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 “180 over 100.” But what do these numbers actually mean?

Understanding the numbers

Blood pressure is recorded as mm Hg (millimeters of mercury) because the traditional measuring device, called a sphygmomanometer, uses a glass column that’s filled with mercury (whose chemical symbol is Hg) 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 standard tool for measuring blood pressure, newer non-mercury 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 mm Hg.

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.

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 x 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 five ounces of blood into the arteries. That adds up to about four to five 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 reduce blood flow.

As cardiac output or arterial resistance increases, so does blood pressure. This is because, in both cases, the heart must pump harder to push the necessary amount of blood through the arteries.

How high is high blood pressure?

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 these 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, the doctor or other 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.

When systolic and diastolic pressures fall into different categories, the JNC advises physicians to rate overall blood pressure by the higher category. For example, 150/85 mm Hg is classified as stage 1 hypertension, not prehypertension. This is also an example of systolic hypertension — defined as a systolic pressure of 140 mm Hg or higher and a diastolic pressure below 90 mm Hg.

The JNC notes that people in the normal category — those with blood pressure below 120/80 mm Hg — have the lowest risk of developing cardiovascular disease. Patients in the “prehypertension” category have a greatly increased risk of developing hypertension and should make changes in their lifestyle to reduce the risk. Patients with stage 1 hypertension generally require medication, although aggressive changes in lifestyle can sometimes eliminate the need for medication.

Table 1: Current blood pressure categories
Category	Systolic blood pressure (mm Hg)		Diastolic blood pressure (mm Hg)
Normal	Less than 120	And	Less than 80
Prehypertension	120-139	Or	80-89
Stage 1 hypertension	140-159	Or	90-99
Stage 2 hypertension	160 or higher	Or	100 or higher

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. The layer of cells lining the inner wall of blood vessels (known as the endothelium) produces an enormous number of vasodilators and vasoconstrictors — chemicals that cause the vessels to widen or narrow. The endothelium helps maintain the tone of your blood vessels by releasing these substances as your body’s needs change. 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.

Figure 1: Measuring blood pressure

Typically, a health care professional measures a patient’s blood pressure using a stethoscope and a cuff that is inflated until the pressure it exerts is greater than the patient’s systolic pressure (the pressure when the heart contracts). The cuff compresses the arm until the brachial artery is squeezed shut. At first, the artery walls will be closed, and the clinician will not hear anything through the stethoscope. As air is released from the cuff, he or she will hear a thump. This is the moment when the clinician records the systolic blood pressure — the first and higher of the two numbers in a person’s blood pressure. As the cuff pressure continues to drop below the level of systolic pressure, the artery will begin to open and close, and the clinician will hear a thumping noise. When the rhythmic sound becomes faint, he or she records the diastolic pressure — the second, lower figure. As the cuff pressure declines below the diastolic pressure in the artery (the pressure between heartbeats), the vessel remains open, and the sounds disappear completely.

A complex chain reaction

The autonomic nervous system is divided into two parts: the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system prepares the body for action by quickening heart rate and breathing, while the parasympathetic nervous system has the opposite effect. The sympathetic nervous system rules during times of stress or fear. The parasympathetic governs during sleep.

When your blood pressure drops suddenly, the sympathetic nervous system compensates by releasing two neurotransmitters, or chemical messengers, from nerve endings: norepinephrine and epinephrine (also called adrenaline). These substances stimulate your heart muscle and cause your blood vessels to tighten. This reaction speeds your heart, increases cardiac output, and raises your blood pressure. To lower the pressure, the parasympathetic nervous system releases acetylcholine, a neurotransmitter that slows the heart.

The autonomic nervous system can also trigger specific organs to release chemicals that regulate blood pressure. For example, when blood pressure drops, the sympathetic nervous system signals your kidneys to release an enzyme called renin into the circulatory system. Renin, in turn, triggers the production of angiotensin, a protein that helps increase pressure by constricting the walls of small arteries. Angiotensin also stimulates your adrenal glands to secrete the hormone aldosterone, which causes the kidneys to conserve sodium and water, thereby raising blood volume and blood pressure. Together, this sequence of events is called the renin-angiotensin-aldosterone (RAA) cascade.

Room for error

Given the many mechanisms the body uses to regulate blood pressure, there are a number of ways something could go wrong. Some researchers suggest, for instance, a lack of vasodilators — particularly nitric oxide, which is also known as endothelium-derived relaxing factor — or an overproduction of certain vasoconstrictors, such as endothelin, can cause some cases of hypertension, although this hasn’t been proven.