Blood pressure (BP) is the pressure of circulating blood on the walls of blood vessels. Most of this pressure is due to the work done by the heart in pumping blood round the circulation. 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 (16kPa) systolic, and 80 millimetres of mercury (11kPa) diastolic, abbreviated "120/80mmHg".
Traditionally, blood pressure was measured non-invasively using ausculation with a mercury-tube sphygmomanometer.[1] 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 average difference between two standardized reading methods of 5mm Hg or less and a standard deviation of less than 8mm Hg.[4] 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 nervous and 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 115/75mmHg,[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 UK, to  advocate 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 10mmHg 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

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 30mmHg at 20 weeks of gestation, and increases to approximately 45mmHg at 40 weeks of gestation.[24]
The average blood pressure for full-term infants:[25]
Systolic 65–95mmHg
Diastolic 30–60mmHg
Childhood

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.[30]  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,[30] 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

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 less than arterial pressure, with common values of 5mmHg in the right atrium and 8mmHg in the left atrium.
Variants of venous pressure include:
Central venous pressure, which is a good approximation of right atrial pressure,[34] which is a major determinant of right ventricular end diastolic volume. (However, there can be exceptions in some cases.)[35]
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 5–10mmHg[36]
Pulmonary pressure

Normally, the pressure in the pulmonary artery is about 15mmHg at rest.[37]
Increased blood pressure in the capillaries of the lung causes pulmonary hypertension, leading to interstitial edema if the pressure increases to above 20mmHg, and to pulmonary edema at pressures above 25mmHg.[38]
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;[39] typically this is of the order of ~7mm Hg.[39]
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.[40]
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 (>140mmHg) with a normal diastolic blood pressure (<90mmHg), it is called "isolated systolic hypertension" and may present a health concern.[41][42]
For those with heart valve 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 2 weeks apart for each person, there was an increased severity of aortic and mitral regurgitation when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.[43]
Low blood pressure

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.[44]
Causes of low arterial pressure include:[45]
Sepsis
Hemorrhage – blood loss
Cardiogenic shock
Neurally mediated hypotension (or reflex syncope)
Toxins including toxic doses of blood pressure medicine
Hormonal abnormalities, such as Addison's disease
Eating disorders, particularly anorexia nervosa and bulimia
Orthostatic hypotension
A large fall in blood pressure upon standing (persistent systolic/diastolic blood pressure decrease of >20/10mm 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.[46] 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.[47] Usually this failure of compensation is due to diseases or drugs that affect the sympathetic nervous system.[46] 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[48] and a heightened risk of cerebrovascular disease.[49]  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.[49]  Older individuals and those who had received blood pressure medications were more likely to exhibit larger fluctuations in pressure.[49]
Physiology

During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure.[50] The blood pressure in the circulation is principally due to the pumping action of the heart.[51] 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.[52] Pulsatility also diminishes in the smaller elements of the arterial circulation, although some transmitted pulsatility is observed in capillaries.[53]  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.[51]
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[54] 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.[55][56][57] In the longer-term the relationship between volume and blood pressure is more complex.[58] 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)[59] 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.[60] 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.[61][62] Reductions in capillary density, termed capillary rarefaction, may also contribute to increased resistance in some circumstances.[63]
In practice, each individual's autonomic nervous system and other systems regulating blood pressure, notably the kidney,[64] 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)):[65]

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}}}

[65]

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,[66]

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.[66]
Regulation of blood pressure

The endogenous 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.[67]
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.[1] The most common automated blood pressure measurement technique is based on the oscillometric method.[68] Fully automated oscillometric measurement has been available since 1981.[69] This principle has recently been used to measure blood pressure with a smartphone.[70] Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting.
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).[71]  As in humans, blood pressure in animals differs by age, sex, time of day and circumstances:[72][73]  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.[74]

Hypertension in cats and dogs
Hypertension in cats and dogs is diagnosed if the blood pressure is greater than 150mm Hg (systolic) and/or 95mm Hg (diastolic).[73]

Further reading

, American Heart Association
, Toronto General Hospital
, Vaughn's Summaries

What is considered high for blood pressure?


The borders of the oceans are the limits of the Earth's oceanic waters. The definition and number of oceans can vary depending on the adopted criteria.
Overview
Though generally described as several separate oceans, the world's oceanic waters constitute one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean.[1][2]  This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[3]
The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. The principal divisions (in descending order of area) are the: Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays, straits, and other names.
Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle. Continental crust is thicker but less dense. From this perspective, the Earth has three oceans: the World Ocean, the Caspian Sea, and the Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is at times a discrete ocean, because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is a natural canal cut through continental rock some 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.
Despite their names, some smaller landlocked "seas" are not connected with the World Ocean, such as the Caspian Sea (which is nevertheless, geologically, itself a full-fledged ocean—see above) and numerous salt lakes such as the Aral Sea.
A complete hierarchy showing which seas belong to which oceans, according to the International Hydrographic Organization (IHO) and for the whole planet, is available at the European Marine Gazetteer website.[4]  See also the list of seas article for the seas included in each ocean area. Also note there are many varying definitions of the world's seas and no single authority.

