TheInfoList

Blood pressure (BP) is the
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

of circulating
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

against the walls of
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino ...
s. Most of this pressure results from the heart pumping blood through the
circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertai ...
. When used without qualification, the term "blood pressure" refers to the pressure in the large
arteries An artery (plural arteries) () is a blood vessel that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pulmonary arteries, pulmonary ...

. Blood pressure is usually expressed in terms of the
systolic pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" ref ...
(maximum pressure during one heartbeat) over
diastolic pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , ...
(minimum pressure between two heartbeats) in the
cardiac cycle The cardiac cycle is the performance of the human heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxy ...
. It is measured in millimeters of
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

(
mmHg A millimetre of mercury is a manometric unit of pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its M ...
) above the surrounding
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. ...
. Blood pressure is one of the
vital signs Vital signs (also known as vitals) are a group of the four to six most important that indicate the status of the body’s (life-sustaining) functions. These measurements are taken to help assess the general physical health of a person, give clu ...
—together with
respiratory rate The respiratory rate is the rate at which breathing File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497.s009.ogv, upright=1.4, X-ray video of a female American alligator while breathing. ...
,
heart rate Heart rate is the speed of the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the heart, or a perceived effect of it, such as: **Heart sounds, the noises gene ...

,
oxygen saturation Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is Dissolution (chemistry), dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium. It can ...
, and
body temperature Thermoregulation is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a sy ...
—that
healthcare professional A health professional (or healthcare professional) may provide health care Health care, health-care, or healthcare is the maintenance or improvement of health via the preventive healthcare, prevention, diagnosis, therapy, treatment, recovery, or ...
s use in evaluating a patient's
health Health, according to the World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each ...

. Normal resting blood pressure, in an
adult Biologically Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

is approximately systolic over diastolic, denoted as "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, although these average data mask quite large divergent regional trends. Traditionally, blood pressure was measured
non-invasive A medical procedure is defined as ''non-invasive'' when no break in the skin is created and there is no contact with the mucosa, or skin break, or internal body cavity beyond a natural or artificial body orifice. For example, deep palpation and per ...
ly using
auscultation:''For the ancient monasterial worker, see Auscultare'' Auscultation (based on the Latin verb ''auscultare'' "to listen") is listening to the internal sounds of the body, usually using a stethoscope. Auscultation is performed for the purposes of exa ...

with either an aneroid gauge, or a
sphygmomanometer A sphygmomanometer ( ), also known as a blood pressure monitor, or blood pressure gauge, is a device used to measure blood pressure, composed of an inflatable cuff to collapse and then release the artery under the cuff in a controlled manner, and ...

. Auscultation is still generally considered to be the gold standard of accuracy for non-invasive blood pressure readings in clinic. However, semi-automated methods have become common, largely due to concerns about potential mercury toxicity, although cost, ease of use and applicability to
ambulatory blood pressure Ambulatory blood pressure monitoring (ABPM) measures blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ ...
or home blood pressure measurements have also influenced this trend. 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. Blood pressure is influenced by
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
,
systemic vascular resistance Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow.The resistance offered by the systemic circulation is known as the systemic vascular resistance (SVR) or may sometimes be ca ...
and
arterial stiffness Arterial stiffness occurs as a consequence of biological aging and arteriosclerosis. Inflammation plays a major role in arteriosclerosis development, and consequently it is a major contributor in large arteries stiffening. Increased arterial stiff ...
and varies depending on situation, emotional state, activity, and relative health/disease states. In the short term, blood pressure is
regulated Regulation is the management of complex systems A complex system is a system A system is a group of interacting Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way e ...
by
baroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s which act via the brain to influence the and the
endocrine The endocrine system is a messenger system comprising feedback loops of the hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Re ...

systems. Blood pressure that is too low is called
hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the Systole, systolic blood pressure (the top number) and ...
, pressure that is consistently too high is called
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not ...

, and normal pressure is called normotension. Both hypertension and hypotension have many causes and may be of sudden onset or of long duration. Long-term hypertension is a risk factor for many diseases, including
stroke A stroke is a medical condition A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not due to any immediate external injury. Di ...

