High Blood Pressure
(Hypertension)
What it is, what causes it, why it matters, and why knowing your numbers can help.
Written by Ian Murray, Cardiac Physiologist
High blood pressure — also known as hypertension — is a common condition in which the pressure inside your arteries remains higher than is healthy over time.
It often causes no obvious symptoms. You can feel completely well and still have high blood pressure without knowing it. Measuring your blood pressure may therefore be the only way to discover that it is raised. [1,2]
Over time, persistently high blood pressure can damage blood vessels and place extra strain on the heart and other organs. It also increases the likelihood of serious problems such as heart attack and stroke. The important point is that high blood pressure can be detected, assessed and managed. [1–3]
This page explains what blood pressure means, why it can become high, how it can affect the body and why repeated measurements are more useful than one isolated reading.
Quick overview
Use the links below to jump to the section you need:
What is blood pressure?
Every time your heart beats, it pushes blood into your arteries. This creates pressure inside the blood vessels as blood travels around your body.
That pressure is not constant. It rises and falls with every heartbeat.
Video showing blood flow through the heart. Blood fills the upper chambers before flowing into the lower chambers. The lower chambers then contract, pumping blood out of the heart.
Two numbers describe your blood pressure
A blood pressure reading is written as two numbers,
for example: 120/80 mmHg
Systolic blood pressure — the top number — is the pressure in your arteries when the heart contracts and pumps blood out.
Diastolic blood pressure — the bottom number — is the pressure between heartbeats, when the heart relaxes and refills. [1,2]
The unit mmHg means millimetres of mercury, the traditional unit used for measuring blood pressure.
For a simple refresher on how each heartbeat creates pressure inside the arteries, see our guide to
How the Heart Works.
How healthy arteries help
Arteries are not rigid pipes. Healthy artery walls have some elasticity, allowing them to expand as each pressure wave passes and then recoil.
This helps cushion the pressure created by each heartbeat and supports blood flow around the body. [2,3]
Video showing blood flow through an artery. Notice how the artery expands as a pressure wave passes and then recoils.
Why does blood pressure often rise with age?
As we get older, arteries commonly become stiffer. When they are less able to expand and cushion each pressure wave, systolic blood pressure in particular tends to rise. [2,3]
Ageing is only part of the picture, however. Getting older does not mean that high blood pressure is inevitable.
Blood pressure is controlled by several interacting systems. Changes in blood vessels, kidney function, hormones, nerve signals and other factors can gradually shift this control towards a higher pressure. Genetics, ageing, lifestyle and environment can all influence this process. [2,3]
What causes high blood pressure?
For most adults with hypertension, there is no single identifiable cause. Instead, several influences gradually interact. This is known as primary hypertension, and it accounts for most cases. [2,3]
Your body continually adjusts blood pressure so that enough blood reaches the brain, heart, kidneys and other organs. Several parts of this system are involved:
arteries and smaller blood vessels influence how easily blood can flow;
the kidneys regulate salt, fluid balance and blood volume;
hormonal and nerve signals help control blood-vessel tone, heart activity and fluid balance;
ageing and inherited characteristics can alter these systems over time.
Lifestyle and environmental influences, including salt intake, body weight, physical activity, alcohol, sleep, stress and some medicines or substances, can also affect blood pressure. [2,3]
For many people, several influences overlap over many years. This is why primary hypertension usually develops gradually rather than having one obvious starting point.
What is secondary hypertension?
In some people, high blood pressure has a more specific underlying cause. This is known as secondary hypertension.
Possible causes include certain kidney conditions, hormone disorders, obstructive sleep apnoea, narrowing of an artery supplying a kidney, and some medicines or other substances. [1–3]
A healthcare professional may consider further investigation when there is a reason to suspect an underlying cause.
Where does atherosclerosis fit in?
High blood pressure and atherosclerosis are closely connected, but they are not the same condition.
Atherosclerosis is the gradual build-up of fatty material within artery walls. Over time, plaques can narrow arteries. If a plaque ruptures, a blood clot can form and block blood flow — one way a heart attack or ischaemic stroke can happen. [2,3]
High blood pressure can contribute to damage and changes within artery walls, while atherosclerosis can develop alongside hypertension, particularly as people get older. [2,3]
When they occur together, they increase the chance of serious events such as a heart attack or stroke.
