conditions affecting the pumping of your heart

There are some conditions which can damage your heart muscle, making it weak and unable to pump as efficiently as before:

  • heart attack 
  • high blood pressure (hypertension)
  • heart valve problems - see Chest Heart & Stroke Scotland
  • cardiomyopathy - this is a general term for diseases of the heart muscle. Sometimes these diseases are inherited from your family. Sometimes they are caused by other things, like viral infections.

There are also conditions - like high blood pressure (hypertension) - which mean your heart has to work harder.

When your heart muscle can’t meet your body’s demands for blood and oxygen, you can develop various symptoms, like breathlessness, extreme tiredness and ankle swelling. This is called heart failure because of the failure of your heart to pump blood around the body and work efficiently.


Your heart can’t function normally if the heart valves aren't working properly, as it can affect the flow of blood through the heart.

There are two main ways that the valves can be affected:

  • valves can leak - this is called valve regurgitation or valve incompetence
  • valves can narrow and stiffen - this is called valve stenosis

Further information

Try the British Heart Foundation's Know your heart, an interactive tool narrated and presented by Dr Hilary Jones.