blood pressure

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the fluid that circulates through the heart, arteries, capillaries, and veins and is the chief means of transport within the body. It transports oxygen from the lungs to the body tissues, and carbon dioxide from the tissues to the lungs. It transports nutritive substances and metabolites to the tissues and removes waste products to the kidneys and other organs of excretion. It has an essential role in the maintenance of fluid balance.

In an emergency, blood cells and antibodies carried in the blood are brought to a point of infection, or blood-clotting substances are carried to a break in a blood vessel. The blood distributes hormones from the endocrine glands to the organs they influence. It also helps regulate body temperature by carrying excess heat from the interior of the body to the surface layers of the skin, where the heat is dissipated to the surrounding air.

Blood varies in color from a bright red in the arteries to a duller red in the veins. The total quantity of blood within an individual depends upon body weight; a person weighing 70 kg (154 lb) has about 4.5 liters of blood in the body.

Blood is composed of two parts: the fluid portion is called plasma, and the solid portion or formed elements (suspended in the fluid) consists of the blood cells (erythrocytes and leukocytes) and the platelets. Plasma accounts for about 55 per cent of the volume and the formed elements account for about 45 per cent. ( and table.)

Chemical analyses of various substances in the blood are invaluable aids in (1) the prevention of disease by alerting the patient and health care provider to potentially dangerous levels of blood constituents that could lead to more serious conditions, (2) diagnosis of pathologic conditions already present, (3) assessment of the patient's progress when a disturbance in blood chemistry exists, and (4) assessment of the patient's status by establishing baseline or “normal” levels for each individual patient.

In recent years, with the increasing attention to preventive health care and rapid progress in technology and automation, the use of a battery of screening tests performed by automated instruments has become quite common. These instruments are capable of performing simultaneously a variety of blood chemistry tests. Some of the more common screening tests performed on samples of blood include evaluation of electrolyte, albumin, and bilirubin levels, blood urea nitrogen (BUN), cholesterol, total protein, and such enzymes as lactate dehydrogenase and aspartate transaminase. Other tests include electrophoresis for serum proteins, blood gas analysis, glucose tolerance tests, and measurement of iron levels.
Composition of the blood, which constitutes 8% of total body weight. From Applegate, 2000.
blood bank
1. a place of storage for blood.
2. an organization that collects, processes, stores, and transfuses blood. In most health agencies the blood bank is located in the pathology laboratory. It is operated by medical technologists under the direction of a pathologist.
blood bank technologist a clinical laboratory scientist/medical technologist who has postgraduate education in blood banking and is certified by the Board of Registry of the American Society of Clinical Pathologists; designated as MT(ASCP)SBB. Specialists in blood bank technology perform both routine and specialized tests in blood bank immunohematology and perform transfusion services. The address of the American Association of Blood Banks is 8101 Glenbrook Road, Bethesda, MD 20814 (telephone 301-907-6582). The address of the Board of Registry of the American Society of Clinical Pathologists is P.O. Box 12270, Chicago, IL 60612. Their telephone number is 312-738-1336 and their web site is
blood-brain barrier BBB; the barrier separating the blood from the brain parenchyma everywhere except in the hypothalamus. It is permeable to water, oxygen, carbon dioxide, and nonionic solutes, such as glucose, alcohol, and general anesthetics, and is only slightly permeable to electrolytes and other ionic substances. Some small molecules, e.g., amino acids, are taken up across the barrier by specific transport mechanisms.
citrated blood blood treated with sodium citrate or citric acid to prevent its coagulation.
cord blood the blood contained in the umbilical vessels at the time of delivery of the infant. It is rich in stem cells that could be used in place of bone marrow for a transplant; thus, it is sometimes collected and stored for future use.
blood count determination of the number of blood cells in a given sample of blood, usually expressed as the number in a cubic millimeter; it may be either a complete blood count or a count of just one of the elements such as an erythrocyte count, leukocyte count or a platelet count. Methods include manual counts using a hemacytometer and automated counts using a flow cytometer, a Coulter counter, or other means. The blood count is useful in the diagnosis of various blood dyscrasias, infections, or other abnormal conditions and is one of the most common tests done on the blood. Called also blood cell count. (See accompanying table.)
defibrinated blood whole blood from which fibrin has been separated during the clotting process.
blood gas analysis laboratory studies of arterial and venous blood for the purpose of measuring oxygen and carbon dioxide levels and pressure or tension, and hydrogen ion concentration (pH). (See accompanying table.) Analyses of blood gases provide the following information:ƒ

PaO2—partial pressure (P) of oxygen (O2) in the arterial blood (a)

SaO2—percentage of available hemoglobin that is saturated (Sa) with oxygen (O2)

PaCO2—partial pressure (P) of carbon dioxide (CO2) in the arterial blood (a)

pH—an expression of the extent to which the blood is alkaline or acidic

HCO3—the level of plasma bicarbonate; an indicator of the metabolic acid-base status

These parameters are important tools for assessment of a patient's acid-base balance. They reflect the ability of the lungs to exchange oxygen and carbon dioxide, the ability of the kidneys to control the retention or elimination of bicarbonate, and the effectiveness of the heart as a pump. Because the lungs and kidneys act as important regulators of the respiratory and metabolic acid-base balance, assessment of the status of a patient with any disorder of respiration and metabolism includes periodic blood gas measurements.

The partial pressure of a particular gas in a mixture of gases, as of oxygen in air, is the pressure exerted by that gas alone. It is proportional to the relative number of molecules of the gas, for example, the fraction of all the molecules in the air that are oxygen molecules. The partial pressure of a gas in a liquid is the partial pressure of a real or imaginary gas that is in equilibrium with the liquid.

PaO2 measures the oxygen content of the arterial blood, most of which is bound to hemoglobin, forming oxyhemoglobin. The SaO2 measures the oxygen in oxyhemoglobin as a percentage of the total hemoglobin oxygen-carrying capacity.

