Blood pressure

Definition

Blood pressure refers to pressure against the walls of blood vessels when the heart pumps blood through the arteries, the main oxygen-carrying vessels of the circulatory system.

Description

Blood pressure is considered a vital sign and is important to overall health because of its direct relationship to heart function, the functioning of arteries and other blood vessels, and maintaining blood flow so that all limbs and organs receive oxygen carried in the blood. Two opposite forces create blood pressure: 1) the pumping force of the heart as it sends blood to the arteries and into circulation in the body, and 2) the force arteries exert against the blood flow. The two types of pressure are measured as two separate numbers. Pressure measured as the heart contracts to send blood out through arteries to the body is systolic pressure, the upper number; and pressure measured as the heart rests in-between beats is diastolic pressure, the lower number. For example, 120/80, described as 120 over 80, is the systolic reading first and then the diastolic, each representing millimeters of mercury (mmHg). A reading less than 120mmHg systolic and less than 80mmHg diastolic is considered normal. Systolic pressure is known to increase in men and women as they age, increasing especially over age 60. Diastolic pressure usually increases before age 50, either by itself or as the systolic pressure increases. Both are important indicators of health status and risk for heart disease .

For each individual, blood pressure depends on the interaction of several body conditions, including total blood volume, amount of blood being pumped by the heart, amount of resistance from blood vessel walls, distribution of blood to body parts, nervous system condition and activity, and influence of certain body hormones. Blood pressure abnormalities include both high blood pressure (hypertension ) and low blood pressure (hypotension).

High blood pressure or hypertension

Treatment for high blood pressure is one of the most common reasons for doctor's office visits but, according to the American Heart Association, only 27% of people with high blood pressure are able to keep it under control. High blood pressure can have a significant impact on health, especially heart health. Higher pressure increases the heart's workload and stresses arteries while causing growth of tissue in artery walls (atheroma); as pressure climbs, the heart walls become thicker and the heart enlarges and becomes weaker. High blood pressure can be primary, occurring alone and leading to other health problems; or it can be secondary, the result of another condition or disease that affects the vascular system. High blood pressure is an indication of risk for heart problems (cardiovascular disease). The American Heart Association reports a direct correlation between high blood pressure and heart attack , heart failure, stroke and kidney disease. High blood pressure can also lead to other diseases and conditions such as obesity , diabetes and thyroid problems.

Low blood pressure or hypotension

Low blood pressure occurs less often as an ongoing condition requiring treatment. A momentary drop in blood pressure can occur simply by changing position from sitting to standing (postural hypotension), making the individual feel dizzy. In some individuals, it may be normal to have a lower-than-normal blood pressure without also having blood supply problems; this kind of low pressure may actually help protect against heart disease. Other individuals with consistently low blood pressure may develop a type of heart disease (ischemic heart disease) in which not enough oxygen is supplied to the heart muscle. Low blood pressure occurs most often in crisis situations when an individual has a severe condition or disease state, including hemorrhage, overdose of drugs, hormone abnormalities, or systemic blood poisoning (sepsis). Low arterial pressure is a sign that blood flow has decreased significantly, resulting in not enough blood and oxygen being supplied to the brain. This can cause dizziness , weakness, loss of consciousness and, with failure of other organ systems, death .

Demographics

The U.S. Public Health Service reports that half of all Americans over age 65 have high blood pressure, about 50 million altogether. The World Health Organization (WHO) reports that one billion people around the world have high blood pressure and 7.1 million die of blood pressure-related disease each year. Men are more often affected than women, except in women after menopause when incidence is equal to men. African-Americans are estimated to be 18 times more likely than Caucasians to develop high blood pressure in later adult life. Low blood pressure can occur in any adult of any age, gender or race, usually in conjunction with a severe disease state; WHO reports that low blood pressure is the number one risk factor associated with deaths around the world.

Causes and symptoms

High blood pressure often has no symptoms and, for this reason, is sometimes called the silent killer. Having blood pressure measured once or twice does not always reveal high blood pressure, since the level changes from hour to hour, day to day. Some people have higher blood pressure when they visit their doctor, a condition called “the white coat effect”; it happens simply because the visit may be associated with some degree of anxiousness. Blood pressure normally increases as people age, and the normal range becomes higher for people over age 65 because flexibility of the arteries decreases.

Risk factors identified for high blood pressure include cigarette smoking , use of alcohol or drugs, lack of exercise, stress , obesity, frequent use of stimulants such as caffeine , and excessive use of salt. The intake of sodium or salt in the diet increases risk by increasing water retention in body tissues, which in turn creates greater pressure on blood vessel walls. During stress, the stress response causes artery walls to constrict, disturbing their function. People with a high incidence of high blood pressure in their families are also at greater risk of developing it themselves.

