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
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
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.
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
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
https://youtu.be/ep4cQrYFL0w Check out this video from the British Heart Foundation on how Blood circulates around the heart.
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