A PaO2 of 60 mm Hg represents an SaO2 of 90 per cent, which is sufficient to meet the needs of the body's cells. However, as the PaO2 falls, the SaO2 decreases rapidly. A PaO2 below 55 indicates a state of hypoxemia that requires correction. Normal PaO2 values at sea level are 80 mm Hg for elderly adults and 100 mm Hg for young adults.

However, some patients with chronic obstructive pulmonary disease can tolerate a PaO2 as low as 70 mm Hg without becoming hypoxic. In caring for patients with this condition, it is important to know that attempts to elevate the PaO2 level to the normal level can be dangerous and even fatal. It is best to establish a baseline for each individual patient before supplementary oxygen is given, and then to assess his condition and the effectiveness of his therapy according to this baseline.

The PaCO2 gives information about the cellular production of carbon dioxide through metabolic processes, and the removal of it from the body via the lungs. The normal range is 32 to 45 mm Hg. Values outside this range indicate a primary respiratory problem associated with pulmonary function, or a metabolic problem for which there is respiratory compensation.

In the newborn the normal PaO2 is 50 to 80 mm Hg. At 40 to 50 mm Hg cyanosis may become apparent. Respiratory distress in an infant who is unable to ventilate the lungs adequately will produce a drop in PaO2 level. However, there is no marked increase in PaCO2 level in some infants as in adults with respiratory distress because many infants can still eliminate carbon dioxide from the lungs even though weakness prevents inhaling an adequate oxygen supply. All infants being ventilated and receiving oxygen therapy require frequent blood gas analyses and also pH, base excess, and oxygen saturation levels to avoid oxygen toxicity and acid-base imbalance.

Blood pH gives information about the patient's metabolic state. A pH of 7.4 is considered normal; a value lower than 7.4 indicates acidemia and one higher than 7.4 alkalemia.

Because the amount of CO2 in the blood affects its pH, abnormal PaCO2 values are interpreted in relation to the pH. If the PaCO2 value is elevated, and the pH is below normal, respiratory acidosis from either acute or chronic hyperventilation is suspected. Conversely, a PaCO2 below normal and a pH above normal indicates respiratory alkalosis. When both the PaCO2 and the pH are elevated, there is respiratory retention of CO2 to compensate for metabolic acidosis. If both values are below normal, there is respiratory elimination of CO2 (hyperventilation) to compensate for metabolic acidosis.

Abnormal levels of bicarbonate (HCO3) in the plasma are also interpreted in relation to the pH in the diagnosis of disturbances in the metabolic component of the acid-base balance. The normal range for HCO3 is 22 to 26 mEq per liter. Abnormally low levels of both HCO3 and pH indicate acidosis of metabolic origin. Conversely, elevations of both of these values indicate metabolic alkalosis. The kidneys maintain bicarbonate levels by filtering bicarbonate and returning it to the blood; they also produce new bicarbonate to replace that which is used in buffering. Therefore, a decreased HCO3 and an increased pH level indicate either retention of hydrogen ions by the kidneys or the elimination of HCO3 in an effort to compensate for respiratory alkalosis. Conversely, if the HCO3 level is increased and the pH is decreased, the kidneys have compensated for respiratory acidosis by retaining HCO3 or by eliminating hydrogen ions.
blood gas analysis, mixed venous blood gas analysis performed on a blood sample obtained from the pulmonary artery.
blood gas analysis, transcutaneous the determination of PO2 and PCO2 by placement of a heated electrode over the skin to get an inference of PaO2 and PaCO2.
blood group the phenotype of erythrocytes defined by one or more cellular antigenic structural groupings under the control of allelic genes. In clinical practice there are four main blood groups or blood types: A, B, O, and AB (see table). In addition to this major grouping there is an Rh-hR system that is important in the prevention of erythroblastosis fetalis resulting from incompatibility of blood groups in mother and fetus.

The ABO blood group system was first introduced in 1900 by Karl Landsteiner; in 1920 group AB was discovered by van Descatello and Sturli. Identification of these four major blood groups represented a major step toward resolving the problem of blood transfusion reactions resulting from donor-recipient incompatibility. In 1938 Landsteiner and Weiner discovered another blood factor related to maternal-fetal incompatibility. The factor was named Rh because the researchers were using rhesus monkeys in their studies. Further research has uncovered additional factors in the Rh group.

Although more than 90 factors have been identified, many of these are not highly antigenic and are not, therefore, a cause for concern in the typing of blood for clinical purposes.

The term factor, in reference to blood groups, is synonymous with antigen, and the reaction occurring between incompatible blood types is an antigen-antibody reaction. In cases of incompatibility, the antigen, located on the red blood cells, is an agglutinogen and the specific antibody, located in the serum, is an agglutinin. These are so named because whenever red blood cells with a certain factor come in contact with the agglutinin specific for it, there is agglutination or clumping of the erythrocytes.

In determining blood group, a sample of blood is taken and mixed with specially prepared sera. One serum, anti-A agglutinin, causes blood of group A to agglutinate; another serum, anti-B agglutinin, causes blood of group B to agglutinate. Thus, if anti-A serum alone causes clumping, the blood is group A; if anti-B serum alone causes clumping, it is group B. If both cause clumping, the blood group is AB, and if it is not clumped by either, it is identified as group O.
occult blood that present in such small amounts as to be detectable only by chemical tests or by spectroscopic or microscopic examination.
peripheral blood that obtained from acral areas, or from the circulation remote from the heart; the blood in the systemic circulation.
blood poisoning popular term for septicemia.
blood pressure
1. the pressure of the blood against the walls of any blood vessel.
2. the term usually refers to the pressure of the blood within the arteries, or arterial blood pressure. This pressure is determined by several interrelated factors, including the pumping action of the heart, the resistance to the flow of blood in the arterioles, the elasticity of the walls of the main arteries, the blood volume and extracellular fluid volume, and the blood's viscosity, or thickness.

The pumping action of the heart refers to how hard the heart pumps the blood (force of heartbeat), how much blood it pumps (the cardiac output), and how efficiently it does the job. Contraction of the heart, which forces blood through the arteries, is the phase known as systole. Relaxation of the heart between contractions is called diastole.

