Blood pressure

Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure comes from the heart pumping blood through the circulatory system. When we talk about blood pressure without any other details, we usually mean the pressure in a brachial artery, which is where it is most often measured. This pressure is described using 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". On average, 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 serious health problems over time, like stroke, heart disease, and kidney failure.

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.

Having higher blood pressure over time can increase the chance of heart problems. Keeping blood pressure lower is linked to better heart health over many years. 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.

Systemic venous pressure

Blood pressure usually means the pressure in the arteries of the systemic circulation. But looking at 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 around 15 mmHg when a person is resting. If the pressure in the lung's tiny blood vessels, called capillaries, gets too high, it can cause problems. When the pressure goes above 20 mmHg, it may lead to swelling between the lung layers. If it rises above 25 mmHg, it can cause fluid to 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 can help doctors understand heart health better than other blood pressure measurements, like those taken at the arm using the brachial artery. While measuring it used to need special medical tools, new ways allow doctors to check it without those tools. Some experts think doctors should start using aortic pressure instead of other measurements to help make important health decisions.

Mean systemic pressure

Main article: Mean systemic pressure

When the heart stops beating, the pressure of blood in your body does not drop to zero. 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, keeping 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, larger, and weaker over time.

Continuously high blood pressure is a risk factor for strokes, heart attacks, heart failure, and arterial aneurysms, and it is a leading cause of chronic kidney failure. Even a moderate 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. Elevated pulse pressure has been found to better predict heart issues, especially in older people, compared to other pressure measures.

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 (usually a drop of more than 20/10 mmHg) 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, such as increasing heart rate and narrowing blood vessels. If these balance mechanisms fail, it can cause lightheadedness, dizziness, weakness, or fainting. This failure is often due to illness or medicines that affect certain body systems. Similar effects can happen when experiencing strong forces, like during certain flying activities.

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, problems in the brain’s small blood vessels, and memory loss, regardless of the average blood pressure level. Recent studies have also connected big changes in blood pressure to death, strokes, heart failure, and heart changes that can lead to heart failure. This has led to discussions about whether these big changes in blood pressure should be treated, even in older adults who do not usually have high blood pressure.

Older people and those who take blood pressure medicine are more likely to have bigger changes in their blood pressure. There is some evidence that different blood pressure medicines affect these changes differently, but it is not clear if these differences lead to better health outcomes.

Physiology

Every time your heart beats, the blood pressure changes between a high point (called systolic) and a low point (called diastolic). This pressure is mostly created by the heart pumping blood through your body. But other things, like signals from the brain and the kidneys, also help control blood pressure. The pressure helps blood flow through your body, and it changes depending on where the blood is and how much it has to push against.

Hemodynamics

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). The heart’s output depends on how much blood fills it, how strong the heartbeat is, and how much the blood has to push against. Small blood vessels, especially tiny ones called arterioles, play a big role in resistance. Things like the width of the vessels, their length, and how thick the blood is can also change resistance. Some substances can make vessels narrower, raising blood pressure, while others can make them wider, lowering it.

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. A normal pulse pressure is around 40 mmHg. If it’s too high or too low, it can be a sign of health problems.

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. Doctors often estimate MAP using the high and low blood pressure numbers.

Regulation of blood pressure

See also: Renin–angiotensin system

The body has several ways to keep blood pressure steady. Special sensors in the arteries can tell the brain when blood pressure changes. The brain then helps adjust the heart’s strength and the width of the blood vessels. The kidneys also help by controlling the amount of fluid in the body. When blood pressure drops, the kidneys can make the body hold onto more salt and water, which helps raise blood pressure again.

Measurement

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

In doctors' offices, it is common for blood pressure readings to often end in the number zero, which may not always be accurate.

In animals

Blood pressure in animals can change based on what kind of animal it is. Larger animals, like giraffes, have slower heart rates and need higher blood pressure to send blood all the way up to their heads. This helps them get enough blood flow.

Just like in humans, blood pressure in animals can also change depending on their age, sex, and the time of day. Scientists often study blood pressure in animals like rats, mice, dogs, and rabbits to learn more about how it works.

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 for seeing, can have even higher blood pressure than others.