Blood pressure

Blood pressure (BP) is the force of circulating blood pushing against the walls of blood vessels. This pressure mostly comes from the heart pumping blood through the circulatory system. When we talk about blood pressure, we usually mean the pressure in a brachial artery, where it is most often measured. We describe this pressure with two numbers: the systolic pressure (the highest pressure during a heartbeat) over the diastolic pressure (the lowest pressure between beats). We measure these numbers in millimetres of mercury (mmHg) or sometimes in kilopascals (kPa).

Blood pressure is one of the vital signs that healthcare workers check to see how healthy someone is. The other signs include respiratory rate, heart rate, oxygen saturation, and body temperature. A normal resting blood pressure for an adult is about 120 millimetres of mercury systolic over 80 millimetres of mercury diastolic, written as "120/80 mmHg". People around the world have had similar blood pressure levels since 1975.

Traditionally, healthcare workers measured blood pressure by listening with a stethoscope as they squeezed an arm with a sphygmomanometer. While this method is still trusted, newer machines that work on their own have become common. This change happened because of worries about mercury, cost, and making it easier to measure blood pressure at home or while moving around.

Blood pressure changes based on many things, like how hard the heart is working, the tightness of blood vessels, how much blood there is, and how stiff the arteries are. It can also go up or down depending on what a person is doing, how they are feeling, and their overall health. If blood pressure is too low for a long time, it is called hypotension. If it is too high, it is called hypertension. Both can lead to health problems over time.

Classification, normal and abnormal values

Blood pressure measurements can change depending on where and how they are taken. Doctors use different levels for blood pressure measured at a office, at home, or over a whole day using a special machine.

A digital sphygmomanometer used for measuring blood pressure

Having higher blood pressure over time can increase the chance of heart problems. Keeping blood pressure lower helps heart health. Blood pressure changes throughout the day, usually being highest in the morning and evening, and lowest at night. Changes in blood pressure can also happen with age, temperature, noise, stress, food, activity, standing up, certain medicines, and illness.

Different factors such as age influence blood pressure. Usually, blood pressure is not considered too low unless a person feels unwell. In children, normal blood pressure ranges are lower and depend on their height. As adults get older, blood pressure often goes up, especially the pressure when the heart beats, which can be a sign of unhealthy arteries.

Blood pressure classifications
Categories	Systolic blood pressure, mmHg			and/or	Diastolic blood pressure, mmHg
Method	Office	Home	24h ambulatory	and/or	Office	Home	24h ambulatory
American College of Cardiology/American Heart Association (2017)
Normal				and
Elevated	120–129	120–129	115–124	and
Hypertension, stage 1	130–139	130–134	125–129	or	80–89	80–84	75–79
Hypertension, stage 2	≥140	≥135	≥130	or	≥90	≥85	≥80
European Society of Cardiology (2024)
Non-elevated				and
Elevated	120–139	120–134	115–129	and	70–89	70–84	65–79
Hypertension	≥140	≥135	≥130	or	≥90	≥85	≥80
European Society of Hypertension/International Society of Hypertension (2023)
Optimal		—N/a	—N/a	and		—N/a	—N/a
Normal	120–129	—N/a	—N/a	and/or	80–84	—N/a	—N/a
High normal	130–139	—N/a	—N/a	and/or	85–89	—N/a	—N/a
Hypertension, grade 1	140–159	≥135	≥130	and/or	90–99	≥85	≥80
Hypertension, grade 2	160–179	—N/a	—N/a	and/or	100–109	—N/a	—N/a
Hypertension, grade 3	≥180	—N/a	—N/a	and/or	≥110	—N/a	—N/a

Reference ranges for blood pressure (BP) in children
Stage	Approximate age	Systolic BP, mmHg	Diastolic BP, mmHg
Infants	0–12 months	75–100	50–70
Toddlers and preschoolers	1–5 years	80–110	50–80
School age	6–12 years	85–120	50–80
Adolescents	13–18 years	95–140	60–90

Systemic venous pressure

Blood pressure usually means the pressure in the arteries of the systemic circulation. But checking pressure in the veins and pulmonary vessels is also important in special medical care, though it needs special tools to measure.

Venous pressure is the pressure in the veins or the atria of the heart. It is much lower than arterial pressure, with typical values around 5 mmHg in the right atrium and 8 mmHg in the left atrium. Types of venous pressure include central venous pressure, jugular venous pressure, and portal venous pressure in the portal vein.

Pulmonary pressure

Main article: Pulmonary artery pressure

The pressure in the pulmonary artery is usually about 15 mmHg when a person is resting. If this pressure gets too high in the tiny blood vessels of the lungs, called capillaries, it can cause problems. When the pressure rises above 20 mmHg, it may cause swelling. If it goes above 25 mmHg, fluid can build up in the lungs.

Aortic pressure

Main article: Aortic pressure

Aortic pressure is the pressure of blood at the root of the aorta, the main blood vessel leaving the heart. It helps doctors learn about heart health better than other blood pressure checks, like those done at the arm using the brachial artery. Measuring it used to need special tools, but now doctors can check it without them. Some experts think doctors should start using aortic pressure to help make important health choices.