The main arteries leading from the heart have walls with strong elastic fibers capable of expanding and absorbing the pulsations generated by the heart. At each pulsation the arteries expand and absorb the momentary increase in blood pressure. As the heart relaxes in preparation for another beat, the aortic valves close to prevent blood from flowing back to the heart chambers, and the artery walls spring back, forcing the blood through the body between contractions. In this way the arteries act as dampers on the pulsations and thus provide a steady flow of blood through the blood vessels. Because of this, there are actually two blood pressures within the blood vessels during one complete beat of the heart: a higher blood pressure during systole (the contraction phase) and a lower blood pressure during diastole (the relaxation phase). These two blood pressures are known as the systolic pressure and the diastolic pressure, respectively.

It is generally agreed that a reading of 120 mm Hg systolic and 80 mm Hg diastolic are the norms for a blood pressure reading; that is, it represents the average blood pressure obtained from a large sampling of healthy adults. In general, a blood pressure of 95 mm Hg systolic and 60 mm Hg diastolic indicates hypotension. However, a reading equal to or below this level must be interpreted in the light of each patient's “normal” reading as determined by baseline data.

On the basis of validated research on the long-term effects of an elevated blood pressure, it is generally agreed that some degree of risk for major cardiovascular disease exists when the systolic pressure is greater than or equal to 140 mm Hg, and the diastolic pressure is greater than or equal to 90 mm Hg. Life expectancy is reduced at all ages and in both males and females when the diastolic pressure is above 90 mm Hg. (See accompanying table.)
Measurement of the Blood Pressure. The blood pressure is usually measured in the artery of the upper arm, with a sphygmomanometer.
Measurement of blood pressure. From Applegate, 2000.
This consists of a rubber cuff and a gauge or column of mercury for measuring pressure. The rubber cuff is wrapped about the patient's arm, and then air is pumped into the cuff by means of a rubber bulb. As the pressure inside the rubber cuff increases, the flow of blood through the artery is momentarily checked.

A stethoscope is placed over the artery at the elbow and the air pressure within the cuff is slowly released. As soon as blood begins to flow through the artery again, Korotkoff sounds are heard. The first sounds heard are tapping sounds that gradually increase in intensity. The initial tapping sound that is heard for at least two consecutive beats is recorded as the systolic blood pressure.

The first phase of the sounds may be followed by a momentary disappearance of sounds that can last from 30 to 40 mm Hg as the gauge needle (or mercury column) descends. It is important that this auscultatory gap not be missed; otherwise, either an erroneously low systolic pressure or high diastolic pressure will be obtained.

During the second phase following the temporary absence of sound there are murmuring or swishing sounds. As deflation of the cuff continues, the sounds become sharper and louder. These sounds represent phase three. During phase four the sounds become muffled rather abruptly and then are followed by silence, which represents phase five.

Although there is disagreement as to which of the latter phases should represent the diastolic pressure, it is usually recommended that phase five, the point at which sounds disappear, be used as the diastolic pressure for adults, and phase four be used for children. The reason for this is that children, having a high cardiac output, often will continue to produce sounds when the gauge is at a very low reading or even at zero. In some adult patients whose arterioles have lost their elasticity, the fifth phase is also extremely low or nonexistent. In these cases, it is recommended that three readings be recorded: phase one and phases four and five. For example, the blood pressure would be written as 140/96/0. On most occasions, however, the blood pressure is written as a fraction. The systolic pressure is written as the top number, a line is drawn, and the diastolic pressure is written as the bottom number.

Errors in blood pressure measurement can result from failure of the cuff to reach and compress the artery. The cuff diameter should be 20 per cent greater than the diameter of the limb, the bladder of the cuff must be centered over the artery, and the cuff must be wrapped smoothly and snugly to ensure proper inflation. When a mercury gauge is used, the meniscus should be at eye level to avoid a false reading.
Direct Measurement of Blood Pressure. Critically ill patients who require continuous monitoring of the blood pressure may have a catheter inserted into an artery and attached to a catheter-monitor-transducer system. The blood pressure is displayed on an oscilloscope at the bedside so that the patient's pressure can be determined at a glance. This intra-arterial technique of blood pressure monitoring provides accurate, objective, and continuous data on the patient's status.
blood pressure, mean arterial MAP; the average pressure within an artery over a complete cycle of one heartbeat; in the brachial artery, calculated to be the diastolic pressure plus 1/3 of the difference between the systolic and diastolic pressures.
blood stream bloodstream.
blood urea nitrogen see urea nitrogen.
blood volume
1. the total quantity of blood in the body; the plasma volume added to the red cell volume.
2. a laboratory test performed to determine this. The indicators used to determine these measurements are 125I-labeled human serum albumin for plasma volume and 51Cr-labeled erythrocytes for red cell volume. The regulation of blood volume in the circulatory system is affected by the intrinsic mechanism for fluid exchange at the capillary membranes and by hormonal influences and nervous reflexes that affect the excretion of fluids by the kidneys. A rapid decrease in the blood volume, as in hemorrhage, greatly reduces the cardiac output and creates a condition called shock or circulatory shock. Conversely, an increase in blood volume, as when there is retention of water and salt in the body because of renal failure, results in an increase in cardiac output. The eventual outcome of this situation is increased arterial blood pressure.

The blood volume in the pulmonary circulation is approximately 12 per cent of the total blood volume. Such conditions as left-sided heart failure and mitral stenosis can greatly increase the pulmonary blood volume while decreasing the systemic volume. As would be expected, right-sided heart failure has the opposite effect. The latter condition has less serious effects because the volume of the systemic circulation is about seven times that of the pulmonary circulation and it is therefore better able to accommodate a change in fluid volume.
Tests. Clinical assessment of blood volume can be accomplished in a number of ways, for example, by measuring the patient's blood pressure while he is lying down, sitting, and standing. The quality and volume of peripheral pulses will give information about blood volume, as does determining the ease and speed with which a compressed vein will refill after pressure is released. Neck veins that are engorged indicate hypervolemia; the collapse of these veins indicates hypovolemia. A more accurate assessment can be done through the use of intravascular catheters such as the central venous pressure catheter, which measures pressure in the right atrium, and the swan-ganz catheter, which measures pressure on both sides of the heart.

