How your heart works
heart is a pump that keeps blood moving around your body. It
delivers oxygen and nutrients to all parts of your body, and
carries away unwanted carbon dioxide and waste products.
Play our interactive game, Know Your Heart, to help you
understand it better. Or read more detailed information
on the structure of the heart and how blood travels around
your body below.
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
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
- The two larger lower chambers are called the
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
- 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
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
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
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
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.