,
heart disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina pectoris, angina and myocardial infarction (commonly known as a heart attack). Other CVDs inc ...
, and
kidney failure Kidney failure, also known as end-stage kidney disease, is a medical condition in which the kidneys are functioning at less than 15% of normal levels. Kidney failure is classified as either acute kidney failure, which develops rapidly and may res ...
. Long-term hypertension is more common than long-term hypotension.

# Classification, normal and abnormal values

## Systemic arterial pressure

The risk of cardiovascular disease increases progressively above 115/75 mmHg, below this level there is limited evidence. 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. 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). Similar thresholds had been adopted by the American Heart Association for adults who are 18 years and older, 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. Blood pressure fluctuates from minute to minute and normally shows a circadian rhythm over a 24-hour period, with highest readings in the early morning and evenings and lowest readings at night. 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. Blood pressure varies over longer time periods (months to years) and this variability predicts adverse outcomes. 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 A drug is any chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched ...

, and disease. The variability in blood pressure and the better predictive value of ambulatory blood pressure measurements has led 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. Various other factors, such as age and
sex Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual r ...
, 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 A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatica ...
or obstructive arterial disease. There is no accepted diagnostic standard for hypotension, although pressures less than 90/60 are commonly regarded as hypotensive. In practice blood pressure is considered too low only if
symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showi ...
are present.

## Systemic arterial pressure and age

### Fetal blood pressure

In
pregnancy Pregnancy, also known as gestation, is the time during which one or more offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual repr ...

, it is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive blood through the fetal circulation. The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to approximately 45 mmHg at 40 weeks of gestation. The average blood pressure for full-term infants: * Systolic 65–95 mmHg * Diastolic 30–60 mmHg

### Childhood

In children, the normal ranges for blood pressure are lower than for adults and depend on height. Reference blood pressure values have been developed for children in different countries, based on the distribution of blood pressure in children of these countries.

In adults in most societies, systolic blood pressure tends to rise from early adulthood onward, up to at least age 70; 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. An age-related rise in blood pressure is not considered healthy and is not observed in some isolated unacculturated communities.

# Systemic venous pressure

Blood pressure generally refers to the arterial pressure in the
systemic circulation The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and bloo ...
. However, measurement of pressures in the venous system and the pulmonary vessels plays an important role in
intensive care medicine Intensive care medicine, also called critical care medicine, is a medical Specialty (medicine), specialty that deals with seriously or critically ill patients who have, are at risk of, or are recovering from conditions that may be life-threate ...
but requires invasive measurement of pressure using a
catheter In medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge) ...

. Venous pressure is the vascular pressure in a
vein Veins are blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A b ...

or in the
atria of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ...
. It is much lower than arterial pressure, with common values of 5 mmHg in the
right atrium The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the ventricles of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood ...
and 8 mmHg in the left atrium. Variants of venous pressure include: *
Central venous pressureCentral venous pressure (CVP) is the blood pressure in the venae cavae, near the right atrium of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulato ...
, which is a good approximation of right atrial pressure, which is a major determinant of right ventricular end diastolic volume. (However, there can be exceptions in some cases.) * The
jugular venous pressure and marked jugular venous distention. External jugular vein marked by an arrow; however, JVP is not measured by looking at the external jugular vein even but is instead measured by pulsations of the skin from the internal jugular vein, which is not ...
(JVP) is the indirectly observed pressure over the venous system. It can be useful in the differentiation of different forms of
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...
and
lung disease The lungs are the primary Organ (anatomy), organs of the respiratory system in humans and many other animals including a few fish and some snails. In mammals and most other vertebrates, two lungs are located near the vertebral column, backbone ...
. * The portal venous pressure is the blood pressure in the
portal vein The portal vein or hepatic portal vein (HPV) is a blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that ...
. It is normally 5–10 mmHg

# Pulmonary pressure

Normally, the pressure in the
pulmonary artery A pulmonary artery is an artery An artery (plural arteries) () is a blood vessel that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are t ...

is about 15 mmHg at rest. Increased blood pressure in the capillaries of the lung causes
pulmonary hypertension Pulmonary hypertension (PH or PHTN) is a condition of increased blood pressure within the arteries of the lungs. Symptoms include shortness of breath, syncope, tiredness, chest pain, pedal edema, swelling of the legs, and a fast heartbeat. The ...
edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid in the body's tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a ...

if the pressure increases to above 20 mmHg, and to
pulmonary edema Pulmonary edema is fluid accumulation in the tissue and air spaces of the lungs. It leads to impaired gas exchange and may cause respiratory failure. It is due to either failure of the left ventricle of the heart to remove blood adequately ...

at pressures above 25 mmHg.