Measurement of blood volume is accomplished by using substances that combine with red blood cells, for example, iron, chromium, and phosphate, or substances that combine with plasma proteins. In either case the measurement of the blood volume is based on the “dilution” principle. That is, the volume of any fluid compartment can be measured if a given amount of a substance is dispersed evenly in the fluid within the compartment, and then the extent of dilution of the substance is measured.

For example, a small amount of radioactive chromium (51Cr), which is widely used to determine blood volume, is mixed with a sample of blood drawn from the patient. After about 30 minutes the 51Cr will have entered the red blood cells. The sample with the tagged red blood cells is then returned by injection into the patient's bloodstream. About 10 minutes later a sample is removed from the patient's circulating blood and the radioactivity level of this sample is measured. The total blood volume is calculated according to this formula:
When volume is used to arrive at the total blood volume, a dye (usually T-1824, also known as Evans blue) is injected into the circulating blood. The dye immediately combines with the blood proteins and within 10 minutes is dispersed throughout the circulatory system. A sample of blood is then drawn and the exact quantity of dye is measured. Using the information about plasma volume obtained by applying the above formula, the total blood volume can be calculated, provided the hematocrit is also known. The formula for this calculation is:
whole blood that from which none of the elements has been removed, sometimes specifically that drawn from a selected donor under aseptic conditions, containing citrate ion or heparin, and used as a blood replenisher.


 (P) [presh´ur]
force per unit area.
arterial pressure (arterial blood pressure) blood pressure (def. 2).
atmospheric pressure the pressure exerted by the atmosphere, usually considered as the downward pressure of air onto a unit of area of the earth's surface; the unit of pressure at sea level is one atmosphere. Pressure decreases with increasing altitude.
barometric pressure atmospheric p.
blood pressure
2. pressure of blood on walls of any blood vessel.
capillary pressure the blood pressure in the capillaries.
central venous pressure see central venous pressure.
cerebral perfusion pressure the mean arterial pressure minus the intracranial pressure; a measure of the adequacy of cerebral blood flow.
cerebrospinal pressure the pressure of the cerebrospinal fluid, normally 100 to 150 mm Hg.
continuous positive airway pressure see continuous positive airway pressure.
filling pressure see mean circulatory filling pressure.
high blood pressure hypertension.
intracranial pressure see intracranial pressure.
intraocular pressure the pressure exerted against the outer coats by the contents of the eyeball.
intrapleural pressure (intrathoracic pressure) pleural pressure.
intrinsic positive end-expiratory pressure elevated positive end-expiratory pressure and dynamic pulmonary hyperinflation caused by insufficient expiratory time or a limitation on expiratory flow. It cannot be routinely measured by a ventilator's pressure monitoring system but is measurable only using an expiratory hold maneuver done by the clinician. Its presence increases the work needed to trigger the ventilator, causes errors in the calculation of pulmonary compliance, may cause hemodynamic compromise, and complicates interpretation of hemodynamic measurements. Called also auto-PEEP and intrinsic PEEP.
maximal expiratory pressure maximum expiratory pressure.
maximal inspiratory pressure the pressure during inhalation against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning from mechanical ventilation. A maximum inspiratory pressure above −25 cm H2O is associated with successful weaning.
maximum expiratory pressure (MEP) a measure of the strength of respiratory muscles, obtained by having the patient exhale as strongly as possible against a mouthpiece; the maximum value is near total lung capacity.
maximum inspiratory pressure (MIP) the inspiratory pressure generated against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning from mechanical ventilation. A maximum inspiratory pressure above −25 cm H2O is associated with successful weaning.
mean airway pressure the average pressure generated during the respiratory cycle.
mean circulatory filling pressure a measure of the average (arterial and venous) pressure necessary to cause filling of the circulation with blood; it varies with blood volume and is directly proportional to the rate of venous return and thus to cardiac output.
negative pressure pressure less than that of the atmosphere.
oncotic pressure the osmotic pressure of a colloid in solution.
osmotic pressure the pressure required to stop osmosis through a semipermeable membrane between a solution and pure solvent; it is proportional to the osmolality of the solution. Symbol π.
partial pressure the pressure exerted by each of the constituents of a mixture of gases.
peak pressure in mechanical ventilation, the highest pressure that occurs during inhalation.
plateau pressure in mechanical ventilation, the pressure measured at the proximal airway during an end-inspiratory pause; a reflection of alveolar pressure.
pleural pressure the pressure between the visceral pleura and the thoracic pleura in the pleural cavity. Called also intrapleural or intrathoracic pressure.
positive pressure pressure greater than that of the atmosphere.
positive end-expiratory pressure (PEEP) a method of control mode ventilation in which positive pressure is maintained during expiration to increase the volume of gas remaining in the lungs at the end of expiration, thus reducing the shunting of blood through the lungs and improving gas exchange. A PEEP higher than the critical closing pressure prevents alveolar collapse and can markedly improve the arterial Po2 in patients with a lowered functional residual capacity, as in acute respiratory failure.
Effects of the application of positive end-expiratory pressure (PEEP) on the alveoli. A, Atelectatic alveoli before PEEP application. B, Optimal PEEP application has reinflated alveoli to normal volume. C, Excessive PEEP application overdistends the alveoli and compresses adjacent pulmonary capillaries, creating dead space with its attendant hypercapnia. From Pierce, 1995.
pulmonary artery wedge pressure (PAWP) (pulmonary capillary wedge pressure (PCWP)) intravascular pressure, reflecting the left ventricular end diastolic pressure, measured by a swan-ganz catheter wedged into a small pulmonary artery to block the flow from behind.
pulse pressure the difference between the systolic and diastolic pressures. If the systolic pressure is 120 mm Hg and the diastolic pressure is 80 mm Hg, the pulse pressure is 40 mm Hg; the normal pulse pressure is between 30 and 40 mm Hg.
urethral pressure the pressure inwards exerted by the walls of the urethra, which must be counteracted in order for urine to flow through; see also urethral pressure profile.
venous pressure the blood pressure in the veins; see also central venous pressure.
water vapor pressure the tension exerted by water vapor molecules, 47 mm Hg at normal body temperature.
wedge pressure blood pressure measured by a small catheter wedged into a vessel, occluding it; see also pulmonary capillary wedge pressure and wedged hepatic vein pressure.
wedged hepatic vein pressure the venous pressure measured with a catheter wedged into the hepatic vein. The difference between wedged and free hepatic vein pressures is used to locate the site of obstruction in portal hypertension; it is elevated in that due to cirrhosis, but low in cardiac ascites or portal vein thrombosis.