Low blood pressure can be a characteristic of the individual that is tolerated and not treated, but is more often caused by a severe, life-threatening condition related to a disease state. Hemorrhage, drug overdoses, heart or kidney failure, blood poisoning, or another condition that overwhelms the body can cause blood pressure to drop dramatically. Symptoms of low blood pressure range from light-headedness to dizziness to weakness and unconsciousness.

Diagnosis

The initial diagnostic test for high blood pressure is usually an elevated blood pressure discovered in a regular doctor's office visit. If an abnormal blood pressure is suspected, the doctor completes a health history and history of family blood pressure problems. A complete physical examination with laboratory tests (blood count, blood chemistries and urinalysis) and cardiac testing (electrocardiogram) are done to determine overall health, especially risk of heart disease. Arterial blood pressure is measured as is blood pressure in the arms and legs (peripheral blood pressure). Several types of blood pressure measurements may be used, including the familiar mercurybased method (sphygmomanometer), which requires wrapping a pressure cuff around the arm and tightening it by squeezing a bulb, then reading the results on a meter. Other digital and mechanical methods are used, primarily in hospitals. Home methods are available for self-monitoring.

Treatment

Treatment for high blood pressure involves use of pressure-lowering drugs and changes in diet and lifestyle. Lifestyle changes are usually recommended first, and diet is often a significant factor. A vegetarian diet has been shown to be associated with lower blood pressure. A way of eating shown to reduce blood pressure is described in Dietary Approaches to Stop Hypertension, known as the “DASH Diet.” It is recommended by the National Institutes of Health (NIH), the American Heart Association, and most heart doctors. The DASH diet consists of fruits, vegetables, low-fat dairy, whole grains, poultry, fish and nuts. A group of adults tested after consuming this diet for a period of time all experienced significant reductions in blood pressure. Other recommended changes include reducing salt intake, caffeine and alcohol. Increasing physical activity is often recommended, including exercise as simple as walking. Weight loss may be an important factor for some individuals. Clinical trials have shown that reducing weight is able to reduce both systolic and diastolic blood pressure levels. Weight loss programs are a combination of low-fat-diet and an exercise regimen that involves use of the long muscles in the legs. It has not been shown that weight loss prevents the rise in blood pressure that occurs normally as people age, even if these individuals are on a diet that restricts salt intake. Individuals with diabetes or heart disease need to treat the underlying disease as well. Physicians may recommend stress reduction for some individuals.

If weight loss, exercise and dietary changes alone do not make a significant difference in blood pressure, the physician may prescribe drugs to reduce fluid levels (diuretics ) and blood pressure reducing medications (anti-hypertensive drugs) such as beta blockers .

Nutrition/Dietetic concerns

Reducing salt intake and increasing potassium in foods or with supplements is part of any diet for lowering blood pressure. Whether an individual should follow a salt-free, low-salt, or no-salt-added diet depends on the degree of elevation of blood pressure. For some, it may be sufficient to avoid adding table salt to foods for taste. Eliminating hidden salt found in a wide variety of foods can also be avoided. Canned and commercially prepared foods such as packaged meals and meats, pizza, entrees and soups, including bouillon are examples. Other high-salt items to avoid include chips, sauerkraut, anchovies and some ocean fish, olives and pickles, cheeses, and most meats, including pork, beef and all meats preserved with sodium. Anything containing sodium should be avoided, including food additives MSG (monosodium glutamate) and Accent, baking soda, diet sodas, softened water, meat tenderizer, soy sauce, and sodium-based medications such as naproxen sodium (Advil and Nuprin).

Other foods and nutritional supplements known to help manage blood pressure include:

• spices and herbs: cayenne, parsley, rosemary and chamomile
• a high-fiber diet with plenty of whole grains such as brown rice and oatmeal
• plant proteins (e.g., peas, beans, soy) rather than animal proteins
• whole fruits and vegetables, especially apples, bananas, broccoli, leafy greens, squash and sweet potatoes
• foods containing potassium such as dairy products, fish, beans poultry, bananas, avocados, potatoes, vegetables and whole grains
• maitake, shitake and reishi mushrooms
• garlic as a food or supplement: contains quercitin, shown to be effective in reducing blood pressure
• calcium and magnesium taken as supplements
• essential fatty acids as in flax oil or fish oil, which help reduce blood pressure and improve circulation by aiding digestion of fats and reducing cholesterol
• Coenzyme Q10, which helps to improve heart function and lower blood pressure
• Vitamin C , shown to reduce blood pressure
• Vitamin B complex: important for blood circulation; vitamin B6 is a mild diuretic that helps reduce fluid content in body tissues, relieving pressure in the vascular system

QUESTIONS TO ASK YOUR DOCTOR

• How often should I have my blood pressure checked?
• Is there something specific I can do to lower my blood pressure?
• What circumstances or habits might raise my blood pressure?
• How many of your patients have successfully lowered their blood pressure and what helped them most?