Arctic Ocean
The Arctic Ocean covers much of the Arctic and washes upon northern North America and Eurasia and is sometimes considered a sea or estuary of the Atlantic.[6][7]
The International Hydrographic Organization (IHO) defines the limits of the Arctic Ocean (excluding the seas it contains) as follows:[8]
Between Greenland and West Spitzbergen [sic] — The Northern limit of Greenland Sea.[Ar 1]
Between West Spitzbergen and North East Land — the parallel of lat. 80°N.
From Cape Leigh Smith to Cape Kohlsaat — the Northern limit of Barentsz Sea [sic].[Ar 2]
From Cape Kohlsaat to Cape Molotov — the Northern limit of Kara Sea.[Ar 3]
From Cape Molotov to the Northern extremity of Kotelni Island — the Northern limit of Laptev Sea.[Ar 4]
From the Northern extremity of Kotelni Island to the Northern point of Wrangel Island — the Northern limit of East Siberian Sea.[Ar 5]
From the Northern point of Wrangel Island to Point Barrow — the Northern limit of Chuckchi Sea [sic].[Ar 6]
From Point Barrow to Cape Land's End on Prince Patrick Island — the Northern limit of Beaufort Sea,[Ar 7] through the Northwest coast of Prince Patrick Island to Cape Leopold M'Clintock, thence to Cape Murray (Brook Island) and along the Northwest coast to the extreme Northerly point; to Cape Mackay (Borden Island); through the Northwesterly coast of Borden Island to Cape Malloch, to Cape Isachsen (Ellef Ringnes Island); to the Northwest point of Meighen Island to Cape Stallworthy (Axel Heiberg Island) to Cape Colgate the extreme West point of Ellesmere Island; through the North shore of Ellesmere Island to Cape Columbia thence a line to Cape Morris Jesup (Greenland).
Note that these definitions exclude any marginal waterbodies that are separately defined by the IHO (such as the Kara Sea and East Siberian Sea), though these are usually considered to be part of the Arctic Ocean.
The CIA defines the limits of the Arctic Ocean differently, as depicted in the map comparing its definition to the IHO's definition.
Notes

Atlantic Ocean

The Atlantic Ocean separates the Americas from Europe and Africa. It may be further subdivided by the equator into northern and southern portions.
North Atlantic
The 3rd edition, currently in force, of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the North Atlantic Ocean (excluding the seas it contains) as follows:[8]
On the West. The Eastern limits of the Caribbean Sea,[At 1] the Southeastern limits of the Gulf of Mexico[At 2] from the North coast of Cuba to Key West, the Southwestern limit of the Bay of Fundy[At 3] and the Southeastern and Northeastern limits of the Gulf of St. Lawrence.[At 4]
On the North. The Southern limit of Davis Strait[At 5] from the coast of Labrador to Greenland and the Southwestern limit of the Greenland Sea[At 6] and Norwegian Sea[At 7] from Greenland to the Shetland Islands.
On the East. The Northwestern limit of the North Sea,[At 8] the Northern and Western limits of the Scottish Seas,[At 9] the Southern limit of the Irish Sea,[At 10] the Western limits of the Bristol[At 11] and English Channels,[At 12] of the Bay of Biscay[At 13] and of the Mediterranean Sea.[At 14]
On the South. The equator, from the coast of Brazil to the Southwestern limit of the Gulf of Guinea.[At 15]
South Atlantic
The 3rd edition (currently in force) of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the South Atlantic Ocean (excluding the seas it contains) as follows:[8]
On the Southwest. The meridian of Cape Horn, Chile (67°16'W) from Tierra del Fuego to the Antarctic Continent; a line from Cape Virgins () to Cape Espiritu Santo, Tierra del Fuego, the Eastern entrance to Magellan Strait, Chile
On the West. The limit of the Rio de La Plata.[At 16]
On the North. The Southern limit of the North Atlantic Ocean.
On the Northeast. The limit of the Gulf of Guinea.[At 15]
On the Southeast. From Cape Agulhas along the meridian of 20° East to the Antarctic continent.
On the South. The Antarctic Continent.
Note that these definitions exclude any marginal waterbodies that are separately defined by the IHO (such as the Bay of Biscay and Gulf of Guinea), though these are usually considered to be part of the Atlantic Ocean.
In its 2002 draft, the IHO redefined the Atlantic Ocean, moving its southern limit to 60°S, with the waters south of that line identified as the Southern Ocean. This new definition has not yet been ratified (and, in addition, a reservation was lodged in 2003 by Australia.)[10] While the name "Southern Ocean" is frequently used, some geographic authorities such as the 10th edition of the World Atlas from the U.S. National Geographic Society generally show the Atlantic, Indian, and Pacific Oceans continuing to Antarctica.  If and when adopted, the 2002 definition would be published in the 4th edition of Limits of Oceans and Seas, re-instituting the 2nd edition's "Southern Ocean", omitted from the 3rd edition.
Notes