blood pres·sure (BP),

the pressure or tension of the blood within the systemic arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric pressure.
Synonym(s): piesis

blood pressure

n. Abbr. BP
The pressure exerted by the blood against the walls of the blood vessels, especially the arteries. It varies with the strength of the heartbeat, the elasticity of the arterial walls, the volume and viscosity of the blood, and a person's health, age, and physical condition.

blood pressure (BP)

Etymology: AS, blod + L, premere, to press
the pressure exerted by the circulating volume of blood on the walls of the arteries and veins and on the chambers of the heart. Blood pressure is regulated by the homeostatic mechanisms of the body by the volume of the blood, the lumen of the arteries and arterioles, and the force of cardiac contraction. In the aorta and large arteries of a healthy young adult, blood pressure is approximately 120 mm Hg during systole and 70 mm Hg during diastole. See also hypertension, hypotension.
method The indirect blood pressure is most often measured by auscultation, using an aneroid or mercury sphygmomanometer, a stethoscope, and a blood pressure cuff. With the upper arm at the level of the heart, the cuff is placed around the upper arm and inflated to a pressure greater than the systolic pressure, occluding the brachial artery. The diaphragm of the stethoscope is placed over the artery in the antecubital space, and the pressure in the cuff is slowly released. No sound is heard until the cuff pressure falls below the systolic pressure in the artery; at that point a pulse is heard. As the cuff pressure continues to fall slowly, the pulse continues, first becoming louder, then dull and muffled. These sounds, called sounds of Korotkoff, are produced by turbulence of the blood flowing through a vessel that is partially occluded as the arterial pressure falls to the low pressure of diastole. When the cuff pressure is less than the diastolic pressure, no pulse is heard. Thus the cuff pressure at which the first sound is heard is the systolic blood pressure, indicative of the pressure in the large arteries during systole; the cuff pressure at which the sounds stop is the diastolic blood pressure, indicative of the pressure in the arteries during diastole. A variation of this method involves the use of palpation in place of auscultation in the antecubital space to determine the systolic pressure (the pressure at which a pulse is first palpated). Another variation uses a transducer in the cuff to translate changes in ultrasound frequency caused by blood movement within the artery to audible sounds. Blood pressure may be monitored directly by means of a strain gauge or mercury manometer after a cannula has been placed in an artery.
interventions The intervals at which the patient's blood pressure is to be taken are specified. The pressure in both arms is taken the first time the procedure is performed; persistent major differences between the two readings is indicative of a vascular occlusion. Alternatively, the blood pressure may be taken using the thigh and the popliteal space when the leg is at the level of the heart. The width of the cuff should be one third to one half the circumference of the limb used. Thus, a larger cuff is required for a large patient or for any patient if the pressure is taken at the thigh.
outcome criteria Any factor that increases peripheral resistance or cardiac output increases the blood pressure. Therefore, it is important to obtain a blood pressure reading when the patient is at rest. Increased peripheral resistance usually increases the diastolic pressure, and increased cardiac output tends to increase the systolic pressure. Blood pressure increases with age, primarily as a result of the decreased distensibility of the veins. As a person grows older, an increase in systolic pressure precedes an increase in diastolic pressure.
enlarge picture
Measurement of blood pressure

blood pressure

The force that blood in the circulation exerts on arterial walls, 2º to myocardial contraction, in response to various demands (e.g., exercise, stress, sleep), which is divided into systolic (due to heart contractions) and diastolic (relaxation phases). Blood pressure (BP) varies with age and sex.
Standard level for normal systolic BP
Adults—BP = 100 + age;
Children—BP = 2 x age + 80.

Standard level for normal diastolic BP
± 2/3 of above.

Normal BP
120/80 mm Hg.

blood pressure

Cardiology The force that blood in the circulation exerts on arterial walls, 2º to myocardial contraction in response to various demands–eg, exercise, stress, sleep, which is divided into systolic–due to heart contractions and diastolic–relaxation phases; BP varies with age and sex Rule of thumb for normal systolic BP–Adults BP = 100 + age; Children BP = 2 x age + 80; Diastolic BP should be ±2/3 Normal BP 120/80 mm Hg. See Hypertension, Hypotension, Sphygmomanometer–blood pressure cuff.

blood pres·sure

(BP) (blŭd presh'ŭr)
The pressure or tension of the blood within the systemic arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric pressure.

blood pressure



Enlarge picture
BLOOD PRESSURE: Relationship of blood pressure to changes in cuff pressure and the first and fifth Korotkoff sounds (BP 120/80)
The tension exerted on the walls of arteries by: the strength of the contraction of the heart; the resistance of arterioles and capillaries; the elasticity of blood vessels; the blood volume; and blood viscosity.

