What is blood pressure?

Quick Answer
The force exerted by blood upon the walls of blood vessels as the heart pumps blood through the circulatory system.
Expert Answers
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Structure and Functions

When the left ventricle of the heart beats, it pumps five liters of blood per minute to the aorta, through the arteries, and into the arterioles. As these blood vessels decrease in diameter, they create resistance to blood flow and the pressure of the blood against their walls increases. This blood pressure is expressed as two numbers measured in millimeters of mercury (mmHg) by a sphygmomanometer. The first number, called the systolic pressure, is the maximum pressure that occurs when the heart contracts and the ventricle is emptying its blood. The second number, called the diastolic pressure, is the minimum pressure that occurs when the heart relaxes and the ventricle is filling with blood before the next contraction.

Blood pressure depends on the strength of the heart muscle, the volume and thickness of blood being pumped, and the diameter and flexibility of the blood vessels, all of which may vary with age, health, and physical condition. Blood pressure is also affected by activity, diet, hydration, emotional stress, physical pain, tobacco use, weight (including pregnancy), abrupt changes in body position, and medication and drugs (including caffeine and alcohol).

Blood pressure is a commonly measured indicator of the body’s state of health, along with body temperature, pulse rate (number of heartbeats per minute), and respiratory rate (number of breaths per minute). It can be measured noninvasively using a sphygmomanometer, which may be electronic and automatic or mercury-based and require manual inflation and deflation. With the patient seated or lying down, an inflatable cuff is wrapped firmly around the upper arm at the level of the heart with the lower edge of the cuff 1 inch above the crease in the elbow. The arm should be bare and any sleeve should be pushed or rolled up without constricting circulation. If the meter is electronic, then the cuff will automatically inflate and deflate when activated and a digital reading will be displayed. If the meter is manual, then a health care provider will place the bell of a stethoscope over the large artery on the inside of the elbow, just below the cuff. The cuff is then inflated by quickly squeezing a rubber bulb until the gauge reads 10–30 mmHg higher than the expected systolic pressure but no higher than 210 mmHg. No sound should be heard through the stethoscope. A valve is opened so that the cuff deflates slowly; the pressure reading should drop 2–3 mmHg per second. When the sound of pulsing blood is first heard through the stethoscope, the reading on the gauge is the systolic pressure. As the cuff continues to deflate, this sound will disappear and the reading on the gauge at that point is the diastolic pressure. Blood pressure should be checked at least every year or two. It should be checked more often during illnesses and medical treatments.

Disorders and Diseases

The average blood pressure for healthy adults is 120 over 80 mmHg (written as 120/80). A systolic pressure of 120 to 139 mmHg or a diastolic pressure of 80 to 89 mmHg is considered to be slightly elevated, a condition called prehypertension. A systolic pressure of at least 140 mmHg or a diastolic pressure of at least 90 mmHg is considered to be elevated, a condition called hypertension. In some cases, hypertension has no known direct medical cause; however, in other cases, it is secondary to other health conditions such as kidney disease. Potential risk factors for hypertension include obesity; anxiety, trauma, or pain; poor cardiovascular fitness; obstetric disorders such as preeclampsia; sickle cell crisis; pancreatitis; medications such as oral contraceptives, beta-2 agonists, and monoamine oxidase inhibitors; drugs such as caffeine, cocaine, amphetamines; and withdrawal from alcohol or opiates. Chronic hypertension is a risk factor for heart attack, stroke, and aneurysm.

The goal of hypertension treatment is to get the resting blood pressure below 140/90. In some cases, this may be accomplished solely with lifestyle changes, which should be tried before drug therapy is begun. These lifestyle changes include discontinuing alcohol consumption and tobacco use, reducing dietary salt and sugar intake, eating foods low in saturated fat, performing regular low-intensity exercise such as walking, and getting sufficient sleep and stress relief. When these changes alone are insufficient, one or more medications may be prescribed, such as diuretics, beta-blockers, calcium-channel blockers, or angiotensin-converting enzyme (ACE) inhibitors.

While hypertension is diagnosed from specific elevated blood pressure measurements, hypotension is diagnosed by symptoms of low blood flow rather than solely by blood pressure numbers. These symptoms include light-headedness, weakness, visual disturbance, and fainting.

To increase the volume and thickness of blood being pumped, patients with hypotension are typically instructed to drink more liquids (excluding caffeinated and alcoholic beverages) and ingest more salt. They must also avoid dehydration from excess sweating and hot showers or baths. To improve the resistance in blood vessels, patients may wear compression stockings and should regularly perform moderate exercise. Little is known about the causes of hypotension and few medications are available to raise low blood pressure safely.

Perspective and Prospects

Direct measurement of arterial blood pressure was first reported by Reverend Stephen Hales in 1733. He inserted a glass tube in a horse’s artery and found that the column of blood rose to a vertical height of more than eight feet. Pressures were later measured with columns of water and saline, but they still required an unwieldy length of tube. Eventually, a mercury column was used because mercury is more than thirteen times as dense as water and the column length was more manageable. Sphygmomanometers today no longer use mercury, but the standard unit of mmHg remains.


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