Blood pressure explained

Blood pressure

Blood pressure (BP) is the pressure of circulating blood on the walls of blood vessels. Most of this pressure is due to work done by the heart by pumping blood through the circulatory system. Used without further specification, "blood pressure" usually refers to the pressure in large arteries of the systemic circulation. Blood pressure is usually expressed in terms of the systolic pressure (maximum during one heartbeat) over diastolic pressure (minimum in between two heartbeats) and is measured in millimeters of mercury (mmHg), above the surrounding atmospheric pressure.

Blood pressure is one of the vital signs, along with respiratory rate, heart rate, oxygen saturation, and body temperature. Normal resting blood pressure in an adult is approximately systolic, and diastolic, abbreviated "120/80 mmHg". Globally, the average blood pressure, age standardized, has remained about the same since 1975 to the present, at approx. 127/79 mmHg in men and 122/77 mmHg in women.[1]

Traditionally, blood pressure was measured non-invasively using ausculation with a mercury-tube sphygmomanometer. Ausculation is still generally considered to be the gold standard of accuracy for non-invasive blood pressure readings in clinic.[2] However, semi-automated methods have become common, largely due to concerns about potential mercury toxicity,[3] although cost, ease of use and applicability to ambulatory blood pressure or home blood pressure measurements have also influenced this trend.[4] Early automated alternatives to mercury-tube sphygmomanometers were often seriously inaccurate, but modern devices validated to international standards achieve an average difference between two standardized reading methods of 5 mm Hg or less and a standard deviation of less than 8 mm Hg. Most of these semi-automated methods measure blood pressure using oscillometry.[5]

Blood pressure is influenced by cardiac output, total peripheral resistance and arterial stiffness and varies depending on situation, emotional state, activity, and relative health/disease states. In the short term, blood pressure is regulated by baroreceptors which act via the brain to influence the nervous and the endocrine systems.

Blood pressure that is too low is called hypotension, and pressure that is consistently high is hypertension. Both have many causes and may be of sudden onset or of long duration. Long-term hypertension is a risk factor for many diseases, including heart disease, stroke and kidney failure. Long-term hypertension is more common than long-term hypotension, which is usually only diagnosed when it causes symptoms.

Classification, normal and abnormal values

Systemic arterial pressure

The Task Force for the management of arterial hypertension of the European Society of Cardiology (ESC) and the European Society of Hypertension (ESH) classification of office blood pressure (BP)a and definitions of hypertension gradeb. The same classification is used for all ages from 16 years.a BP category is defined according to seated clinic BP and by the highest level of BP, whether systolic or diastolic.

b Isolated systolic hypertension is graded 1, 2, or 3 according to systolic BP values in the ranges indicated.

Categorysystolic BP, mmHgdiastolic BP, mmHg
< 120
< 80
High normal
Grade 1 hypertension
Grade 2 hypertension
Grade 3 hypertension
≥ 180
≥ 110
Isolated systolic hypertensionb
≥ 140
< 90
The risk of cardiovascular disease increases progressively above 115/75 mmHg,[6] below this level there is limited evidence.[7]

Observational studies demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long-term cardiovascular health. There is an ongoing medical debate over what is the optimal level of blood pressure to target when using drugs to lower blood pressure with hypertension, particularly in older people.[8]

The table shows the most recent classification (2018) of office (or clinic) blood pressure by The Task Force for the management of arterial hypertension of the European Society of Cardiology (ESC) and the European Society of Hypertension (ESH).[9] Similar thresholds had been adopted by the American Heart Association for adults who are 18 years and older,[10] but in November 2017 the American Heart Association announced revised definitions for blood pressure categories that increased the number of people considered to have high blood pressure.[11]