Normal blood pressure is defined as a systolic BP between 100 and 120 mm Hg and a diastolic BP below 80 mm Hg (in adults over age 18). Prehypertension is present when measured blood pressures are between 120 and 140 mm Hg systolic or between 80 and 90 mm Hg diastolic. When either the systolic pressure exceeds 140 mm Hg or the diastolic exceeds 90 mm Hg, and these values are confirmed on two additional visits, stage I hypertension (high blood pressure) is present. See: illustration

Low blood pressure is sometimes present in healthy individuals, but it indicates shock in patients with fever, active bleeding, allergic reactions, active heart disease, spinal cord injuries, or trauma. Blood pressure should be checked routinely whenever a patient sees a health care provider because controlling abnormally high blood pressure effectively prevents damage to the heart and circulatory system as well as the kidneys, retina, brain, and other organs.

Patient care

Elevated blood pressures should first be addressed by giving advice to patients about lifestyle modifications, such as limiting the intake of alcohol, following a diet approved by the American Heart Association, and increasing the level of physical exercise. Weight loss in obese patients is also advisable. Medications are added to lifestyle instructions most of the time. Antihypertensive medications are used according to evidence-based guidelines and the side effects these drugs may cause in particular patients. Diuretics, for example, are esp. helpful in blacks and elderly patients (but may be inadvisable in patients with gout); beta blockers are the drugs of choice in patients with a history of myocardial infarction (but would be contraindicated in patients with advanced heart block); alpha blockers are well suited for men with prostatic hypertrophy; and angiotensin-converting enzyme inhibitors prevent kidney disease in patients with diabetes mellitus. Other antihypertensive drug classes include the angiotensin II receptor antagonists, centrally active alpha antagonists, and calcium channel blockers. Low blood pressure is not treated in healthy patients; in patients with acute illnesses, it is often corrected with hydration or pressor agents.

augmented diastolic blood pressure

An increase in diastolic pressure, usually by an artificial device, such as an intra-aortic balloon pump.
See: intra-aortic balloon counterpulsation

central blood pressure

Blood pressure in the heart chambers, in a great vein, or close to the heart. If determined in a vein, it is termed central venous pressure; if in the aorta or a similar large artery close to the heart, it is designated central arterial pressure.

chronic low blood pressure

A condition in which the systolic blood pressure is consistently less than 100 mm Hg. In the absence of associated disease, low blood pressure is often a predictor of longevity and continued health.
See: hypotension; orthostatic hypotension

diastolic blood pressure

The blood pressure when the ventricles of the heart are filling with blood. In health this equals about 60 to 80 mm Hg.

direct measurement of blood pressure

Determination of the blood pressure within the lumen of an artery or within a chamber of the heart with a catheter introduced into the organ and attached to a pressure-monitoring transducer. It is done by placing a sterile needle or small catheter inside an artery and having the blood pressure transmitted through that system to a suitable recorder. As the blood pressure fluctuates, the changes are recorded graphically.

high blood pressure


indirect measurement of blood pressure

A simple external method for measuring blood pressure.

Palpation method: The same arm, usually the right, should be used each time the pressure is measured. The arm should be raised to heart level if the patient is sitting, or kept parallel to the body if the patient is recumbent. The patient's arm should be relaxed and supported in a resting position. Exertion during the examination could result in a higher blood pressure reading. Either a mercury-gravity or aneroid-manometer type of blood pressure apparatus may be used. The blood compression cuff should be the width and length appropriate for the size of the subject's arm: narrow (2.5 to 6 cm) for infants and children and wide (13 cm) for adults. The inflatable bag encased in the cuff should be 20% wider than one third the circumference of the limb used. The deflated cuff is placed evenly and snugly around the upper arm so that its lower edge is about 1 in above the point of the brachial artery where the bell of the electronic sensor will be applied. While feeling the radial pulse, inflate the cuff until the pressure is about 30 mm above the point where the radial pulse was no longer felt. Deflate the cuff slowly and record as accurately as possible the pressure at which the pulse returns to the radial artery. Systolic blood pressure is determined by this method; diastolic blood pressure cannot be determined by this method.

This method is used for both continuous and intermittent readings, and while it formerly was used primarily in ICUs, it now is used routinely by nursing assistants on units throughout health care agencies and in clinics and physicians' offices. Measuring blood pressure at the wrist is more comfortable than a conventional BP cuff because it derives readings without pumping a bladder full of air, and with accuracy rivaling direct measurement from an arterial catheter. The sensor is placed directly over the radial artery and connected to an electronic monitor. Pressure is monitored every 15 heartbeats and systolic, diastolic, mean arterial pressure, waveforms, and pulse rate are displayed. The first reading appears in 15 seconds, and the sensor measures pressures from 40 to 240 mm Hg, with preset alarms to alert the nurse to extreme highs and lows. Results are not affected by low cardiac output, arrhythmias, hypothermia, or obesity, and this method is being used increasingly on adults in hospital special care units where frequent serial readings are required.

Auscultatory method: Begin as above. After inflating the cuff until the pressure is about 30 mm above the point where the radial pulse disappears, place the bell of the stethoscope over the brachial artery just below the blood pressure cuff. Then deflate the cuff slowly, about 2 to 3 mm Hg per heartbeat. The first sound heard from the artery is recorded as the systolic pressure. The point at which sounds are no longer heard is recorded as the diastolic pressure. For convenience the blood pressure is recorded as figures separated by a slash. The systolic value is recorded first.

Sounds heard over the brachial artery change in quality at some point prior to the point the sounds disappear. Some physicians consider this the diastolic pressure. This value should be noted when recording the blood pressure by placing it between the systolic pressure and the pressure noted when the sound disappears. Thus, 120/90/80 indicates a systolic pressure of 120 with a first diastolic sound change at a pressure of 90 and a final diastolic pressure of 80. The latter pressure is the point of disappearance of all sounds from the artery. When the values are so recorded, the physician may use either of the last two figures as the diastolic pressure. When the change in sound and the disappearance of all sound coincide, the result should be written as follows: 120/80/80.

mean blood pressure

The sum of twice the diastolic blood pressure plus the systolic blood pressure, all divided by 3.

negative blood pressure

Blood pressure that is less than atmospheric pressure, as in the great veins near the heart.

normal blood pressure

A blood pressure between 100 and 120 mm Hg systolic and < 80 mm Hg diastolic.

