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.
Valves
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
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
Read more from Chest Heart & Stroke Scotland on how the heart works.
What can go wrong?
Structure
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.
Valves
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.
The heart is a pump, usually beating about 60 to 100 times per minute. With each heartbeat, the heart sends blood throughout our bodies, carrying oxygen to every cell. After delivering the oxygen, the blood returns to the heart. The heart then sends the blood to the lungs to pick up more oxygen. This cycle repeats over and over again.
What Does the Circulatory System Do?
The circulatory system is made up of blood vessels that carry blood away from and towards the heart. Arteries carry blood away from the heart and veins carry blood back to the heart.
The circulatory system carries oxygen, nutrients, and hormones to cells, and removes waste products, like carbon dioxide. These roadways travel in one direction only, to keep things going where they should.
The heart has four chambers — two on top and two on bottom:
- The two bottom chambers are the right ventricle and the left ventricle. These pump blood out of the heart. A wall called the interventricular septum is between the two ventricles.
- The two top chambers are the right atrium and the left atrium. They receive the blood entering the heart. A wall called the interatrial septum is between the atria.
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Animation showing the normal heart anatomy and blood pumping through pulmonary and systemic circulation.
The atria are separated from the ventricles by the atrioventricular valves:
- The tricuspid valve separates the right atrium from the right ventricle.
- The mitral valve separates the left atrium from the left ventricle.
Two valves also separate the ventricles from the large blood vessels that carry blood leaving the heart:
- The pulmonic valve is between the right ventricle and the pulmonary artery, which carries blood to the lungs.
- The aortic valve is between the left ventricle and the aorta, which carries blood to the body.
What Are the Parts of the Circulatory System?
Two pathways come from the heart:
- The pulmonary circulation is a short loop from the heart to the lungs and back again.
- The systemic circulation carries blood from the heart to all the other parts of the body and back again.
In pulmonary circulation:
- The pulmonary artery is a big artery that comes from the heart. It splits into two main branches, and brings blood from the heart to the lungs. At the lungs, the blood picks up oxygen and drops off carbon dioxide. The blood then returns to the heart through the pulmonary veins.
In systemic circulation:
- Next, blood that returns to the heart has picked up lots of oxygen from the lungs. So it can now go out to the body. The aorta is a big artery that leaves the heart carrying this oxygenated blood. Branches off of the aorta send blood to the muscles of the heart itself, as well as all other parts of the body. Like a tree, the branches gets smaller and smaller as they get farther from the aorta.
At each body part, a network of tiny blood vessels called capillaries connects the very small artery branches to very small veins. The capillaries have very thin walls, and through them, nutrients and oxygen are delivered to the cells. Waste products are brought into the capillaries.
Capillaries then lead into small veins. Small veins lead to larger and larger veins as the blood approaches the heart. Valves in the veins keep blood flowing in the correct direction. Two large veins that lead into the heart are the superior vena cava and inferior vena cava. (The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.)
Once the blood is back in the heart, it needs to re-enter the pulmonary circulation and go back to the lungs to drop off the carbon dioxide and pick up more oxygen.
How Does the Heart Beat?
The heart gets messages from the body that tell it when to pump more or less blood depending on a person's needs. For example, when we're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by our bodies at rest. But when we're exercising, the heart pumps faster so that our muscles get more oxygen and can work harder.
How the heart beats is controlled by a system of electrical signals in the heart. The sinus (or sinoatrial) node is a small area of tissue in the wall of the right atrium. It sends out an electrical signal to start the contracting (pumping) of the heart muscle. This node is called the pacemaker of the heart because it sets the rate of the heartbeat and causes the rest of the heart to contract in its rhythm.
These electrical impulses make the atria contract first. Then the impulses travel down to the atrioventricular (or AV) node, which acts as a kind of relay station. From here, the electrical signal travels through the right and left ventricles, making them contract.
One complete heartbeat is made up of two phases:
- The first phase is called systole (SISS-tuh-lee). This is when the ventricles contract and pump blood into the aorta and pulmonary artery. During systole, the atrioventricular valves close, creating the first sound (the lub) of a heartbeat. When the atrioventricular valves close, it keeps the blood from going back up into the atria. During this time, the aortic and pulmonary valves are open to allow blood into the aorta and pulmonary artery. When the ventricles finish contracting, the aortic and pulmonary valves close to prevent blood from flowing back into the ventricles. These valves closing is what creates the second sound (the dub) of a heartbeat.
- The second phase is called diastole (die-AS-tuh-lee). This is when the atrioventricular valves open and the ventricles relax. This allows the ventricles to fill with blood from the atria, and get ready for the next heartbeat.
How Can I Help Keep My Child's Heart Healthy?
To help keep your child's heart healthy:
- Encourage plenty of exercise.
- Offer a nutritious diet.
- Help your child reach and keep a healthy weight.
- Go for regular medical checkups.
- Tell the doctor about any family history of heart problems.
Let the doctor know if your child has any chest pain, trouble breathing, or dizzy or fainting spells; or if your child feels like the heart sometimes goes really fast or skips a beat.