# 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; typically this is of the order of ~7mm Hg.

# Disorders of blood pressure

Disorders of blood pressure control include
high blood pressure Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term Long-Term Capital Management L.P. (LTCM) was a hedge fund''A financial History of the United States Volume II: 1970–2001'', Jerry W. Markham, Chapter 5: "Bank ...

,
low blood pressure Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dia ...
, and blood pressure that shows excessive or maladaptive fluctuation.

## High blood pressure

Arterial hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a Chronic (medicine), long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure typically does n ...
can be an indicator of other problems and may have long-term adverse effects. Sometimes it can be an acute problem, for example
hypertensive emergency A hypertensive emergency is high blood pressure Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High ...
. Levels of arterial pressure put mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (
atheroma An atheroma, or atheromatous plaque ("plaque"), is an abnormal accumulation of material in the inner layer of the wall of an artery An artery (plural arteries) () is a blood vessel that takes blood away from the heart to one or more parts of ...

) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the
heart muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...

tends to thicken, enlarge and become weaker over time. Persistent
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not ...

is one of the risk factors for
stroke A stroke is a medical condition A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not due to any immediate external injury. Di ...

s,
heart attacks A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

,
heart failure Heart failure (HF), also known as congestive heart failure (CHF) and (congestive) cardiac failure (CCF), is a set of manifestations caused by the failure of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in mo ...
, and arterial aneurysms, and is the leading cause of
chronic kidney failure Chronic kidney disease (CKD) is a type of kidney disease The kidneys are two reddish-brown bean-shaped organs found in vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukary ...
. Even moderate elevation of arterial pressure leads to shortened
life expectancy Life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of its birth, its current age, and other demographic Demography (from prefix ''demo-'' from Ancient Greek Ancien ...

. At severely high pressures, mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated. In the past, most attention was paid to
diastolic Image:Heart diastole.png, upright=1.5, Heart performance during ventricular diastole: early diastole is a suction mechanism that draws blood 'down' from the left atrium (pink) and right atrium (blue) into each of the two ventricles. Then, in late ve ...
pressure; but nowadays it is recognized that both high
systolic to supply all body systems; 2) oxygen-depleted blood (blue arrow) in the right ventricle begins pulsing through the pulmonic (pulmonary) valve en route to the lungs for reoxygenation. depolarization is the start-point of the atrial stage of syst ...
pressure and high
pulse pressure Pulse pressure is the difference between systolic and diastolic blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμ ...
(the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures (see the article on
pulse pressure Pulse pressure is the difference between systolic and diastolic blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμ ...
). If systolic blood pressure is elevated (>140 mmHg) with a normal diastolic blood pressure (<90 mmHg), it is called "isolated systolic hypertension" and may present a health concern. For those with
heart valve A heart valve is a one-way valve check valve Image:Check Valve.svg, 140px, Check valve symbol on piping and instrumentation diagrams. The arrow shows the flow direction. A check valve, non-return valve, reflux valve, retention valve, foot valv ...
regurgitation, a change in its severity may be associated with a change in diastolic pressure. In a study of people with heart valve regurgitation that compared measurements two weeks apart for each person, there was an increased severity of
aortic The aorta ( ) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart and extending down to the abdomen, where it aortic bifurcation, splits into two smaller arteries (the common ilia ...

and
mitral regurgitation Mitral regurgitation (MR), also known as mitral insufficiency, or mitral incompetence is a form of valvular heart disease in which the mitral valve is insufficient and does not close properly when the heart pumps out blood. Section: Valvular Hear ...

when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.

