Blood pressure | Wikipedia audio article

Blood pressure | Wikipedia audio article

November 1, 2019 0 By Jose Scott


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 120 millimetres of mercury (16 kPa) systolic, and 80 millimetres of mercury
(11 kPa) 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.Traditionally,
blood pressure was measured non-invasively using auscultation 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. 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 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, 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 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, and disease. 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. 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.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 are present.===Systemic arterial pressure and age=======
Fetal blood pressure====In pregnancy, 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.====Aging adults====
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. However, measurement of pressures in the venous
system and the pulmonary vessels plays an important role in intensive care medicine
but requires invasive measurement of pressure using a catheter. 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: Central venous pressure, 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 (JVP) is the indirectly
observed pressure over the venous system. It can be useful in the differentiation of
different forms of heart and lung disease. The portal venous pressure is the blood pressure
in the portal vein. It is normally 5–10 mmHg==Pulmonary pressure==Normally, the pressure in the pulmonary artery
is about 15 mmHg at rest.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.==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, low blood pressure, and
blood pressure that shows excessive or maladaptive fluctuation.===High blood pressure===Arterial hypertension 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. Levels of arterial pressure put mechanical
stress on the arterial walls. Higher pressures increase heart workload and
progression of unhealthy tissue growth (atheroma) 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 tends to thicken, enlarge
and become weaker over time. Persistent hypertension is one of the risk
factors for strokes, heart attacks, heart failure and arterial aneurysms, and is the
leading cause of chronic kidney failure. Even moderate elevation of arterial pressure
leads to shortened life expectancy. 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 pressure;
but nowadays it is recognized that both high systolic pressure and high pulse pressure
(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). If systolic blood pressure is elevated (>140
mmHg) with a normal diastolic blood pressure (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. 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. 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 and a heightened risk of cerebrovascular
disease. 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. Older individuals and those who had received
blood pressure medications were more likely to exhibit larger fluctuations in pressure.==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. 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. 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, 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 (MAP) and pulse pressure. Most influences on blood pressure can be understood
in terms of their effect on cardiac output 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. 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 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) 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. 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. 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 and is determined by the cardiac
output (CO), systemic vascular resistance (SVR), and central venous pressure (CVP)): MAP=
( CO ⋅ SVR )
+ CVP {\displaystyle \!{\text{MAP}}=({\text{CO}}\cdot
{\text{SVR}})+{\text{CVP}}} In practice, the contribution of CVP (which
is small) is generally ignored and so MAP=CO ⋅ SVR {\displaystyle \!{\text{MAP}}={\text{CO}}\cdot
{\text{SVR}}} MAP can be estimated from measurements of
the systolic pressure P sys {\displaystyle \!P_{\text{sys}}}
and the diastolic pressure P dias {\displaystyle \!P_{\text{dias}}} MAP ≊ P dias + 1
3 ( P sys − P dias ) {\displaystyle \!{\text{MAP}}\approxeq P_{\text{dias}}+{\frac
{1}{3}}(P_{\text{sys}}-P_{\text{dias}})}===Pulse pressure===The
pulse pressure is the difference between the measured systolic and diastolic pressures, P pulse=P sys − P dias . {\displaystyle \!P_{\text{pulse}}=P_{\text{sys}}-P_{\text{dias}}.} 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.===Regulation of blood pressure===The endogenous, homeostatic regulation of
arterial pressure is not completely understood, but the following mechanisms of regulating
arterial pressure have been well-characterized: Baroreceptor reflex: Baroreceptors in the
high pressure receptor zones detect changes in arterial pressure. These baroreceptors send signals ultimately
to the medulla of the brain stem, specifically to the rostral ventrolateral medulla (RVLM). The medulla, by way of the autonomic nervous
system, 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. 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 or drops in arterial pressure by activating an endogenous vasoconstrictor
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. Baroreceptors in low pressure receptor zones
(mainly in the venae cavae and the pulmonary veins, and in the atria) result in feedback
by regulating the secretion of antidiuretic hormone (ADH/Vasopressin), renin and aldosterone. The resultant increase in blood volume results
in an increased cardiac output 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 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 which
will increase blood flow to the heart and subsequently the preload, ultimately increasing
the cardiac output. 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 because if blocked, individuals may suffer from orthostatic hypotension
and fainting.==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. The most common automated blood pressure measurement
technique is based on the 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 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==
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). As in humans, blood pressure in animals differs
by age, sex, time of day and circumstances: 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.===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