Prognosis

High blood pressure is treatable and can be reduced or reversed with appropriate treatment and lifestyle changes. Any elevation of blood pressure can shorten an individual's life expectancy. Severely high blood pressure (twice normal) can not be tolerated for more than one to two years. Extremely low blood pressure associated with life-threatening conditions stemming from a severe disease state may result in critical oxygen depletion (hypoxia) and death.

Prevention

Lifestyle measures shown to help prevent high blood pressure include those recommended for reducing high blood pressure:

• not smoking
• reducing caffeine and alcohol consumption
• reducing dietary fat, especially unsaturated fats
• avoiding prepared foods (e.g., packaged dinners and pizza) and refined foods such as white flour and sugar
• eating a whole foods diet consisting of fruits, vegetables, low-fat dairy, whole grains, poultry, fish and nuts
• exercising regularly

Resources

BOOKS

Balch, P. A. “High Blood Pressure.” Prescription for Nutritional Healing. Garden City Park, NY: Avery, 1997.

Heller, Marla. The DASH Diet Action Plan. Deerfield, IL: Amidon Press, 2007.

Oparil S. “Arterial Hypertension.” Cecil Textbook of Medicine, 21st ed. Edited by Goldman et al. Philadelphia: W.B. Saunders, 2000.

KEY TERMS

atheroma —Deposits of fat accumulating inside arteries.

atherosclerosis (arteriosclerosis) —A condition in which fat (lipids) deposits (plaque) in medium and large arteries encourage hardening (fibrosis) and calcification of artery walls that may eventually block blood flow.

beta blocker —A type of drug that acts on the heart to slow the heart rate and reduce the force of theheart's contraction in order to reduce blood pressure.

cardiovascular —Descriptive term referring to anything related to the heart, blood vessels (veins, arteries), and blood circulation.

circulatory system —The circular course followed by the blood when it leaves the heart through arteries and returns through veins.

diuretic —A substance or drug that encourages urinary output. Used to remove excess fluid from the body to relieve extra strain on the heart and reduce high blood pressure.

hypoxia —A condition characterized by reduced oxygen levels in the blood and/or body tissues. Can be the result of a drop in blood pressure.

ischemia (ischemic) —A form of anemia in specific areas where blood flow has been blocked or reduced, such as a blocked artery or reduced blood flow in a specific organ. An ischemic process can cause pain and organ dysfunction.

vascular system —A term that describes the network of veins and arteries in the body.

PERIODICALS

Chobanian, A. V., et al. “Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure.” Hypertension Dallas: American Heart Association, 2003.

OTHER

“Healthy Eating.” Your Guide to Lowering High Blood Pressure. 2006 [cited April 5, 2008]. National Heart Lung and Blood Institute. http://www.nhlbi.nih.gov/hbp/prevent/h_eating/h_eating.htm.

ORGANIZATIONS

American Heart Association, 7272 Greenville Avenue, Dallas, TX, 75231-4596, (888) AHA-2453, (214) 706-5262, www.americanheart.org.

L. Lee Culvert

Blood Pressure

Definition

Blood pressure is the pressure of the blood against the blood vessel walls as blood flows through the vessels.

Description

The heart beats about 60 to 70 times a minute. With each beat as the heart contracts, a surge of blood is pumped from the heart into the arteries. The pressure in the artery walls during this surge is measured as the systolic blood pressure (a higher number). Between beats, the heart is relaxed and there is much less pressure on the artery walls. This is measured as the diastolic blood pressure (a lower number). Blood pressure is given as two numbers written as 120/80 mm Hg and is measured with a device called a sphygmomanometer in millimeters (mm) of mercury (Hg). The pressure depends on the amount of blood pumped through the heart in addition to the resistance and elasticity of the blood vessels to the amount of blood flowing.

Function

Blood pressure is necessary to sustain life. It continuously forces blood carrying oxygen and nutrients from the heart to the organs and tissues of the body. Blood pressure levels can go up or down in the course of a day depending on activity and stress levels, medications, or diet.

A person's blood pressure is determined by the contraction of the heart's ventricles, which pump blood into the aorta and subsequently throughout the body. The normal adult blood pressure has a systolic number of 120 and a diastolic number of 80. Systolic pressure is taken when the heart contracts; diastolic pressure is taken when the heart is relaxed.