## Low blood pressure

Blood pressure that is too low is known as
hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the Systole, systolic blood pressure (the top number) and ...
. This is a medical concern if it causes signs or symptoms, such as dizziness, fainting, or in extreme cases,
circulatory shock Shock is the state of insufficient blood flow to the tissues of the body as a result of problems with the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, org ...
. Causes of low arterial pressure include: *
Sepsis Sepsis is a life-threatening condition that arises when the body's response to infection An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction ...
*
Hemorrhage Bleeding, also known as a hemorrhage, haemorrhage, or simply blood loss, is blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrient A nutrient is a substance used by an organism to survi ...
– blood loss *
Cardiogenic shock Cardiogenic shock (CS) is a medical emergency resulting from inadequate blood flow due to the dysfunction of the ventricles of the heart.Textbooks of Internal MedicinHarrison's Principles of Internal Medicine 16th Edition, The McGraw-Hill McG ...
* Neurally mediated hypotension (or reflex syncope) *
Toxins A toxin is a harmful substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived from the word toxi ...
including toxic doses of blood pressure medicine *
Hormonal A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for ...

abnormalities, such as *
Eating disorder An eating disorder is a mental disorder defined by abnormal eating behaviors that negatively affect a person's health, physical or mental health, mental health. Only one eating disorder can be diagnosed at a given time. Types of eating disorders ...
s, particularly
anorexia nervosa Anorexia nervosa, often referred to simply as anorexia, is an eating disorder An eating disorder is a mental disorder defined by abnormal eating behaviors that negatively affect a person's health, physical or mental health, mental health. On ...
and
bulimia Bulimia nervosa, also known as simply bulimia, is an eating disorder characterized by binge eating followed by purging. Binge eating refers to eating a large amount of food in a short amount of time. Purging refers to the attempts to get rid of ...

### 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 Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (fr ...
(postural hypotension) and represents a failure of the body to compensate for the effect of
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

on the circulation. Standing results in an increased
hydrostatic Fluid statics or hydrostatics is the branch of fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical object ...
pressure in the blood vessels of the lower limbs. The consequent distension of the veins below the
diaphragm Diaphragm may refer to: * Diaphragm (anatomy) or thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Diaphragm (optics), a stop in the light path of a lens, having an aperture that regulates the amount of light that pass ...
(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 The autonomic nervous system (ANS), formerly the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of viscera, internal organs. The autonomic nervous ...

which increases
heart rate Heart rate is the speed of the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the heart, or a perceived effect of it, such as: **Heart sounds, the noises gene ...

,
myocardial contractilityMyocardial contractility represents the innate ability of the heart muscle (cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all species of animal Animals ...
and systemic arterial
vasoconstriction Vasoconstriction is the narrowing of the blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of th ...

to preserve blood pressure and elicits
venous Veins are blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circu ...

vasoconstriction to decrease venous
compliance Compliance can mean: Healthcare * Compliance (medicine) In medicine, patient compliance (also adherence, capacitance) describes the degree to which a patient correctly follows medical advice. Most commonly, it refers to medication or drug compli ...
. Decreased venous compliance also results from an intrinsic
myogenicThe myogenic mechanism is how artery, arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow constant within the blood vessel. Myogenic response refers to a contraction initiated by the myocyte itself instea ...
increase in venous
smooth muscle Smooth muscle is an involuntary non-striated muscle Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. It is also referred to as myo ...

tone in response to the elevated pressure in the veins of the lower body. Other compensatory mechanisms include the veno-arteriolar
axon reflex The axon reflex (or the flare response) is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making ...
, the ' skeletal muscle pump' and ' respiratory pump'. Together these mechanisms normally stabilize blood pressure within a minute or less. If these compensatory mechanisms fail and arterial pressure and blood
flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on ...
decrease beyond a certain point, the
perfusion Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ (anatomy), organ or a tissue (biology), tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as ...
of the brain becomes critically compromised (i.e., the blood supply is not sufficient), causing
lightheadedness Lightheadedness is a common and typically unpleasant sensation of dizziness or a feeling that one may faint. The sensation of lightheadedness can be short-lived, prolonged, or, rarely, recurring. In addition to dizziness, the individual may feel ...
,
dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling such as giddiness or foolishness. Dizziness is a common medical c ...

