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An A to Z of the Circulatory System

Illustration of a heart and blood vessels

Sometimes referred to as the ‘motorway’ of the body, the aorta is your body’s biggest blood vessel, measuring about an inch in diameter (1). Lying on your back, you may see your stomach softly pulsate - this is your abdominal aortic artery in action.

illustration of the inside of an artery
Blood vessels

Arteries carry oxygen-rich blood to all tissues; veins carry deoxygenated blood back to the heart. (Veins appear blue even though their blood is actually maroon in colour, because of the way that they reflect and absorb the red and blue wavelengths of visible light (2)). Some 10 billion capillaries, each one-tenth the diameter of a human hair, lace all body tissues, connecting arteries to veins.

Illustration of a heart with artery coming towards you and cells flowing through

William Harvey (1578-1657) was the first to describe how blood flowed in a closed circuit within the body, with the heart acting as the pump.

Illustration of an heart and set of lungs
Double circuit

Ours is a heart of two halves (3). The right side receives blood from the body and propels it to the lungs; the left side receives blood from the lungs and ejects it at high pressure to all body cells. At rest, it takes a minute for your blood to circumnavigate the body; during exercise, as little as ten seconds.

Human need image showing higher levels of pain on the bottom of the foot

Left on its own, under the pull of gravity, blood would struggle to get back from the feet to the heart. The contraction of leg muscles, with valves in leg veins acting like closed doors to prevent backflow, aids upward mobility.

Illustration of blood cells flowing through the body
False ideas

For centuries the teachings of Galen, a surgeon to the gladiators in the Roman Empire, prevailed. He thought that blood ebbed and flowed in sea-like waves from the liver to body organs, where it was consumed. The heart was simply, and falsely, seen as a furnace ‘boiling the blood’.

Illustration of blood cells and oxygen flowing through the body

Your body transports oxygen to every living cell, mostly carrying it bound to haemoglobin in red blood cells. Being so vital to life, one billion molecules of oxygen pack into each red blood cell.

Illustration of a hormone molecular structure

Blood hormone levels vary over time, in particular patterns, with stress hormones peaking each morning, and female reproductive hormones showing a 28 day cycle. Insulin levels also show a daily rhythm, dropping overnight and between meals and beginning to increase when food is seen or smelt, peaking after the meal.

Illustration of white and red blood cells

White blood cells provide a line of defence against injury and infection second only to skin.

Image of a persons neck, they are holding on to their shoulders
Jugular vein

On both sides of the neck, these veins carry blood back from the head to the heart. Their pulsations can tell doctors how healthy your heart is (4).

Image of Karl Landsteiner
Karl Landsteiner (1868-1943)

Estimated to have saved more than a billion lives, Dr Karl Landsteiner made blood donation possible through the discovery of blood groups in the early 20th century (5).

Human need image showing pain in calf, ankle, heal, and bottom of foot
Leg pain

May be an indicator of poor circulation, especially if you have high blood pressure, high cholesterol, diabetes, or you smoke. If you experience aches or cramps in the calf, thigh or buttock while walking, consult your doctor.

Image of elderly couple walking through the park with their grandchild
Moving matters

Research in the British Medical Journal suggests that around 70,000 deaths each year in the UK are linked to spending too much time sitting down.

EMS icon
Neuromuscular electrical stimulation

Simulating the effect of exercise by artificially activating the calf muscle pump, through electrical impulses delivered to the feet, this emerging technology has been shown to improve circulation and aid exercise.

Illustration of a blocked artery

In much the same way that water pipes become choked with calcium salts, your arteries can become furred up and blocked by fatty plaques; blood clots are another culprit. When this happens in the heart or brain, the result may be angina, heart attack, vascular dementia or stroke.

Image of a woman holding on to the back of her calf
Peripheral arterial disease (PAD)

When the narrowing or blockage is in an extremity, the disease is called PAD. Often painless, but symptoms include pain while walking, severe cases can result in gangrene or amputation.

Image of of couple hugging
Quality of life

Poor circulation, depending on where it’s worst, can lead to feelings of breathlessness, light-headedness, aches and pains and cold. Erectile dysfunction in men, leg ulcers and even depression can develop, with vascular surgeons calling for more emphasis on mental health for their patients (6).