Normally, about 5.5 quarts (5.25 liters) of blood goes through the heart and blood vessels each minute, an amount called cardiac output. The body is dependent on its volume of blood to maintain blood pressure. If a person experiences heavy blood loss, blood pressure will plunge. Similarly, an increase in blood volume, in cases like water retention, will cause blood pressure to rise.

The brain's medulla contains a cluster of nerves, called the cardiovascular center, that control heart rate, the contraction of the ventricles, and blood vessel diameter. Sensory receptors monitor the stretching of blood vessel walls. During exercise, the heart rate rises and the ventricles contract more forcefully. The cardiovascular center then monitors the dilation (expansion) or constriction of peripheral blood vessels. For example, the blood vessels to organs directly involved the exercise will expand. Blood flow to skeletal muscles may increase by a factor of 10 and that to the heart and skin can triple. Simultaneously, constriction will occur in the blood vessels of the digestive system.

The sensory receptors in the walls of blood vessels continually monitor blood pressure. When the receptors detect an increase in aortic pressure, for example, the cardiovascular center directs the lowering of the heart rate and the stretching of blood vessels, which decreases the blood pressure. A decrease in blood pressure causes an increased heart rate and vasoconstriction.

As people age, the blood vessels become less flexible and the heart muscle is less strong, resulting in a smaller output and lower maximum heart rate. Systolic pressure tends to rise as a person ages. Coronary artery disease, which causes the blood vessels in the heart to receive inadequate oxygenation, can cause chest pain or heart attack. Atherosclerosis (clogging of the arteries) can also cause an increase in blood pressure.

Role in human health

The Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC) develops high blood pressure prevention and control recommendations for healthcare providers. According to the JNC, optimal blood pressure (BP) measurement is a systolic blood pressure (SBP) of 120 mm Hg or less and a diastolic blood pressure (DBP) of 80 mm Hg or less. Blood pressure is still considered normal at levels of 130 mm Hg SBP or less and 85 mm Hg DBP or less.

Periodic blood pressure measurement is recommended every one to two years for adults with normal blood pressure. A healthcare provider should determine the frequency of blood pressure measurement based on each patient's individual risk factors for high blood pressure. Individual risk factors that contribute to high blood pressure, such as diabetes, a family history of high blood pressure, a diet high in fat and cholesterol, being African-American, elderly, overweight, a smoker, or heavy drinker, are important to consider when advising patients on the frequency of periodic blood pressure measurement. Prevention and management of high blood pressure requires not only active participation by the patient but also education and support from health care providers. Patient education is a shared responsibility among physicians, nurses, dietitians, and allied health professionals. While patient education is time-consuming, it is very important to the process of maintaining health and preventing disease.

Common diseases and disorders

High blood pressure, also called hypertension, is a cardiovascular disease affecting nearly 50 million Americans. The higher than normal pressure pushes blood against the artery walls causing the heart to work harder in order to pump blood to the body. The JNC defines high blood pressure as a systolic blood pressure (SBP) of 140 mm Hg or greater, a diastolic blood pressure (DBP) of 90 mm Hg or greater, or taking high blood pressure (antihypertensive) medications.

High blood pressure often has no warning signs or symptoms. So, if it is not identified or treated, high blood pressure can damage the arteries and organs causing serious medical problems over time. If not properly managed, high blood pressure can increase the risk of developing, among other problems, the following:

• Atherosclerosis, also called "hardening of the arteries"—High blood pressure can cause atherosclerosis or a thickening and narrowing of the blood vessel walls. This can slow or prevent blood flow through the arteries and may lead to heart attack or stroke.
• Stroke—High blood pressure can cause the arteries to narrow and lead to a stroke if a blood clot blocks one of the narrowed arteries (thrombolytic stroke) or if a weakened blood vessel in the brain ruptures (hemorrhagic stroke).
• Coronary heart disease—High blood pressure can cause the coronary arteries to narrow and harden. The coronary arteries carry oxygen to the heart muscle so it can function to pump blood to the body. If blood cannot flow properly through the coronary arteries to the heart, the heart cannot get enough oxygen. This can cause chest pain (angina). If the blood flow to the heart muscle is blocked, it can cause a heart attack. Heart disease is the leading cause of death in the United States.
• Congestive heart failure—Over years, uncontrolled high blood pressure can cause the heart muscle to compensate by becoming larger (dilatation) to allow more blood to fill it, by thickening the heart muscle (hypertrophy) to pump more forcefully, or by beating faster to increase circulation. According to the National Institutes of Health, uncontrolled high blood pressure increases the risk of heart failure by 200%, compared with those who do not have high blood pressure.
• Kidney failure—Over years, high blood pressure can damage the blood vessels of the kidney. The damage may cause the kidneys to no longer filter waste from the blood adequately, which could require dialysis treatment or possibly a kidney transplant.