, weakness or
fainting Syncope, commonly known as fainting, is a loss of consciousness Consciousness, at its simplest, is or of internal and external existence. Despite millennia of analyses, definitions, explanations and debates by philosophers and scienti ...
. Usually this failure of compensation is due to disease, or drugs that affect the
sympathetic nervous system The sympathetic nervous system (SNS) is one of two divisions of the autonomic nervous system, along with the parasympathetic nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes co ...
. A similar effect is observed following the experience of excessive gravitational forces (G-loading), such as routinely experienced by aerobatic or combat pilots '' where the extreme hydrostatic pressures exceed the ability of the body's compensatory mechanisms.

## Variable or fluctuating blood pressure

Some fluctuation or variation in blood pressure is normal. Variations in pressure that are significantly greater than the norm are associated with increased risk of cardiovascular disease brain small vessel disease, and dementia independent of the average blood pressure level. Recent evidence from
clinical trial Clinical trials are experiments or observations done in . Such prospective biomedical or behavioral research studies on are designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as no ...

s has also linked variation in blood pressure to stroke, heart failure, and cardiac changes that may give rise to heart failure. These data have prompted discussion of whether excessive variation in blood pressure should be treated, even among normotensive older adults. Older individuals and those who had received blood pressure medications are more likely to exhibit larger fluctuations in pressure, and there is some evidence that different antihypertensive agents have different effects on blood pressure variability; whether these differences translate to benefits in outcome is uncertain.

# Physiology

During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. The blood pressure in the circulation is principally due to the pumping action of the heart. However, blood pressure is also regulated by neural regulation from the brain (see Hypertension and the brain), as well as osmotic regulation from the kidney. 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 Fluid statics or hydrostatics is the branch of fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical object ...
effects (e.g. standing), mean blood pressure decreases as the circulating blood moves away from the
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...

through arteries and
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessel ...

due to
viscous The viscosity of a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

losses of energy. Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and
arterioles An arteriole is a small-diameter blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to c ...
. Pulsatility also diminishes in the smaller elements of the arterial circulation, although some transmitted pulsatility is observed in capillaries. Gravity affects blood pressure via hydrostatic forces (e.g., during standing), and valves in veins,
breathing Breathing (or ventilation) is the process of moving air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of th ...

, and pumping from contraction of skeletal muscles also influence blood pressure, particularly in veins.

## Hemodynamics

A simple view of the hemodynamics of systemic arterial pressure is based around
mean arterial pressure In medicine, the mean arterial pressure (MAP) is an average blood pressure in an individual during a single cardiac cycle. Calculation The Systemic Vascular Resistance (SVR), denoted R is represented mathematically by the formula :R = \Delta P/ ...

(MAP) and pulse pressure. Most influences on blood pressure can be understood in terms of their effect on
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
,
systemic vascular resistance Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow.The resistance offered by the systemic circulation is known as the systemic vascular resistance (SVR) or may sometimes be ca ...
, or
arterial stiffness Arterial stiffness occurs as a consequence of biological aging and arteriosclerosis. Inflammation plays a major role in arteriosclerosis development, and consequently it is a major contributor in large arteries stiffening. Increased arterial stiff ...
(the inverse of arterial compliance). Cardiac output is the product of stroke volume and heart rate. Stroke volume is influenced by 1) the
end diastolic volume In cardiovascular physiology, end-diastolic volume (EDV) is the volume of blood in the right and/or left ventricle at end load or filling in (diastole) or the amount of blood in the ventricles just before systole. Because greater EDVs cause greater ...
or filling pressure of the ventricle acting via the Frank Starling mechanism - this is influenced by
blood volume Blood volume is the volume Volume is a scalar quantity expressing the amount Quantity or amount is a property that can exist as a multitude Multitude is a term for a group of people who cannot be classed under any other distinct category, ex ...
; 2) cardiac contractility; and 3)
afterload Afterload is the pressure that the heart must work against to eject blood during systole (ventricular contraction). Afterload is proportional to the average arterial pressure. As aortic and pulmonary pressures increase, the afterload increases on ...
, the impedance to blood flow presented by the circulation. In the short-term, the greater the blood volume, the higher the cardiac output. This has been proposed as an explanation of the relationship between high dietary salt intake and increased blood pressure; however, responses to increased dietary sodium intake vary between individuals and are highly dependent on autonomic nervous system responses and the
renin–angiotensin system The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physic ...
, changes in plasma osmolarity may also be important. In the longer-term the relationship between volume and blood pressure is more complex. 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 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) and the number of vessels, particularly the smaller numerous, arterioles and capillaries. The presence of a severe arterial
stenosis A stenosis (from Ancient Greek στενός, "narrow") is an abnormal narrowing in a blood vessel or other tubular Organ (anatomy), organ or structure such as foramina and canals. It is also sometimes called a stricture (as in urethral stricture) ...
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. Substances called
vasoconstrictor Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels ...
s reduce the caliber of blood vessels, thereby increasing blood pressure.
Vasodilator Vasodilation is the widening of blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. ...
s (such as
nitroglycerin Nitroglycerin (NG), also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive An explosive (or explosive material) is a reactive substance that contains a grea ...