Mean systemic pressure

Main article: Mean systemic pressure

When the heart stops beating, the pressure of blood in your body does not disappear. There is still some pressure left, called mean systemic pressure. This pressure is usually around 7 mmHg. It stays even when the heart is not working, helping to keep blood moving through the body.

Disorders of blood pressure

Disorders of blood pressure control include high blood pressure, low blood pressure, and blood pressure that changes more than it should.

High blood pressure

Main article: Hypertension

Arterial hypertension can show that there are other health problems and may cause harm over time. Sometimes, it can become a sudden issue, such as in a hypertensive emergency when blood pressure gets very high.

When blood pressure is high, it puts extra pressure on the walls of the arteries. This extra pressure makes the heart work harder and can cause unhealthy growth inside the arteries. The higher the pressure, the more stress there is and the more this unhealthy growth can progress. The heart muscle can also become thicker and weaker over time.

Continuously high blood pressure can increase the risk of strokes, heart attacks, heart failure, and arterial aneurysms. It can also lead to chronic kidney failure. Even a small increase in blood pressure can shorten life expectancy. At very high pressures, a person may only live a few more years without treatment. For people with high blood pressure, changes in heart rate variability can increase the risk of atrial fibrillation.

Both high systolic pressure and high pulse pressure are risk factors for heart problems.

Blood pressure that is too low is known as hypotension. This can be a concern if it causes dizziness, fainting, or, in severe cases, circulatory shock. Causes of low blood pressure include sepsis, hypovolemia, bleeding, cardiogenic shock, reflex syncope, hormonal issues, and eating disorders.

Orthostatic hypotension

Main article: Orthostatic hypotension

A big drop in blood pressure when standing up is called orthostatic hypotension and happens when the body cannot adjust properly to the effects of gravity. Standing up moves blood to the lower parts of the body, which can reduce the blood flow to the brain. Normally, the body has ways to balance this. If these balance mechanisms fail, it can cause lightheadedness, dizziness, weakness, or fainting. This failure is often due to illness or medicines.

Variable or fluctuating blood pressure

Main article: Labile hypertension

Some change in blood pressure is normal. But when blood pressure changes a lot more than usual, it is known as labile hypertension and is linked to a higher risk of heart disease and problems in the brain’s small blood vessels. Older people and those who take blood pressure medicine are more likely to have bigger changes in their blood pressure.

Physiology

Cardiac systole and diastole

Every time your heart beats, your blood pressure changes between a high point (called systolic) and a low point (called diastolic). This pressure is mostly created by your heart pumping blood. The pressure helps blood move through your body, and it changes depending on where the blood is.

Hemodynamics

Blood flow velocity waveforms in the central retinal artery (red) and vein (blue), measured by laser Doppler imaging in the eye fundus of a healthy volunteer.

Main article: Hemodynamics

Blood pressure depends on a few main things: how much blood the heart pumps out (cardiac output), how much the blood vessels resist the flow (systemic vascular resistance), and how stretchy the arteries are (arterial stiffness). Small blood vessels, especially tiny ones called arterioles, help control resistance. Things like the width of the vessels and how thick the blood is can change resistance.

Pulse pressure

Main article: Pulse pressure

Pulse pressure is the difference between the high and low blood pressure numbers. It changes with each heartbeat and depends on how much blood the heart pumps and how stretchy the big arteries are.

Mean arterial pressure

Main article: Mean arterial pressure

Mean arterial pressure (MAP) is the average blood pressure over one heartbeat. It depends on how much blood the heart pumps and how much the blood vessels resist the flow.

Regulation of blood pressure

Measurement

Taking blood pressure with a sphygmomanometer

Blood pressure is often measured with a tool called a sphygmomanometer. This tool can show the pressure using a column of mercury or a special gauge. There are also automatic ways to measure blood pressure, like using a smartphone. In hospitals, blood pressure can sometimes be measured by putting a small tube inside an artery, but this is not common. Researchers are looking for new ways to measure blood pressure without using devices on the body.

In doctors' offices, blood pressure readings often end in the number zero, which might not always be correct.

In animals

Blood pressure in animals can change depending on the type of animal. Larger animals, like giraffes, have slower heart rates and need higher blood pressure to push blood up to their heads. This helps them get enough blood flow.

Like in humans, blood pressure in animals can also change with age, sex, and time of day. Scientists study blood pressure in animals such as rats, mice, dogs, and rabbits to learn more about it.

Hypertension in cats and dogs

In cats and dogs, high blood pressure is usually found if the pressure is above a certain level. Some dog breeds, especially those used as seeing dogs, can have even higher blood pressure than others.

Blood pressure and heart rate of various mammals
Species	Blood pressure mmHg		Heart rate beats per minute
Species	Systolic	Diastolic	Heart rate beats per minute
Calves	140	70	75–146
Cats	155	68	100–259
Dogs	161	51	62–170
Goats	140	90	80–120
Guinea-pigs	140	90	240–300
Mice	120	75	580–680
Pigs	169	55	74–116
Rabbits	118	67	205–306
Rats	153	51	305–500
Rhesus monkeys	160	125	180–210
Sheep	140	80	63–210