The important distinction is that atherosclerosis is not the usual explanation for why most people develop high blood pressure. Primary hypertension generally develops through several interacting influences.
Video showing gradual plaque build-up within the artery wall. Over time, this can narrow the blood vessel and restrict blood flow.
How can high blood pressure affect the body?
The concern is not usually one isolated high reading. It is blood pressure that remains raised over time.
Because high blood pressure often causes no obvious symptoms, changes can develop gradually without being noticed.
The heart
Persistently raised blood pressure means the heart must pump blood into a higher-pressure arterial system.
Over time, the heart muscle can thicken and work less effectively. High blood pressure also increases the likelihood of coronary heart disease, heart attack and heart failure. [1–3]
The brain
High blood pressure is an important contributor to stroke.
It is associated with strokes caused by a blocked blood vessel and strokes caused by bleeding in or around the brain. Long-standing high blood pressure is also associated with damage to small blood vessels in the brain and changes in memory and thinking later in life. [2,3]
The kidneys
The kidneys contain many small blood vessels and also play an important role in controlling blood pressure.
High blood pressure can gradually damage the kidneys, while kidney disease can itself contribute to high blood pressure. [1–3]
The eyes
Persistently high blood pressure can damage the small blood vessels supplying the retina at the back of the eye and may affect eyesight. [2,3]
Blood vessels
High blood pressure can gradually damage blood vessels throughout the body and commonly occurs alongside other vascular changes such as arterial stiffness and atherosclerosis. [2,3]
Finding high blood pressure earlier gives you the opportunity to have it properly assessed and, where necessary, brought under better control.
Why is one blood pressure reading not enough?
Blood pressure is not a fixed number. It naturally changes as your body responds to activity, rest, stress, sleep, caffeine, pain and many other everyday influences.
One reading is only a snapshot.
A result may be temporarily higher because you have rushed to an appointment, recently exercised or feel anxious. Some variation is normal even when measurements are taken under similar conditions.
That is why hypertension is generally assessed using repeated measurements, rather than making a judgement from one isolated result.
Depending on the circumstances, this may involve repeat measurements in a healthcare setting, a planned series of readings taken at home, or a portable monitor that records blood pressure over 24 hours. [1–4]
Why can measuring blood pressure at home help?
Repeated home measurements taken with good technique can provide a useful picture of your usual blood pressure at rest.
They can also reduce the influence of factors such as feeling rushed or unfamiliar with the setting during a healthcare appointment.
The aim is not to uncover one fixed or “true” blood pressure number. Blood pressure varies naturally throughout the day. Instead, good technique and consistent conditions reduce avoidable influences so that readings taken at different times can be compared more meaningfully.
For some people, blood pressure is noticeably higher during a healthcare appointment than it is at home. This is commonly known as the white-coat effect.
The opposite can also occur: blood pressure may appear within the expected range during an appointment but be consistently higher at home or on a 24-hour monitor. When this pattern is confirmed, it is known as masked hypertension. [1,2]
Home monitoring can help reveal these patterns, but taking more readings is not automatically better. What matters is using a suitable monitor, following a consistent technique and keeping a useful record.
If you want to start measuring at home, see our step-by-step guide, How to Measure Blood Pressure at Home.
Already have a series of home readings?
Our Understanding Your Home Blood Pressure Readings guide explains how to look at the overall pattern, what a home blood pressure average means, and why a diagnostic threshold is not necessarily your personal treatment target.
Why should I know my numbers?
You cannot reliably tell what your blood pressure is from how you feel.
Someone with high blood pressure may have no obvious symptoms, while feeling stressed, tired or unwell does not necessarily mean that your blood pressure is high.
Measuring gives you information that symptoms cannot.
If you have never checked your blood pressure, knowing your numbers provides a useful starting point.
If you have previously had a higher-than-expected reading, repeat measurements can help show whether it was an isolated result or part of a more consistent pattern.