The cause of high blood pressure is usually unknown, in which case it is called primary or essential hypertension. This cannot be cured. However, it can be easily diagnosed and, in most cases, controlled with lifestyle modifications and/or medications.

Lifestyle modifications

Some of the lifestyle modifications for high blood pressure prevention and management include:

• Weight loss if the patient is overweight. As weight increases, blood pressure rises.
• Cutting down on alcohol, no more than one drink per day for women and no more than two drinks per day for men.
• Decreasing salt and sodium, saturated fat, and cholesterol.
• Increasing physical activity, especially aerobic activity 30 to 45 minutes on most days.
• Stopping smoking.

Medications

High blood pressure medications work in various ways. They can affect the force of the heartbeat, the blood vessels, and the amount of fluid in the body. Some of the different types of medications prescribed to treat high blood pressure are:

• Diuretics, also called "water pills," decrease the amount of fluid in the body by flushing excess water and sodium from the body through the urine.

• Beta blockers make the heart beat less often and with less force by reducing nerve impulses to the heart and blood vessels.
• Calcium channel blockers relax the blood vessels by preventing calcium from entering the muscle cells of the heart.
• Alpha blockers relax the blood vessels by way of the nervous system. They decrease renin secretion, which is involved in angiotensin II formation.
• Vasodilators widen blood vessels by relaxing the muscle in the vessel walls.
• Angiotensin converting enzyme (ACE) inhibitors relax the blood vessels by preventing angiotensin II from being formed.

High blood pressure can sometimes be traced to a cause such as an adrenal gland tumor, kidney disease, hormone abnormalities, birth control pills, or pregnancy. This is called secondary hypertension and can usually be cured if the cause disappears or is corrected.

KEY TERMS

Angiotensin converting enzyme (ACE) inhibitor— A drug used to decrease pressure inside blood vessels.

Artery— A blood vessel that carries blood from the heart to the body.

Beta blocker— A drug used to slow heart rate and reduce pressure inside blood vessels.

Calcium channel blocker— A drug used to relax blood vessels and the heart muscle.

Cardiovascular— The heart and blood vessels.

Congestive heart failure— A cardiovascular disease that involves the heart muscle's diminished or loss of pumping ability, generally causes fluid that cannot be completely ejected from the heart to back up in the lungs.

Diastolic blood pressure— The lower number of a blood pressure measurement or the pressure when the heart is at rest.

Diuretic— A drug that eliminates excess fluid in the body.

Fat— One of the nutrients that supply calories to the body.

Hypertension— High blood pressure.

Hypertrophy— Enlargement of tissue or an organ.

Millimeter (mm)— A unit of measurement equal to one-thousandth of a meter.

Risk factors— Behaviors, traits, or conditions in a person that are associated with an increased chance (risk) of disease.

Sign— An objective observation of an illness.

Sphygmomanometer— A manual device used to measure blood pressure.

Symptom— Any indication of disease noticed or felt by a patient.

Systolic blood pressure— The higher number of a blood pressure measurement or the pressure when the heart is contracting.

Resources

BOOKS

Report of the United States Preventive Services Task Force. Guide to Clinical Preventive Services. International Medical Publishing, 1996.

Tortora, Gerard, and Sandra Grabowski. Principles of Anatomy and Physiology, 8th ed. New York: John Wiley and Sons, 1996.

ORGANIZATIONS

American College of Cardiology. Heart House. 9111 Old Georgetown Rd., Bethesda, MD 20814-1699. (800) 253-4636. 〈http://www.acc.org〉.

American Heart Association. National Center. 7272 Greenville Ave., Dallas, TX 75231. (800) AHA-USA1. 〈http://www.americanheart.org〉.

American Society of Hypertension. 515 Madison Ave., Ste. 1212, New York, NY 10022. (212) 644-0650. 〈http://www.ash-us.org〉.

National Heart, Lung, and Blood Institute. Information Center. PO Box 30105, Bethesda, MD 20824-0105. (800) 575-WELL. 〈http://www.nhlbi.nih.gov〉.

National High Blood Pressure Education Program. NHLBI Health Information Center. PO Box 30105, Bethesda, Maryland 20824-0105. (301) 592-8573.

OTHER

National Heart, Lung, and Blood Institute. Healthy Heart Handbook for Women. 1997. 〈http://www.nhlbi.nih.gov〉.

The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Pub No. 98-4080. November 1997.

blood pressure In a resting individual the left ventricle of the heart pumps typically 5 litres of blood each minute into the aorta and arteries of the body. Downstream, the small arterioles restrict the outflow of blood from the arteries and are therefore known as the main ‘resistance vessels’. The combined effect of the energy generated by the heart and the outflow restriction results in a distending pressure in the arterial system which is referred to as the blood pressure.