Illustration of the side of someones head, it shows the brain lit up

According to the Stroke Association, around 15% of strokes are caused by bleeding from a ruptured vessel in the brain, or on its surface. Bleeds from a ruptured aorta (usually in the abdomen), meanwhile, cause 5,000 deaths each year in the UK (7): Albert Einstein died this way in 1955.

Image of a surgery room with a person picking up tools

Clipping off an aneurysm or stuffing it with coils can prevent the bulge from bursting (8), whilst manmade tubing repairs a rupture. Where they cause significant symptoms, occlusions are opened using tiny balloons and stents (wire mesh scaffolds). If this is unsuccessful or unsuitable, blockages can instead be bypassed using a vessel from the chest, arm or leg, in another attempt at revascularisation.

Image of a woman having her temperature taken with a Digital thermometer gun

Blood absorbs the heat generated by active muscles and redistributes it around the body, keeping all parts warm. In cold weather, blood flow is directed to the brain, and away from the skin, to conserve heat where it’s most needed (9).

Image of someone having their blood pressure taken
Under pressure

Pressure in your blood vessels helps force nutrient-rich fluid across capillary walls and into tissues. Too high or too low and you could be in trouble, with high blood pressure, for example, leading to strokes and kidney failure (10).

Risk factors for high blood pressure include a sedentary lifestyle, diabetes, smoking and high cholesterol; these are also linked to the poor circulation of peripheral arterial disease.

Image of a womans leg, her leg has varicose veins down the inside
Varicose Vein

If your veins are under chronic pressure due to blood pooling in the legs through poor circulation, this can lead to venous disease (11). Your legs may feel tight and heavy and ache; swelling, varicose veins and leg ulcers may result.

outline of a cloud with CO2 written in it

Carbon dioxide, the waste made during energy production, is carried by the blood to the lungs to be breathed out. Hyperventilating or excessive breath holding are harmful since abnormal carbon dioxide levels alter the acidity of blood, affecting heart and brain function and causing muscle cramps (12).

Image of a man outside stretching

According to the British Heart Foundation, regular exercise can lower your risk of heart and circulatory disease by 35%. Muscles, including the heart, strengthen – and even grow more capillaries – when you lead an active lifestyle.

Image of a group of young people all looking at a phone and laughing
Young people

Every week in the UK at least 12 young people die of undiagnosed heart conditions (13). Case studies exist meanwhile of young people in their twenties with peripheral arterial disease, although it is rare (14).

Image of a zebra fish

These fascinating fish are tiny and transparent – and can regenerate a new fin a fortnight after amputation. Scientists are studying them to work out how humans might grow new vessels in the quest to treat peripheral arterial disease (15).


  1. UF Health (2021) Aorta Anatomy
  2. Science World (2015) Is blood really blue?
  3. Cecconi M, Johnston E, Rhodes A (2006) What role does the right side of the heart play in circulation?
  4. Roland J (2017) Jugular vein distension (JVD): Causes, Assessment, and More.
  5. Nobel Prize Outreach AB (2021). Karl Landsteiner Biographical.
  6. Ramirez J, Grenon M (2019) Depression and Peripheral Arterial Disease: A call to action.
  7. British Heart Foundation (2021) All about aortic aneurysm.,around%201%E2%80%932mm%20per%20year.&text=Ruptured%20aortic%20aneurysms%20cause%205%2C000,the%20aorta%2C%20in%20your%20chest.
  8. Huang J (2021) Microsurgical clipping and endovascular coiling for brain aneurysm.
  9. Cowan H (2015) How does the body survive in extreme cold weather?
  10. American Heart Association (2016) How high blood pressure can lead to kidney damage or failure.
  11. Johns Hopkins Medicine (2021) Chronic venous insufficiency.
  12. Johns Hopkins Medicine (2021) Hyperventilation.
  13. Cardiac Risk in the Young (2021) About Us.
  14. Doraiswamy VA, Giri J, Mohler E (2009) Premature peripheral arterial disease – difficult diagnosis in very early presentation.
  15. Royal Veterinary College (2021) How to grow new blood vessels: the zebrafish as a model to study angiogenesis in development and regeneration.

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