) 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. Reductions in capillary density, termed capillary rarefaction, may also contribute to increased resistance in some circumstances. In practice, each individual's autonomic nervous system and other systems regulating blood pressure, notably the kidney, 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 The cardiac cycle is the performance of the human heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxy ...
and is determined by the
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
(CO),
systemic vascular resistance Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow.The resistance offered by the systemic circulation is known as the systemic vascular resistance (SVR) or may sometimes be ca ...
(SVR), and
central venous pressureCentral venous pressure (CVP) is the blood pressure in the venae cavae, near the right atrium of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulato ...
(CVP)): :::::::::::$\! \text = \left(\text \cdot \text\right) + \text$ In practice, the contribution of CVP (which is small) is generally ignored and so :::::::::::$\! \text = \text \cdot \text$ MAP is often estimated from measurements of the systolic pressure, $\! P_$ and the diastolic pressure, $\! P_$  using the equation: $\! \text \approxeq P_ + k \left(P_ - P_\right)$ where ''k'' = 0.333 although other values for ''k'' have been advocated.

## Pulse pressure

The
pulse pressure Pulse pressure is the difference between systolic and diastolic blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμ ...
is the difference between the measured systolic and diastolic pressures, :::::::::::$\! P_ = P_ - P_.$ The pulse pressure is a consequence of the pulsatile nature of the
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
, i.e. the heartbeat. The magnitude of the pulse pressure is usually attributed to the interaction of the
stroke volume In cardiovascular physiologyCardiovascular physiology is the study of the cardiovascular system, specifically addressing the physiology of the heart ("cardio") and blood vessels ("vascular"). These subjects are sometimes addressed separately, unde ...
of the heart, the compliance (ability to expand) of the arterial system—largely attributable to the
aorta The aorta ( ) is the main and largest artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vas ...

and large elastic arteries—and the
resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''Th ...
to flow in the
arterial tree In anatomy, arterial tree is used to refer to all arteries and/or the branching pattern of the arteries. This article regards the human arterial tree. Starting from the aorta: the following are the parts Ascending aorta It is a portion of the ao ...
.

## Regulation of blood pressure

The
endogenous Endogenous substances and processes are those that originate from within a system such as an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bi ...
,
homeostatic In biology, homeostasis is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning for the organism and includes many variables, such ...
regulation of arterial pressure is not completely understood, but the following mechanisms of regulating arterial pressure have been well-characterized: *
Baroreceptor reflexThe baroreflex or baroreceptor reflex is one of the body's homeostasis, homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure ...
:
BaroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s in the high pressure receptor zones detect changes in arterial pressure. These baroreceptors send signals ultimately to the medulla oblongata, medulla of the brain stem, specifically to the rostral ventrolateral medulla (RVLM). The medulla, by way of the
autonomic nervous system The autonomic nervous system (ANS), formerly the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of viscera, internal organs. The autonomic nervous ...