The first report of a direct measurement of arterial blood pressure was by Revd Stephen Hales in 1733. He inserted a tube into an abnormally exposed artery of a horse and observed that a column of blood rose in a glass tube to a vertical height of 8 ft 3 in. This represents the force generated by the heart and transmitted to all the major arteries in the body. We do not now express blood pressure as height in feet and inches of blood. However, we sometimes use centimeters of water, so that the horse's blood pressure would be 250 cm of water or blood. Such a column has obvious practical problems for measuring arterial pressure. But for venous pressures which are much lower, a column of saline connected to a major vein is often used clinically to assess the degree of filling of the circulation. Arterial pressure is usually expressed as millimetres of mercury (mm Hg) because mercury is 13.6 times as dense as water and a mercury column of that height is more practicable. Thus the horse's pressure blood would be 185 mm Hg. An alternative unit for expressing blood pressures, which has not been widely adopted in clinical practice, is the SI unit, the pascal or kilopascal (kPa). One kPa is approximately 7.5 mm Hg.

Blood pressure is not normally expressed as a single figure but rather as two, for example 120/80. This means that the pressure in the arteries varies with each heart beat to a peak, called systolic pressure, of 120 mm Hg, and then declines to a minimum value, called diastolic pressure, of 80 mm Hg just before the next beat. These phasic values of blood pressure can be recorded accurately using modern transducers (electronic measuring devices) connected to catheters (fine tubes) inserted into arteries. However, except for research and measurements during complex investigations in patients, blood pressure is not usually determined by direct puncture of an artery. The most common method is to use the device known as a sphygmomanometer. This is an inflatable cuff which fits round the upper arm and is connected to a mercury manometer. A stethoscope is applied to listen to the artery below the cuff. The cuff is first inflated with a pressure well above systolic and then slowly deflated. The systolic pressure is taken as the pressure in the cuff when the artery just opens and a sound is first heard. The diastolic pressure is that when the sound either becomes muffled or disappears completely.

Blood pressure, like all biological variables, varies widely in different people and, in the same individual, at different times of the day. Typically a normal value for systolic blood pressure would be 120 mm Hg at age 20, increasing perhaps to 140 mm Hg at 60. Diastolic pressure also increases with age but rather less. Estimates of blood pressure in apparently healthy people show values that can be 20 or even 40 mm Hg higher or lower than the average values. This, and the fact that blood pressure varies considerably during the day, particularly in response to stresses such as visiting a doctor, mean that it is very difficult to decide on the basis of a single measurement whether a patient suffers from hypertension (high blood pressure). Definitions of hypertension are constantly changing but, generally, if systolic pressure is consistently greater than 160 mm Hg or diastolic more than 95, a person is considered to be hypertensive.

At rest, each time the heart contracts, it ejects typically 70 ml of blood into the arterial system. This causes a steep increase in arterial pressure, the magnitude of which is dependent both on the volume ejected and on the distensibility of the arteries. Older people have less distensible arteries, which explains why their systolic blood pressure is usually higher than in younger subjects. Because the shape of the arterial pressure pulse is roughly triangular, the mean level of pressure is nearer to the diastolic value.

The importance of blood pressure is that it effectively provides a store of energy, generated by the heart, available to cause blood to flow through the working tissues. It is actually the flow of blood, providing oxygen and nutrients and removing waste products including carbon dioxide, which is really the important factor, but without pressure there would be no flow. Humans, being upright bipedal animals, have a particular problem in supplying blood to all parts of the body. Due just to gravity, pressure in arteries supplying the head is about 100 mm Hg less than that in arteries in the feet. The fact that the brain must have an adequate arterial pressure places a limitation on the range of effective pressures in the upright person.

Control of blood pressure

Mean blood pressure depends on the flow of blood from the heart (cardiac output) and the resistance to flow in the small arteries and microscopic resistance vessels (arterioles).BP = CO × PVRwhere BP is blood pressure, CO is cardiac output, and PVR is the peripheral vascular resistance or the net resistance to blood flow in all the small arteries and microscopic arterioles.

Peripheral vascular resistance is dependent on the radius (r) of the small blood vessels. In fact it turns out to be proportional to 1/r⁴. The equation for blood pressure can now be changed:BP ∝ CO/r⁴

The importance of the degree of constriction of resistance vessels can be seen from this equation because if cardiac output is unchanged a reduction in the average radius of the resistance vessels of only 10% would increase blood pressure by more than 50%. The physiological control of blood pressure is thus effected mainly by regulating the radius of the resistance vessels and, to a smaller extent, the cardiac output. Baroreceptors provide an effective means for detecting changes in blood pressure and bringing about appropriate responses, via the autonomic nervous system. If blood pressure started to fall the baroreceptor stimulation would decrease and the reflex response would cause the small resistance vessels to constrict and the heart to beat faster and harder, by action of the sympathetic nerves. This negative feedback mechanism largely restores the blood pressure. Conversely, if blood pressure increases, stimulation of baroreceptors gives rise to nerve impulses which run to the brain and stimulate activity in the parasympathetic pathway in the vagus nerves, which slows the heart; also inhibition of activity in sympathetic nerves decreases both the rate and force of contraction of the heart and dilates of both the resistance and the capacitance vessels (veins) (Fig. 1).