For someone already diagnosed with hypertension, home readings may form part of ongoing monitoring alongside regular healthcare review. [1–4]
You do not need to own a home monitor to have your blood pressure checked. Depending on where you live, checks may be available through a GP or family doctor, community pharmacy, routine health check, workplace health service or another local healthcare service.
What happens if high blood pressure is confirmed?
High blood pressure can often be managed.
Depending on the individual, management may include changes to physical activity, diet and alcohol intake, weight management, stopping smoking, and blood-pressure-lowering medicines where appropriate. [1–3]
The approach is not the same for everybody. A healthcare professional can consider your blood pressure readings alongside your general health and other relevant factors before discussing what may be appropriate for you.
Finding high blood pressure is therefore not simply identifying a problem. It creates an opportunity to take action and reduce the likelihood of heart and circulation problems developing later.
Key Takeaways: High blood pressure and long-term health
High blood pressure often causes no noticeable symptoms, so measuring it is the only reliable way to know whether it is raised.
It usually develops through several influences acting together over time, rather than one single cause.
Blood pressure that remains high over time can affect blood vessels and organs, including the heart, brain, kidneys and eyes.
Over time, high blood pressure increases the chance of serious events such as heart attack and stroke.
One reading is only a snapshot. Repeated, well-taken measurements provide a clearer picture than one isolated result.
Do not wait to complete a planned series if you develop emergency symptoms or obtain a very high reading. Follow the safety advice below.
When to seek medical help
If your blood pressure is repeatedly much lower than usual and you feel dizzy, faint or unwell, contact a healthcare professional for advice.
Emergency symptoms
Seek emergency medical help immediately if you think you may be having a heart attack or stroke, have severe difficulty breathing, lose consciousness or have another life-threatening emergency. [1,2,5,6]
Possible warning signs include new chest pain, sudden weakness or numbness on one side of the body, facial weakness, difficulty speaking or sudden severe breathing difficulty.
Do not wait for a particular blood pressure number and do not delay seeking emergency help while repeatedly checking your blood pressure.
A very high reading when you otherwise feel well
A systolic reading of 180 mmHg or higher, or a diastolic reading of 120 mmHg or higher, is very high. [1,2]
If you obtain a reading at or above either of these levels and otherwise feel well, sit quietly for several minutes and repeat the measurement using the correct technique.
If the repeated reading remains this high, seek urgent medical advice.
A single home reading at this level does not by itself diagnose severe hypertension, but it should not be ignored.
Do not take additional blood pressure medication unless a healthcare professional has specifically advised you to do so.
What next?
Understanding high blood pressure is the first step. The next step depends on what you need.
Already have a blood pressure monitor?
Learn how to prepare properly, position yourself correctly and collect a useful series of home measurements.
Need a home blood pressure monitor?
Our Blood Pressure Monitors Guide explains what clinical validation means, how to check cuff size and which additional features may be useful.
Important Information:
This page is provided for general educational purposes only and does not constitute medical advice. Medical knowledge and guidance evolve over time, and information may change. Always seek advice from a qualified healthcare professional regarding personal health concerns or medical decisions.
For more details, please see our full Disclaimer.
References
National Institute for Health and Care Excellence. Hypertension in adults: diagnosis and management (NG136). Last updated 26 February 2026.
McEvoy, J.W., McCarthy, C.P., Bruno, R.M., et al. 2024 ESC Guidelines for the management of elevated blood pressure and hypertension. European Heart Journal. 2024;45(38):3912–4018.
Mancia, G., Kreutz, R., Brunström, M., et al. 2023 ESH Guidelines for the management of arterial hypertension. Journal of Hypertension. 2023;41(12):1874–2071.
Parati, G., Stergiou, G.S., Bilo, G., et al. Home blood pressure monitoring: methodology, clinical relevance and practical application. Journal of Hypertension. 2021;39(9):1742–1767.
NHS. Heart attack. NHS health information.
NHS. Symptoms of a stroke. NHS health information.
Content last reviewed: September 2026
Next scheduled review: March 2027
We also review content sooner when important evidence, clinical guidance or safety information changes.