Blood Pressure Basics

You can’t see your blood pressure or feel it, so you may wonder why this simple reading is so important. The answer is that measuring your blood pressure gives your doctor a peek into the workings of your circulatory system. A high number means that your heart is working overtime to pump blood through your body. This extra work can result in a weaker heart muscle and potential organ damage down the road. Your arteries also suffer when your blood pressure is high. The relentless pounding of the blood against the arterial walls causes them to become hard and narrow, potentially setting you up for stroke, kidney failure, and cardiovascular disease.

Having your blood pressure measured is a familiar ritual at most visits to the doctor’s office. The examiner inflates a cuff around your upper arm, listens through a stethoscope, watches a gauge while deflating the cuff, and then scribbles some numbers on your chart. You may be relieved if you learn your blood pressure is normal or alarmed if the examiner says “150 over 100.” But what do these numbers actually mean?

Figure 1: Measuring blood pressure

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  1. A clinician wraps a cuff around a person’s arm, uses a bulb to inflate the cuff, and listens to the sounds in the brachial (inner arm) artery using a stethoscope.
  2. When the pressure inside the cuff is greater than the pressure the heart exerts when it contracts (systolic pressure), the cuff squeezes the brachial artery shut. Blood flow stops, so no sounds are heard through the stethoscope.
  3. As air is released from the cuff, blood flow resumes through the artery in starts and stops, creating a thumping sound. At that moment, the clinician records the systolic blood pressure.
  4. Once the cuff pressure drops below the pressure during the resting phase between heartbeats (diastolic pressure), the thumping sound disappears and the diastolic reading is recorded.

 

Understanding the numbers

Blood pressure is recorded as millimeters of mercury (mm Hg) because the traditional measuring device, called a sphygmomanometer, uses a glass column that’s filled with mercury and is marked in millimeters. A rubber tube connects the column to an arm cuff. As the cuff is inflated or deflated, mercury rises and falls within the column (see Figure 1). Although mercury gauges are still considered the gold standard for measuring blood pressure, mercury-free devices are available. Many modern instruments use a spring gauge with a round dial or a digital monitor, but even these are calibrated to give readings in millimeters of mercury.

The top number, or systolic pressure, reflects the amount of pressure during the heart’s pumping phase, or systole. As the heart contracts with each beat, pressure in the arteries temporarily increases as blood is forced through them. The bottom number, or diastolic pressure, represents the pressure during the resting phase between heartbeats, or diastole. Hypertension is defined as having a systolic reading of at least 140 mm Hg or a diastolic reading of at least 90 mm Hg, or both (see Table 1).

The Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC), a group of physicians and researchers from across the United States, developed guidelines for classifying blood pressure in 2003. The figures are based on extensive reviews of the scientific literature and are updated periodically to keep pace with new research.

To classify your blood pressure, a health professional averages two or more readings taken after you have been seated quietly for at least five minutes. For example, a patient with a measurement of 135/85 mm Hg on one occasion and 145/95 mm Hg on another has an average blood pressure of 140/90 mm Hg and is said to have stage 1 hypertension.

The JNC guidelines emphasize the importance of tackling escalating blood pressure earlier rather than later, thereby heading off heart disease, stroke, and kidney damage. The consensus among hypertension specialists is that people should do whatever it takes to get their blood pressure numbers down to the healthy range. Whatever works for you is the right strategy to adopt, but it most likely will involve some combination of diet, exercise, stress reduction, and medication. Table 1 offers a brief summary of these strategies, depending on which blood pressure category you fall into; the following chapters discuss lifestyle modifications and antihypertensive medications in greater detail (see “Lifestyle changes to lower your blood pressure” and “Medications for treating hypertension”).

Table 1: Classifying and treating hypertension

Category* Systolic blood pressure
(top number)
Diastolic blood pressure
(bottom number)
What you should do
Normal Less than 120 Less than 80 Stick with a healthy lifestyle, including following a diet rich in fruits and vegetables and low in salt, using alcohol moderately, and maintaining a healthy weight.
Prehypertension 120–139 80–89 Change health habits. If you’re heavy, lose weight. Reduce salt in your diet. Eat more fruits and vegetables, and get more exercise. Drink alcohol only in moderation. You do not need medication at this stage if you don’t have other health conditions. If you have diabetes or kidney disease, begin drug therapy if your blood pressure is at or above 130/80.**
Stage 1 hypertension 140–159 90–99 Change your health habits and take a blood pressure drug. Many people start with one medication, but may need to go to a second or third to find a treatment that works. If you have other health conditions, you may need a different drug or an additional one.
Stage 2 hypertension 160 or higher 100 or higher Change your health habits. It’s likely that you’ll need to take at least two blood pressure medications.
*Note: When systolic and diastolic pressures fall into different categories, physicians rate overall blood pressure by the higher category. For example, 150/85 mm Hg is classified as stage 1 hypertension, not prehypertension.**Because both hypertension and diabetes target the same major organs, drug therapy is generally initiated at an earlier stage in people with diabetes. The goal is to maintain blood pressures below the 130/80 threshold. For people with diabetes or chronic kidney disease, stage 1 hypertension is defined as systolic pressure of 130–159 or diastolic pressure of 80–89.

 

If your reading is normal

If your blood pressure is below 120/80 mm Hg, this is where you want it to stay. If you are already committed to a healthy lifestyle, keep it up. If you’ve managed to keep within the normal range without much thought about your health habits, you might want to think again. Data from the Framingham Heart Study suggest that even if your blood pressure is normal at age 55, you run a 90% risk of developing hypertension within your lifetime. But a combination of exercise, weight loss, limited salt intake, a diet rich in fruits and vegetables, and limits on alcohol consumption can prevent hypertension (see “Lifestyle changes to lower your blood pressure”).