Some factors which affect blood pressure

Baroreceptors are important for minimizing changes in blood pressure: animal studies have shown that blood pressure is much more variable if the influence of baroreceptors is removed. However, they do not prevent all fluctuations from occurring. Continuous 24-hour recordings have been made in healthy volunteers and have shown variations of 30–80 mm Hg in systolic pressure and of 10–80 mm Hg in diastolic pressure. Blood pressure is particularly low during sleep, and high during physical activity or emotional stress.

Physical exercise

causes very major effects on the circulation. Due to the enormously increased blood flow through the exercising muscle, the amount of blood pumped by the heart may increase four-fold, or in elite athletes as much as six-fold. The increased volume of blood ejected at each heart beat causes systolic blood pressure to increase, perhaps to 180 mm Hg. However, because blood flows very rapidly out of the arteries, particularly to the working muscle where the resistance vessels are widely dilated, diastolic pressure remains relatively unchanged or may even decrease. Isometric exercise has quite a different effect. Here there is a much smaller effect on the total amount of blood pumped by the heart, but reflexes, particularly those arising from the contracting muscle itself, cause blood vessels elsewhere to constrict, and consequently both systolic and diastolic blood pressure rise sharply. This response may also be augmented by a straining effect (see below).

Emotional stress

can cause quite large increases in blood pressure. Prominent amongst the physiological responses to stress is an increase in activity in the sympathetic nerves. Sympathetic overactivity increases heart rate and force, and constricts resistance blood vessels (Fig. 1). All these effects increase both systolic and diastolic blood pressure and are augmented by increased secretion into the blood of adrenaline and noradrenaline.

Postural changes

exert stresses on the cardiovascular system requiring effective reflex responses to constrict arteries and veins and stimulate the heart, to control blood pressure, maintain brain blood flow, and prevent loss of consciousness. The upright position means that blood vessels below the level of the heart are subjected to increased distending pressures due to the effects of gravity. Veins are particularly susceptible to gravitational stress due to their distensibility, and blood ‘pools’ in dependent veins when we stand. Because of this, less blood flows back to the heart and, were it not for effective reflexes, involving baroreceptors, blood pressure would fall catastrophically, particularly in the brain, resulting in insufficient brain blood flow and consequent loss of consciousness. Blood pressure frequently falls transiently when we stand. This is particularly noticeable if we stand suddenly when warm, for example on getting out of a hot bath, because the resistance blood vessels initially will be dilated. In some people blood pressure control may be inadequate to counter the stress of postural changes and the result is that they faint.

Straining (the Valsalva manoeuvre)

induces large and complex variations in blood pressure. The sort of stresses that induce these changes include blowing against a resistance, lifting heavy objects, and straining at stool. The effects on the circulation are illustrated in Fig. 2. The primary change is caused by an increase in pressure within the chest (intrathoracic pressure) and within the abdomen. Normally, intrathoracic pressure is lower than atmospheric, due to the tendency of lungs to collapse and their prevention from so doing by the chest wall. This negative intrathoracic pressure aids the flow of blood to the heart from the peripheral veins. Straining causes the pressure in both the chest and the abdomen to become positive. Initially the compression of the heart and large arteries causes an increase in blood pressure. Then, the high pressure in the chest impedes the inflow of blood from peripheral veins (veins in the neck can be seen to distend), so the cardiac output decreases and blood pressure falls. Baroreceptors detect this fall and initiate constriction of blood vessels and an increase in heart rate, so that mean blood pressure is restored. At the end of the strain there is a transient fall in pressure before blood rushes back to the heart, causing an overshoot and often a transient slowing of the heart. In people with some autonomic nerve disorders these responses may be deficient: blood pressure falls continuously, and the overshoot is absent.

Roger Hainsworth

BLOOD PRESSURE

Blood pressure is a physiological variable—like body temperature, respiratory rate, or heart rate. Blood pressure is not constant throughout the day; each time the heart squeezes and relaxes, there is a new blood pressure. It increases before awakening and declines with sleep. The level of blood pressure is regulated by the kidneys, brain, heart, endocrine glands, and blood vessels. In the United States, the actual level of blood pressure gradually increases from birth to adulthood. Due to difference in diet and activity levels in nonindustrialized countries, however, blood pressure does not increase beyond the age of eighteen.