Blood pressure fluctuates from minute to minute and normally shows a circadian rhythm over a 24-hour period,[12] with highest readings in the early morning and evenings and lowest readings at night.[13] [14] Loss of the normal fall in blood pressure at night is associated with a greater future risk of cardiovascular disease and there is evidence that night-time blood pressure is a stronger predictor of cardiovascular events than day-time blood pressure.[15] Blood pressure varies over longer time periods (months to years) and this variability predicts adverse outcomes.[16] Blood pressure also changes in response to temperature, noise, emotional stress, consumption of food or liquid, dietary factors, physical activity, changes in posture, such as standing-up, drugs, and disease.[17] The variability in blood pressure and the better predictive value of ambulatory blood pressure measurements has led to some authorities, such as the National Institute for Health and Care Excellence (NICE) in the UK, to advocate for the use of ambulatory blood pressure as the preferred method for diagnosis of hypertension.[18] Various other factors, such as age and sex, also influence a person's blood pressure. Differences between left and right arm blood pressure measurements tend to be small. However, occasionally there is a consistent difference greater than 10 mmHg which may need further investigation, e.g. for peripheral arterial disease or obstructive arterial disease.[19] [20] [21]

There is no accepted diagnostic standard for hypotension, although pressures less than 90/60 are commonly regarded as hypotensive.[22] In practice blood pressure is considered too low only if symptoms are present.[23]

Systemic arterial pressure and age

Fetal blood pressure


The average blood pressure for full-term infants:[25]


Reference ranges for blood pressure (BP) in children[26]
StageApproximate ageSystolic BP, mmHgDiastolic BP, mmHg
Infants0 to 12 months 75–100 50–70
Toddlers and preschoolers1 to 5 years 80–110 50–80
School age6 to 12 years 85–120 50–80
Adolescents13 to 18 years 95–140 60–90
In children, the normal ranges for blood pressure are lower than for adults and depend on height.[27] Reference blood pressure values have been developed for children in different countries, based on the distribution of blood pressure in children of these countries.[28]

Aging adults

In adults in most societies, systolic blood pressure tends to rise from early adulthood onward, up to at least age 70;[29] [30] diastolic pressure tends to begin to rise at the same time but to start to fall earlier in mid-life, approximately age 55. Mean blood pressure rises from early adulthood, plateauing in mid-life, while pulse pressure rises quite markedly after the age of 40. Consequently, in many older people, systolic blood pressure often exceeds the normal adult range, if the diastolic pressure is in the normal range this is termed isolated systolic hypertension. The rise in pulse pressure with age is attributed to increased stiffness of the arteries.[31] An age-related rise in blood pressure is not considered healthy and is not observed in some isolated unacculturated communities.[32]

Systemic venous pressure


Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much lower than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium.

Variants of venous pressure include:

Pulmonary pressure

Normally, the pressure in the pulmonary artery is about 15 mmHg at rest.[36]

Increased blood pressure in the capillaries of the lung causes pulmonary hypertension, leading to interstitial edema if the pressure increases to above 20 mmHg, and to pulmonary edema at pressures above 25 mmHg.[37]

Mean systemic pressure

See main article: Mean systemic pressure. If the heart is stopped, blood pressure falls, but it does not fall to zero. The remaining pressure measured after cessation of the heart beat and redistribution of blood throughout the circulation is termed the mean systemic pressure or mean circulatory filling pressure;[38] typically this is of the order of ~7mm Hg.[38]

Disorders of blood pressure

Disorders of blood pressure control include high blood pressure, low blood pressure, and blood pressure that shows excessive or maladaptive fluctuation.