systolic blood pressure

Blood pressure during contraction of the ventricles. It is normally 100 to 120 mm Hg. Higher systolic blood pressures are found in prehypertension and hypertension.
Synonym: systolic pressure

blood pressure

The pressure exerted on the artery walls and derived from the force of the contraction of the lower chambers of the heart (the VENTRICLES). Blood pressure changes constantly. Peak pressure is called the systolic pressure and the running pressure between beats is called the diastolic pressure. Blood pressure in measured in millimetres of mercury. A typical normal reading is 120/80. See also HYPERTENSION and KOROTKOFF SOUNDS.
Blood pressureclick for a larger image
Fig. 74 Blood pressure . The variation of blood pressure in different vessels.

blood pressure

the force exerted by blood against the walls of the blood vessels, caused by heart contractions forcing a constant volume of blood round a closed system. Strong contraction of the left ventricle (SYSTOLE) ejects blood at high pressure into the AORTA, stretching the arterial walls. When the heart relaxes (DIASTOLE), force is no longer exerted on the arterial blood so pressure drops, although maintenance of pressure is helped by elastic recoil of the arterial walls. These oscillations of blood pressure are largest in the aorta, gradually diminishing as the blood flows along the arteries, becoming nonexistent in the CAPILLARIES.

The level of blood pressure also decreases from heart to tissue and back to the heart, these differences in pressure enabling the flow of blood around the system.

Blood in the veins is prevented from moving backwards by the presence of one-way valves. Venous circulation is also enhanced by activity of the skeletal muscles, hence leg and arm movements aid blood flow back to the heart.

Note that, although the comments above refer to the systemic circulation, a similar situation also applies in the smaller pulmonary system of mammals (see BLOOD CIRCULATORY SYSTEM).

Several factors control the exact level of blood pressure:

  1. heart action (rate of heartbeat, force per beat, volume per beat);
  2. peripheral resistance to blood flow in the capillary beds, caused by friction;
  3. elasticity of arteries;
  4. total blood volume (the higher the volume the higher the pressure);
  5. viscosity of blood (an increase in viscosity causes an increase in blood pressure but a decrease in flow rate).
Arterial blood pressure as recorded over two cardiac cycles, at heart rate 60 beats per minute.

blood pressure

the pressure exerted on the blood vessel walls. Normally refers to the arterial blood pressure, usually expressed in millimetres of mercury (mmHg) because of traditional sphygomanometry (which measures the height of a column of mercury sustained by the pressure in an inflated cuff around the arm that occludes the blood flow) but now more commonly measured by automated strain-gauge devices. Arterial blood pressure fluctuates with each heart beat between a maximum systolic pressure (SBP) during the ejection of blood from the heart and a minimum diastolic pressure (DBP) when the heart is relaxed with the aortic and pulmonary valves closed; the pulse pressure is the difference between them. When blood pressure is measured, values for both are quoted, e.g. 120/70 mmHg. May also be recorded directly from a cannula in an artery linked to a pressure transducer. mean (arterial) blood pressure (MBP) a mean value averaged over the cardiac cycle, derived from the arterial pulse pressure wave; typically closer to the diastolic than the systolic pressure such that MBP = DBP+(SBP-DBP)/3. See also hypertension, hypotension.


connective tissue, consisting of fluid plasma in which is suspended cells (e.g. erythrocytes, leukocytes, platelets), soluble proteins (e.g. immunoglobulins, enzymes) and salts (e.g. calcium ions) circulating the body through arteries, capillaries and veins, transporting oxygen, nutrients and drugs to tissues and removing carbon dioxide and metabolic byproducts from the tissues for excretion


force per unit area exerted by a gas/liquid against the walls of its container, or a solid (e.g. foot) against the contact/support surface
  • blood pressure; BP pressure/tension of arterial blood, maintained by ventricular contraction, arteriolar and capillary resistance, arterial wall elasticity and circulating blood viscosity and volume; recorded (using sphygmomanometer and stethoscope or automated blood pressure recorder) by occluding the brachial artery at heart level; as cuff pressure is reduced, blood flow gradually restores; systolic and diastolic BP are noted (Korotkoff's sounds) in mmHg, and expressed as a ratio (systolic/diastolic); normal adult BP = 120/80mmHg; BP is often raised in older people and in diabetes (see antihypertensive agents; hypertension)

  • diastolic BP lowest value (in mmHg) of recorded BP and minimum pressure at which the arterial system operates; occurs at the point of heart muscle relaxation; noted as the last audible arterial bruit (when impedance to blood flow [imposed by the deflating sphygmomanometer cuff] has fully reduced), and arterial blood flow is no longer restricted, and therefore silent; see pressure, systolic BP

  • partial pressure pressure exerted by a gas in a liquid, e.g. pressure of oxygen in blood (P O2); see pulse oximeter

  • systolic BP highest value (in mmHg) of recorded BP and maximum pressure at which the arterial system operates; occurs at the point of heart muscle contraction; noted as the first audible arterial bruit (when impedance to blood flow [imposed by the fully inflated sphygmomanometer cuff] has reduced [following opening of the pressure valve screw]) and arterial flow can just occur, at maximal cardiac contraction

blood pressure,

n force applied by circulating blood on the walls of the blood vessels and on the chambers of the heart. The pressure in a healthy individual varies but is usually considered below 120 mm Hg during systole and 80 mm Hg during diastole in adults.
Blood pressure.
CategorySystolic (mm Hg)Diastolic (mm Hg)
High normal130–13985–89
 Stage 1 (mild)140–159or90–99
 Stage 2 (moderate)160–179or100–109
 Stage 3 (severe)≥180or≥110