Indian Ocean

The Indian Ocean washes upon southern Asia and separates Africa and Australia.
The 3rd edition, currently in force, of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the Indian Ocean (excluding the seas it contains) as follows:[8]
On the North. The Southern limits of the Arabian Sea[I 1] and the Lakshadweep Sea,[I 2] the Southern limit of the Bay of Bengal,[I 3] the Southern limits of the East Indian Archipelago,[I 4] and the Southern limit of the Great Australian Bight.[I 5]
On the West. From Cape Agulhas in 20° long. East, Southward along this meridian to the Antarctic Continent.
On the East. From South East Cape, the Southern point of Tasmania down the meridian 146°55'E to the Antarctic Continent.
On the South. The Antarctic Continent.
Note that this definition excludes any marginal waterbodies that are separately defined by the IHO (such as the Bay of Bengal and Arabian Sea), though these are usually considered to be part of the Indian Ocean.
In its 2002 draft, the IHO redefined the Indian Ocean, moving its southern limit to 60°S, with the waters south of that line identified as the Southern Ocean. This new definition has not yet been ratified (and, in addition, a reservation was lodged in 2003 by Australia.)[10] While the name "Southern Ocean" is frequently used, some geographic authorities such as the 10th edition of the World Atlas from the U.S. National Geographic Society generally show the Atlantic, Indian, and Pacific Oceans continuing to Antarctica.  If and when adopted, the 2002 definition would be published in the 4th edition of Limits of Oceans and Seas, re-instituting the 2nd edition's "Southern Ocean", omitted from the 3rd edition.
The boundary of the Indian Ocean is a constitutional issue for Australia. The Imperial South Australia Colonisation Act, 1834, which established and defined the Colony of South Australia defined South Australia’s southern limit as being the “Southern Ocean.”   This definition was carried through to Australian constitutional law upon the Federation of Australia in 1901.
Notes

Pacific Ocean

The Pacific is the ocean that separates Asia and Australia from the Americas. It may be further subdivided by the equator into northern and southern portions.
North Pacific
The 3rd edition, currently in force, of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the North Pacific Ocean (excluding the seas it contains) as follows:[8]
On the Southwest. The Northeastern limit of the East Indian Archipelago from the Equator to Luzon Island.
On the West and Northwest. The Eastern limits of the Philippine Sea[P 1] and Japan Sea[P 2] and the Southeastern limit of the Sea of Okhotsk.[P 3]
On the North. The Southern limits of the Bering Sea[P 4] and the Gulf of Alaska.[P 5]
On the East. The Western limit of Coastal waters of Southeast Alaska and Br. Columbia,[P 6] and the Southern limit of the Gulf of California.[P 7]
On the South. The Equator, but excluding those islands of the Gilbert and Galàpagos Groups which lie to the Northward thereof.
South Pacific
The 3rd edition, currently in force, of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the South Pacific Ocean (excluding the seas it contains) as follows:[8]
On the West. From Southeast Cape, the Southern point of Tasmania, down the meridian of 146°55'E to the Antarctic continent.
On the Southwest and Northwest. The Southern, Eastern and Northeastern limits of the Tasman Sea,[P 8][P 9][P 10] the Southeastern and Northeastern limits of the Coral Sea,[P 11][P 12] the Southern, Eastern and Northern limits of the Solomon[P 13][P 14] and Bismark[sic][P 15] seas, and the Northeastern limit of the East Indian Archipelago from New Guinea to the Equator.
On the North. The Equator, but including those islands of the Gilbert and Galàpagos Groups which lie to the Northward thereof.
On the East. The meridian of Cape Horn (67°16'W) from Tierra del Fuego to the Antarctic continent; a line from Cape Virgins () to Cape Espiritu Santo, Tierra del Fuego, the Eastern entrance to Magellan Strait. (These limits have not yet been officially accepted by Argentina and Chile.)
On the South. The Antarctic continent.
Note that these definitions exclude any marginal waterbodies that are separately defined by the IHO (such as the Gulf of Alaska and Coral Sea), though these are usually considered to be part of the Pacific Ocean.
In its 2002 draft, the IHO redefined the Pacific Ocean, moving its southern limit to 60°S, with the waters south of that line identified as the Southern Ocean. This new definition has not yet been ratified (and, in addition, a reservation was lodged in 2003 by Australia.)[10] While the name "Southern Ocean" is frequently used, some geographic authorities such as the 10th edition of the World Atlas from the U.S. National Geographic Society generally show the Atlantic, Indian, and Pacific Oceans continuing to Antarctica.  If and when adopted, the 2002 definition would be published in the 4th edition of Limits of Oceans and Seas, re-instituting the 2nd edition's "Southern Ocean", omitted from the 3rd edition.
Notes