High blood pressure






Low blood pressure

See main article: Hypotension. Blood pressure that is too low is known as hypotension. This is a medical concern if it causes signs or symptoms, such as dizziness, fainting, or in extreme cases, circulatory shock.[43]

Causes of low arterial pressure include:[44]

Orthostatic hypotension

See main article: Orthostatic hypotension. A large fall in blood pressure upon standing (persistent systolic/diastolic blood pressure decrease of >20/10 mm Hg) is termed orthostatic hypotension (postural hypotension) and represents a failure of the body to compensate for the effect of gravity on the circulation. Standing results in an increased hydrostatic pressure in the blood vessels of the lower limbs. The consequent distension of the veins below the diaphragm (venous pooling) causes ~500 ml of blood to be relocated from the chest and upper body. This results in a rapid decrease in central blood volume and a reduction of ventricular preload which in turn reduces stroke volume, and mean arterial pressure. Normally this is compensated for by multiple mechanisms, including activation of the autonomic nervous system which increases heart rate, myocardial contractility and systemic arterial vasoconstriction to preserve blood pressure and elicits venous vasoconstriction to decrease venous compliance. Decreased venous compliance also results from an intrinsic myogenic increase in venous smooth muscle tone in response to the elevated pressure in the veins of the lower body. Other compensatory mechanisms include the veno-arteriolar axon reflex, the 'skeletal muscle pump' and 'respiratory pump'. Together these mechanisms normally stabilize blood pressure within a minute or less.[45] If these compensatory mechanisms fail and arterial pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically compromised (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness or fainting.[46] Usually this failure of compensation is due to diseases or drugs that affect the sympathetic nervous system. A similar effect is observed following the experience of excessive gravitational forces (G-loading), such as routinely experienced by aerobatic or combat pilots 'pulling Gs' where the extreme hydrostatic pressures exceed the ability of the body's compensatory mechanisms.

Fluctuating blood pressure

Normal fluctuation in blood pressure is adaptive and necessary. Fluctuations in pressure that are significantly greater than the norm are associated with greater white matter hyperintensity, a finding consistent with reduced local cerebral blood flow[47] and a heightened risk of cerebrovascular disease.[48] Within both high and low blood pressure groups, a greater degree of fluctuation was found to correlate with an increase in cerebrovascular disease compared to those with less variability, suggesting the consideration of the clinical management of blood pressure fluctuations, even among normotensive older adults.[48] Older individuals and those who had received blood pressure medications were more likely to exhibit larger fluctuations in pressure.[48]


During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure.[49] The blood pressure in the circulation is principally due to the pumping action of the heart.[50] Differences in mean blood pressure drive the flow of blood around the circulation. The rate of mean blood flow depends on both blood pressure and the resistance to flow presented by the blood vessels. In the absence of hydrostatic effects (e.g. standing), mean blood pressure decreases as the circulating blood moves away from the heart through arteries and capillaries due to viscous losses of energy. Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles.[51] Pulsatility also diminishes in the smaller elements of the arterial circulation, although some transmitted pulsatility is observed in capillaries.[52] Gravity affects blood pressure via hydrostatic forces (e.g., during standing), and valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure, particularly in veins.[50]


See main article: Hemodynamics. A simple view of the hemodynamics of systemic arterial pressure is based around mean arterial pressure (MAP) and pulse pressure. Most influences on blood pressure can be understood in terms of their effect on cardiac output[53] and systemic vascular resistance. Cardiac output is the product of stroke volume and heart rate, and stroke volume is influenced by blood volume. In the short-term, the greater the blood volume, the higher the cardiac output. This may explain in part the relationship between dietary salt intake and increased blood pressure, where increased salt intake may increase blood volume potentially resulting in higher arterial pressure. However, this varies with the individual and is highly dependent on autonomic nervous system response and the renin–angiotensin system.[54] [55] [56] In the longer-term the relationship between volume and blood pressure is more complex.[57] In simple terms systemic vascular resistance is mainly determined by the caliber of small arteries and arterioles. The resistance attributable to a blood vessel depends on its radius as described by the Hagen-Poiseuille's equation (resistance∝1/radius4). Hence, the smaller the radius, the very much higher the resistance. Other physical factors that affect resistance include: vessel length (the longer the vessel, the higher the resistance), blood viscosity (the higher the viscosity, the higher the resistance)[58] and the number of vessels, particularly the smaller numerous, arterioles and capillaries. The presence of an arterial stenosis increases resistance to flow, however this increase in resistance rarely increases systemic blood pressure because its contribution to total systemic resistance is small, although it may profoundly decrease downstream flow.[59] Substances called vasoconstrictors reduce the caliber of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the caliber of blood vessels, thereby decreasing arterial pressure. In the longer term a process termed remodeling also contributes to changing the caliber of small blood vessels and influencing resistance and reactivity to vasoactive agents.[60] [61] Reductions in capillary density, termed capillary rarefaction, may also contribute to increased resistance in some circumstances.[62]