An instrument for measuring the arterial blood pressure. There are various types, the most common consisting of an inflatable cuff that is placed around the upper arm (usually the left) and air pressure within the cuff is balanced against the pressure of the blood in the brachial artery. The pressure is estimated by means of a mercury or an aneroid manometer. A stethoscope is normally used in conjunction with the instrument to listen to the blood pressure sounds (a stethoscope is not needed with an electronic sphygmomanometer). Normal systolic and diastolic blood pressures in a young adult are about 120/80, respectively. The difference between the two pressures is called the pulse pressure. Blood pressure varies with age, gender, altitude, disease, stress, fear, excitement, exercise, etc. A normal range for systolic pressure is usually considered to be 100-140 mmHg and for diastolic pressure below 90 mmHg. See arteriosclerosis; hypertension; hypertensive retinopathy.

blood pres·sure

(BP) (blŭd presh'ŭr)
Pressure or tension of the blood within the systemic arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric pressure.

blood pressure,

n the pressure exerted on the arterial walls by the blood when the heart is in systole (systolic pressure), and the pressure maintained by the elasticity of the arteries when the heart is in diastole (diastolic pressure). A consistent arterial pressure greater than 120 over 80 is considered high and suggestive of hypertensive vascular disease. See also hypertension, systole, diastole.
blood pressure classification,
n the rating system for blood pressure levels in millimeters of mercury (mm Hg), given as the systolic over the diastolic pressure. Both the systolic and diastolic pressure, if at increased levels, are indicators of concern for cardiovascular problems. Normal is less than 120 over 80; prehypertension is 120-139 over 80-89, stage 1 hypertension is 140-159 over 90-99; stage 2 hypertension is 159 or higher over 99 or higher. See also hypertension.
blood pressure, diastolic,
n the pressure in the bloodstream when the heart relaxes and dilates, filling with blood. See also blood pressure; blood, pressure, stages; and diastole.
blood pressure, systolic,
n the pressure exerted on the bloodstream by the heart when it contracts, forcing blood from the ventricles of the heart into the pulmonary artery and the aorta. See also blood pressure; blood, pressure, stages; and systole.
blood pressure cuff,
n a part of a sphygmomanometer that fits over the patient's arm. It comes in four sizes, for children up to obese adults. It should be made of a nonelastic material, and the cuff used should be about 20% bigger than the arm it fits over–an undersized cuff will cause the blood pressure reading to appear higher than it is in reality, whereas an oversized cuff will cause the reading to appear too low.
blood pressure stages,
n any of the three stages of hypertension marked by elevated blood pressure. Stage I is 140-159 over 90-99; Stage II is 160-179 over 100-109; Stage III is 180-209 over 110-119.

blood pressure

the pressure of the blood in the blood vessels. The term usually refers to the pressure of the blood within the arteries, or arterial blood pressure. This pressure is determined by several interrelated factors, including the pumping action of the heart, the resistance to the flow of blood in the arterioles, the elasticity of the walls of the main arteries, the blood volume and extracellular fluid volume, and the blood's viscosity, or thickness.
Relatively simple Doppler instruments can provide accurate blood pressure measurements in dogs and cats. The systolic pressure in dogs is 132±22 mmHg; in cats it is 108±23 mmHg. Thoroughbreds have been shown to be 112/77 mmHg. Indwelling catheters can be used in dogs to monitor central venous pressure.

arterial blood pressure
the common measure of blood pressure. The measurement in animal patients must be by a method that does not require entrance to an artery, i.e. noninvasive. Standard methods use an inflatable cuff around a limb, around the tail in the horse, and measurement of the air pressure required to obliterate the pulse wave—the systolic blood pressure, and permit the re-entry of the pulse wave—the diastolic blood pressure.
blood pressure homeostasis
the maintenance of a steady state of blood pressure. The mechanisms involved include the baroreceptor mechanism, the chemoreceptor mechanism, the ischemic response of the central nervous system (the Cushing response), the renin-angiotensin vasoconstrictor and the renin-angiotensin-aldosterone system, the capillary fluid-shift mechanism, the regulation of body fluid level by the kidney and the stress-relaxation mechanism of the arterial wall.
blood pressure impedance
the resistance to pulsatile flow, as in arteries.
pulmonary wedge blood pressure
see wedge pressure.
blood pressure regulation
the complex regulatory system which controls arterial blood pressure is dependent on sensory inputs related to cardiac output, peripheral resistance to blood flow at the arterioles, the viscosity of the blood, the volume of blood in the arterial system, the elasticity of the arterial walls. Changes in blood pressure are brought about by the control exerted on the same physiological mechanisms.
venous blood pressure
see central venous pressure.

Patient discussion about blood pressure

Q. how can i reduce my blood pressure?

A. The main steps in lowering high blood pressure is to take some very important changes in lifestyle- consuming much less salt in food, losing weight and exercising regulary. If this doesn't help (and usually it doesn't help mainly when people don't try hard enought and make an effort), medications can be added to control the blood pressure.

Q. what do i need to do to bring down my blood pressure? what cause a high blood pressure? what are the risks? of high blood pressure ? how can i deal with it effectively ?

A. here are two really good sites that show you specifics: Hope this helps.

Q. What Are the Complications of High Blood Pressure? My wife suffers from high blood pressure. What are the possible complications that are so dangerous? Why is it important to keep high blood pressure under balance?

A. While elevated blood pressure alone is not an illness, it often requires treatment due to its short and long-term effects on many organs. The risk is increased for: Cerebrovascular accident (CVAs or strokes), myocardial infarction (heart attack), hypertensive cardiomyopathy (heart failure due to chronically high blood pressure),hypertensive retinopathy - damage to the retina, hypertensive nephropathy- chronic renal failure due to chronically high blood pressure and hypertensive encephalopathy- confusion, headache and convulsions due to edema in brain that can lead to death. Therefore, it is considered very important to reduce blood pressure to normal levels with strict medical supervision.

More discussions about blood pressure
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It has long been thought that, in general, lowering alcohol intake reduces cardiovascular risk by reducing blood pressure.
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The average response of heart rate and blood pressure at each hour for each rat was calculated for room air, filtered air, and concentrated particles.
It's great to keep blood pressure down without medication, but if it's still high, people shouldn't avoid drugs," says Meir Stampfer of the Harvard School of Public Health.
High blood pressure makes your heart work harder and, over time, can damage blood vessels throughout your body.