, adjusts the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the systemic vascular resistance. The most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch.Archived version 2009-10-03
/ref> * Renin–angiotensin system (RAS): This system is generally known for its long-term adjustment of arterial pressure. This system allows the kidney to compensate for loss in
blood volume Blood volume is the volume Volume is a scalar quantity expressing the amount Quantity or amount is a property that can exist as a multitude Multitude is a term for a group of people who cannot be classed under any other distinct category, ex ...
or drops in arterial pressure by activating an endogenous
vasoconstrictor Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels ...
known as angiotensin II. * Aldosterone release: This steroid hormone is released from the adrenal cortex in response to angiotensin II or high serum potassium levels. Aldosterone stimulates sodium retention and potassium excretion by the kidneys. Since sodium is the main ion that determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, arterial pressure. *
BaroreceptorBaroreceptors (or archaically, pressoreceptors) are sensors located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch. They sense the blood pressure and relay the information to the brain, so that a ...
s in low pressure receptor zones (mainly in the venae cavae and the pulmonary veins, and in the atrium (heart), atria) result in feedback by regulating the secretion of antidiuretic hormone (ADH/Vasopressin), renin and aldosterone. The resultant increase in
blood volume Blood volume is the volume Volume is a scalar quantity expressing the amount Quantity or amount is a property that can exist as a multitude Multitude is a term for a group of people who cannot be classed under any other distinct category, ex ...
results in an increased
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
by the Frank–Starling law of the heart, in turn increasing arterial blood pressure. These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. When blood pressure falls many physiological cascades commence in order to return the blood pressure to a more appropriate level. # The blood pressure fall is detected by a decrease in blood flow and thus a decrease in glomerular filtration rate (GFR). # Decrease in GFR is sensed as a decrease in Na+ levels by the macula densa. # The macula densa causes an increase in Na+ reabsorption, which causes water to follow in via osmosis and leads to an ultimate increase in blood plasma, plasma volume. Further, the macula densa releases adenosine which causes constriction of the afferent arterioles. # At the same time, the juxtaglomerular cells sense the decrease in blood pressure and release renin. # Renin converts angiotensinogen (inactive form) to angiotensin I (active form). # Angiotensin I flows in the bloodstream until it reaches the capillaries of the lungs where angiotensin converting enzyme (ACE) acts on it to convert it into angiotensin II. # Angiotensin II is a vasoconstrictor that will increase blood flow to the heart and subsequently the preload, ultimately increasing the
cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...
. # Angiotensin II also causes an increase in the release of aldosterone from the adrenal glands. # Aldosterone further increases the Na+ and H2O reabsorption in the distal convoluted tubule of the nephron. 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 Hypertension (HTN or HT), also known as high blood pressure (HBP), is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not ...

because if blocked, individuals may suffer from
orthostatic hypotension Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (fr ...
and fainting.

# Measurement

Arterial pressure is most commonly measured via a
sphygmomanometer A sphygmomanometer ( ), also known as a blood pressure monitor, or blood pressure gauge, is a device used to measure blood pressure, composed of an inflatable cuff to collapse and then release the artery under the cuff in a controlled manner, and ...

, which uses the height of a column of mercury, or an aneroid gauge, to reflect the blood pressure by auscultation. The most common automated blood pressure measurement technique is based on the Blood pressure measurement#Oscillometric, oscillometric method. Fully automated oscillometric measurement has been available since 1981. This principle has recently been used to measure blood pressure with a smartphone. Measuring pressure Invasive blood 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. An example is a cuffless blood pressure monitor at the wrist that uses only optical sensors One common problem in office blood pressure measurement in the United States is terminal digit preference. According to one study, "without bias, 10%-20% of measurements are expected to end in zero" Therefore, addressing digit preference is a key issue for improving blood pressure measurement accuracy.

# In animals

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

## Hypertension in cats and dogs

Hypertension in cats and dogs is generally diagnosed if the blood pressure is greater than 160 mm Hg (systolic), although Sighthound, sight hounds have higher blood pressures than most other dog breeds; a systolic pressure greater than 180 mmHg is considered abnormal in these dogs.

*