Whereas temperature is measured with a thermometer, blood pressure is measured with a sphygmomanometer, preferably a mercury sphygmo-manometer, though aneroid and electronic devices are sometimes used.

Blood pressure should be measured after a five-minute period of rest, with the back supported and the legs uncrossed. Constrictive clothing should be removed from around the upper arm, which must be resting on a table at heart level. The blood pressure cuff is evenly and snugly applied around the upper arm above the elbow, and a stethoscope is placed over the crease of the elbow. The cuff is inflated to 15 millimeters of mercury (mmHg) above the point where radial artery pulse (the artery above the thumb at the wrist) disappears. The pressure in the cuff is then slowly released at 2 mmHg per second. The first of two consecutive sounds as cuff pressure decreases is called the systolic blood pressure—the pressure to open the artery occluded with the cuff. The diastolic blood pressure is recorded at the absence of sounds with continued deflation of the blood pressure cuff. Blood pressure is generally recorded to the nearest 2 mmHg. For example, a blood pressure of 142/86 mmHg indicates a systolic blood pressure of 142 mmHg and a diastolic blood pressure of 86 mmHg. Pain and emotional disturbance, as well as caffeine, tobacco, and alcohol, can elevate systolic blood pressure.

HYPERTENSION

An abnormal blood pressure requires confirmation on two subsequent days. An optimal blood pressure is less than 120/80 mmHg. High blood pressure, or hypertension, is defined as either a systolic blood pressure greater than 140 mmHg or a diastolic blood pressure greater than 90 mmHg. Systolic blood pressure is a more powerful predictor of cardiovascular events than diastolic blood pressure. With increasing age, the diastolic blood pressure may actually decrease while systolic blood pressure increases; this indicates increased stiffening of the arteries throughout the body.

Hypertension is not a nervous disorder or an anxiety state, but rather a disease of the blood vessels that increases blood vessel constriction of the small arteries. It particularly damages the blood vessels inside the brain, heart, kidneys, eyes, and the largest artery, the aorta. Damaged arteries may rupture, thicken, or harden and narrow—resulting in strokes, heart attacks, kidney failure, visual impairment, or tearing or rupture of the aorta. Also, the left heart chamber thickens as a consequence of increased blood pressure. When the heart can no longer thicken or enlarge to overcome the increased pressure in the blood vessels, the squeezing function of the heart decreases, resulting in congestive heart failure.

CAUSES OF HYPERTENSION

Fifty million Americans (about one-fifth of the U.S. population) have hypertension (see Figure 1). Over 90 percent of the causes of hypertension remain unknown. Four groups are predisposed to developing hypertension: the obese, the elderly, diabetics, and African Americans. Certain drugs are known to elevate blood pressure, including most arthritis medications (except acetaminophen and aspirin), many cold remedies, nose sprays, weight-reducing pills, and alcohol. Increased heart rate, anemia, excessive thyroid hormone, or stiff

Figure 1

(nondistendible) arteries can increase systolic blood pressure. Blocked arteries to the kidney, kidney failure, and decreased production of thyroid hormone are common causes of hypertension. Other rare causes include tumors of the adrenal gland.

TREATMENT OF HYPERTENSION

Nondrug treatment of hypertension should include weight loss, salt restriction, smoking cessation, and alcohol restriction. A reduced saturatedand total-fat diet that is rich in fruits, vegetables, and low-fat dairy products lowers blood pressure in some individuals, avoiding the need for drug treatment. The treatment goal for uncomplicated hypertensives is below 140/90 mmHg. To achieve that goal consistently, most individuals will need to be treated with more than one drug. Treatment has been proven to decrease heart attacks, strokes, and heart failure, and is usually required throughout life.

L. Michael Prisant

(see also: Atherosclerosis; Cardiovascular Diseases; Coronary Artery Disease; Foods and Diets; Lifestyle; Nutrition )

Bibliography

"The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure." (1997). Arch Intern Med 157:2413–2446.

blood pressure The pressure exerted by the flow of blood through the major arteries of the body. This pressure is greatest during the contraction of the ventricles of the heart (systolic pressure; see systole), which forces blood into the arterial system. Pressure falls to its lowest level when the heart is filling with blood (diastolic pressure; see diastole). Blood pressure is measured in millimetres of mercury using an instrument called a sphygmomanometer. Normal blood pressure for a young average adult human is in the region of 120/80 mmHg (the higher number is the systolic blood pressure; the lower number the diastolic blood pressure), but individual variations are common. Abnormally high blood pressure (hypertension) may be associated with disease or it may occur without an apparent cause.