Southern or Antarctic Ocean

The Southern Ocean contains the waters that surround Antarctica and sometimes is considered an extension of Pacific, Atlantic and Indian Oceans.[8]
In 1928, the first edition of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas publication included the Southern Ocean around Antarctica.  The Southern Ocean was delineated by land-based limits - the continent of Antarctica to the south, and the continents of South America, Africa, and Australia plus Broughton Island, New Zealand in the north. The detailed land-limits used were Cape Horn in South America, Cape Agulhas in Africa, the southern coast of Australia from Cape Leeuwin, Western Australia, to South East Cape, Tasmania, via the western edge of the water body of Bass Strait, and then Broughton Island before returning to Cape Horn.[14]
The northern limits of the Southern Ocean were moved southwards in the IHO's 1937 second edition of the Limits of Oceans and Seas.  The Southern Ocean then extended from Antarctica northwards to latitude 40° south between Cape Agulhas in Africa (long. 20° east) and Cape Leeuwin in Western Australia (long. 115° east), and extended to latitude 55° south between Auckland Island of New Zealand (long. 165° or 166° east) and Cape Horn in South America (long. 67° west).[15]
The Southern Ocean did not appear in the 1953 third edition because "...the northern limits ... are difficult to lay down owing to their seasonal change ... Hydrographic Offices who issue separate publications dealing with this area are therefore left to decide their own northern limits. (Great Britain uses the Latitude of 55° South)". Instead, in the IHO 1953 publication, the Atlantic, Indian and Pacific Oceans were extended southward, the Indian and Pacific Oceans (which had not previously touched pre 1953, as per the first and second editions) now abutted at the meridian of South East Cape, and the southern limits of the Great Australian Bight and the Tasman Sea were moved northwards.[16]
The IHO readdressed the question of the Southern Ocean in a survey in 2000. Of its 68 member nations, 28 responded, and all responding members except Argentina agreed to redefine the ocean, reflecting the importance placed by oceanographers on ocean currents. The proposal for the name Southern Ocean won 18 votes, beating the alternative Antarctic Ocean. Half of the votes supported a definition of the ocean's northern limit at 60°S (with no land interruptions at this latitude), with the other 14 votes cast for other definitions, mostly 50°S, but a few for as far north as 35°S.
The 4th edition of Limits of Oceans and Seas has yet to be published due to 'areas of concern' by several countries relating to various naming issues around the world. The IHB circulated a new draft of the 4th edition of the publication in August 2002, however there were still various changes, 60 seas were added or renamed from the 3rd edition, and even the name of the publication was changed.[17] A reservation had also been lodged by Australia regarding the Southern Ocean limits.[10] Effectively, the 3rd edition (which did not delineate the Southern Ocean leaving delineation to local hydrographic offices) has yet to be superseded and IHO documents declare that it remains "currently in force."
Despite this, the 4th edition definition has de facto usage by many organisations, scientists and nations - even at times by IHO committees.[18] Some nations' hydrographic offices have defined their own boundaries; the United Kingdom used the 55°S parallel for example.[16]
Other sources, such as the National Geographic Society, show the Atlantic, Pacific and Indian Oceans as extending to Antarctica, although articles on the National Geographic web site have begun to reference the Southern Ocean.[19]
In Australia, cartographic authorities defined the Southern Ocean as including the entire body of water between Antarctica and the south coasts of Australia and New Zealand. This delineation is basically the same as the original (first) edition of the IHO publication and effectively the same as the second edition. In the second edition, the Great Australian Bight was defined as the only geographical entity between the Australian coast and the Southern Ocean. Coastal maps of Tasmania and South Australia label the sea areas as Southern Ocean,[20] while Cape Leeuwin in Western Australia is described as the point where the Indian and Southern Oceans meet.[21]
See also

List of seas
International Maritime Organization
Marginal sea
Oceanography
Polar seas
Sea
Seven Seas
World Ocean Atlas

Where does the Atlantic Ocean begin and end?