In practice, each individual's autonomic nervous system and other systems regulating blood pressure, notably the kidney,[63] respond to and regulate all these factors so that, although the above issues are important, they rarely act in isolation and the actual arterial pressure response of a given individual can vary widely in the short and long term.

Mean arterial pressure

MAP is the average of blood pressure over a cardiac cycle and is determined by the cardiac output (CO), systemic vascular resistance (SVR), and central venous pressure (CVP)):[64] [65] [66]


In practice, the contribution of CVP (which is small) is generally ignored and so


MAP can be estimated from measurements of the systolic pressure


  and the diastolic pressure






Pulse pressure

See main article: Pulse pressure. The pulse pressure is the difference between the measured systolic and diastolic pressures,


The pulse pressure is a consequence of the pulsatile nature of the cardiac output, i.e. the heartbeat. The magnitude of the pulse pressure is usually attributed to the interaction of the stroke volume of the heart, the compliance (ability to expand) of the arterial system—largely attributable to the aorta and large elastic arteries—and the resistance to flow in the arterial tree.[67]

Regulation of blood pressure

See main article: Homeostasis. The endogenous regulation of arterial pressure is not completely understood, but the following mechanisms of regulating arterial pressure have been well-characterized:


  1. The blood pressure fall is detected by a decrease in blood flow and thus a decrease in glomerular filtration rate (GFR).
  7. Angiotensin II is a vasoconstrictor which will increase blood flow to the heart and subsequently the preload, ultimately increasing the cardiac output.

Currently, the RAS is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists, also known as angiotensin receptor blockers (ARBs). The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; the antihypertensive effect of diuretics is due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and fainting.


See main article: Blood pressure measurement. Arterial pressure is most commonly measured via a sphygmomanometer, which uses the height of a column of mercury, or an aneroid gauge, to reflect the blood pressure by auscultation.[69] The most common automated blood pressure measurement technique is based on the oscillometric method.[70] Fully automated oscillometric measurement has been available since 1981.[71] This principle has recently been used to measure blood pressure with a smartphone.[72] Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting.Novel methods to measure blood pressure without penetrating the arterial wall, and without applying any pressure on patient's body are currently being explored. So-called cuffless measurements, these methods open the door to more comfortable and acceptable blood pressure monitors. See by instance, a cuffless blood pressure monitor at the wrist that uses only optical sensors [73]

Blood pressure in other animals

Blood pressure in non-human mammals is similar to human blood pressure. In contrast, heart rate differs markedly, largely depending on the size of the animal (larger animals have slower heart rates).[74] As in humans, blood pressure in animals differs by age, sex, time of day and circumstances:[75] [76] measurements made in laboratories or anesthesia may not be representative of values under free-living conditions. Rats, mice, dogs and rabbits have been used extensively to study the causes of high blood pressure.[77]

Blood pressure and heart rate of various mammals (modified from)!Species!Systolic blood pressure, mm Hg !Diastolic blood pressure, mm Hg!Heart rate, beats per minute
Rhesus monkeys160125180–210

Hypertension in cats and dogs

Hypertension in cats and dogs is diagnosed if the blood pressure is greater than 150 mm Hg (systolic) and/or 95 mm Hg (diastolic).

Further reading

External links

Notes and References

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