 

Prehypertension

You have prehypertension if your systolic blood pressure reading is 120 to 139, your diastolic pressure is 80 to 89, or both. The risk of cardiovascular disease begins climbing at pressures as low as 115/75 mm Hg, and it doubles for every 20-point increase in systolic pressure and each 10-point increase in diastolic pressure. If your blood pressure falls into the prehypertension category and you do not have any other risk factors, lifestyle changes are the recommended treatment at this stage.

If you have diabetes or chronic kidney disease, you should begin using antihypertensive medications at pressures of 130/80 mm Hg.

 

Stage 1 hypertension

You have stage 1 hypertension if your systolic blood pressure is 140 to 159 or your diastolic pressure is 90 to 99, or both. If you don’t have any accompanying conditions such as heart disease, diabetes, kidney disease, or a history of stroke, you will usually start with lifestyle modifications and a single medication. Your doctor may let you try lifestyle modifications alone for two or three months to see if you may be able to avoid medication altogether, but many people find that they need to take some type of medication in order to reduce their blood pressure numbers to healthy levels. You may have to try several drugs to find one that works best.

The initial choice of drug may depend on whether you have other health problems — such as diabetes, migraine headaches, or cardiac arrhythmias — in addition to hypertension. The JNC guidelines also recommend that African Americans, who are at a higher-than-average risk for hypertension-related complications, start with a two-drug regimen if blood pressure readings top 145/90 mm Hg.

 

Stage 2 hypertension

You have stage 2 hypertension if your systolic pressure is at least 160 mm Hg, your diastolic pressure is at least 100 mm Hg, or both. In addition to lifestyle modifications, you will probably need to take at least two medications. If this course of action fails to bring your blood pressure down to your target level (below 140/90 for most individuals and below 130/80 for those with diabetes or chronic kidney disease), your doctor may add additional drugs to the mix.

 

What does blood pressure measure?

Blood pressure reflects both how hard your heart is working and what condition your arteries are in. The formula is as simple as ABC — or actually, C × A = B, that is, cardiac output times arterial resistance equals blood pressure. Cardiac output is the amount of blood your heart pumps per minute. With each beat, your heart propels about 5 ounces of blood into the arteries. That adds up to about 4 to 5 quarts over the course of a minute of normal activity. During strenuous activity, your heart must pump considerably more blood to meet your body’s increased demand for oxygen.

Arterial resistance is the pressure the walls of the arteries exert on the flowing blood. As blood pushes into the arteries with each heartbeat, it forces the artery walls to expand, much like an elastic waistband stretches to accommodate your body. When the blood flow ebbs, the vessel returns to its original shape. The less flexible the vessels are, the greater the arterial resistance. Narrowed, tightened, or inflexible vessels result in a higher pressure at any level of flow. As cardiac output or arterial resistance increases, so does blood pressure.

Figure 2: How the body regulates blood pressure

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Like an expert driver, the body constantly adjusts blood pressure in response to small changes in the environment. The central mechanism for regulating blood pressure is the renin-angiotensin-aldosterone system. This hormonal interaction takes place primarily in the circulatory system, the nervous system, and the kidneys. However, the renin-angiotensin system appears to operate independently within other organs, such as the brain and the blood vessels.

  1. When blood pressure falls, the nervous system sends a signal to the kidneys to release renin into the bloodstream. Renin splits angiotensinogen, a large protein in the bloodstream, to create angiotensin I.
  2. Angiotensin I is then converted into smaller pieces, including angiotensin II, which causes small arteries to constrict, thereby raising blood pressure.
  3. Angiotensin II also spurs the adrenal glands to release another hormone, aldosterone.
  4. Aldosterone causes the kidneys to retain sodium and water, which raises blood volume and blood pressure.

 

Natural blood pressure controls

Your blood pressure is never constant, nor should it be. Your body continually adjusts cardiac output and arterial resistance to deliver oxygen and nutrients to the tissues and organs that most need them — your muscles during a jog or your digestive system at mealtime, for example. Your blood pressure also varies according to the time of day. It’s highest in the morning and lowest at night during sleep.

Your body can make dramatic adjustments in blood pressure within seconds. A sprint for the elevator, the sound of breaking glass, or a confrontation with someone may send blood pressure soaring from an idling 110/70 mm Hg to a racing 180/110 mm Hg or higher. These changes occur without conscious thought and are directed by complex interactions among your central nervous system, hormones, and substances produced in your blood vessels.

A key player in blood pressure regulation is the layer of cells lining the inner wall of blood vessels. This layer, known as the endothelium, encompasses a surface area approximately equivalent to seven tennis courts. Far from being an inert conduit for blood flow, the endothelial lining secretes dozens of substances that interact with the circulating blood as well as with the cell layer that lies below it. Of particular interest in hypertension research are the vasodilators (nitric oxide and prostacyclin) and the vasoconstrictors (angiotensin II and endothelin-1). These chemical messengers instruct your blood vessels to widen or narrow based on your body’s minute-by-minute blood flow requirements.

As long as your blood pressure is in the normal range, healthy vessels tend to be dilated. When blood pressure gets too high (such as during times of stress) or too low (when you’re dehydrated, for example), pressure-sensing nerve cells located throughout your circulatory system relay this information to your autonomic nervous system. The autonomic nervous system manages the involuntary activities of smooth muscles, including those in the intestines, sweat glands, airways, heart, and blood vessels. It responds by setting off a chain of events designed to restore blood pressure to normal levels (see Figure 2).

 

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