You probably recognize the numbers 120/80 as a normal blood pressure. But why two numbers? The explanation is quite simple. Your heart beats about 70 times a minute, and with each beat blood is pumped into the arteries. As this happens, the pressure inside the arteries goes up, until the end of that heartbeat. The peak level of pressure is called the systolic pressure. Then the heart relaxes, and begins filling with blood for the next beat, and the pressure in the arteries starts to fall and reaches a minimum level just before the next heartbeat, which is the diastolic pressure. So the number 120 refers to the systolic pressure, and 80 to the diastolic pressure. Each heartbeat produces a slightly different pressure, but usually the two numbers go up and down together.

The blood pressure is expressed as millimeters of mercury, usually abbreviated as mm Hg (Hg is the shortened version of the Latin name for mercury). The reasons for using mm Hg are both historical and practical. The pressure gauge used by doctors to measure blood pressure is called a sphygmomanometer, which has a column of mercury, the height of which is recorded in millimeters, and is a measure of the pressure inside the cuff.

Although it is so common, hypertension has not traditionally been treated by specialists, but by a variety of physicians, including family practitioners, internists, cardiologists, and nephrologists. This works fine for many patients, but others benefit from more specialized care. Some patients have rare (but curable) causes of hypertension that often go undiagnosed for many years, and others just can’t seem to find the right mix of medicines to keep their blood pressure under control. A recent development has been the recognition of Hypertension specialists whose primary focus is on diagnosing and treating all forms of hypertension and its complications.

The traditional method of measuring blood pressure is with a sphygmomanometer and a stethoscope. The way it works is as follows: the cuff that is wrapped around your upper arm contains a rubber bag, which can be pumped up with air by squeezing a rubber bulb. The bag is also connected via a tube to the column of mercury, which measures the air pressure in the bag. To take a reading of the blood pressure, the cuff is pumped up to a pressure of about 200 mm Hg. This is nearly always higher than the systolic pressure, so that it completely shuts off the circulation of blood in the arm. Then the valve on the rubber bulb is opened a little, and the air in the bag allowed to leak out, and gradually lower the pressure in the cuff. While this is happening the person taking the pressure listens with a stethoscope placed on the elbow crease just below the cuff. When the cuff pressure is greater than the systolic pressure, there's no flow of blood and nothing to hear. But as the pressure is reduced, it gets to a point at which the systolic pressure in the artery is higher than the cuff pressure, so the artery starts to open, and blood to flow. Each spurt of blood makes a whooshing sound, which can be heard with the stethoscope. As the cuff pressure is reduced further the sounds get louder and last longer, but as the cuff pressure approaches the diastolic pressure they start to fade away. The point at which they finally disappear is the diastolic pressure. It may seem puzzling why the sounds come and go in this way, but when the flow of blood in the artery is not interrupted by the occlusion produced by the cuff, it's quite smooth, and makes no noise. What we hear when the cuff pressure is between systolic and diastolic pressure is partly the sound of the artery opening and closing, and partly the sound of turbulent flow.

In practice, there are other ways of measuring blood pressure that provide more information than the traditional stethosocope method. These are 24 hour ambulatory monitoring and home or self monitoring.

Everyone has high blood pressure some of the time, and it only causes a problem when it stays high for long periods. Even then, there are many people who live normal lives with high blood pressure and never know it. Unfortunately, not all are so lucky. The reason that doctors are concerned about high blood pressure is that it increases the risk of a number of serious events, chiefly strokes and heart attacks. Even if these do occur, however, it may be only after ten or twenty years of the pressure being high.

The damage caused by high blood pressure is of three general sorts. The first is the one everyone thinks of - bursting a blood vessel. While this is dramatic and disastrous when it happens, it's actually the least of the three problems. It occurs most frequently in the blood vessels of the brain, where the smaller arteries may develop a weak spot, called an aneurysm. This is an area where the wall is thinner than normal and a bulge develops. When there is a sudden surge of pressure the aneurysm may burst, resulting in bleeding into the tissues of the brain, and hence a stroke.

The second adverse consequence of high blood pressure is that it accelerates the deposition of cholesterol plaque (atheroma) in the arteries. This problem, too, takes many years to develop, and it is very difficult to detect until it causes a major blockage. It affects mainly the larger arteries, but deposition is not uniform. It accumulates most where an artery divides into two smaller branches. The blood flow is normally smooth in the arteries, but where they divide it becomes turbulent, and this turbulence is thought to damage the delicate lining of the arteries. Wherever this damage occurs, cholesterol deposits are more likely to accumulate. The most important sites to be affected are the heart, where atheroma causes angina and heart attacks; the brain, where it causes strokes; the kidneys, where it causes renal failure (and can also make the blood pressure go even higher); and the legs, where it causes a condition known as intermittent claudication, which means pain during walking.

Third, high blood pressure puts a strain on the heart: Because it has to work harder than normal the muscle enlarges, just as any other muscle does when it is used excessively. In people with high blood pressure the volume of the heart doesn't change very much, but the thickness of the muscle increases. Thickening of the heart muscle is bad because the muscle outgrows its blood supply, rendering it more susceptible to the effects of atheroma narrowing the coronary arteries that supply the heart.

Yes. High blood pressure can be classified in two ways, one according to how severe it is (mainly a question of how high the blood pressure is) and the other according to what's causing it. About 95 percent of people with high blood pressure have what is known as essential hypertension, which is really a fancy way of saying that it just happens, and we don't know why. The other 5 percent of cases have secondary hypertension, where there is an identifiable and usually correctable cause. The commonest of these is renovascular hypertension, where there is narrowing of the artery to one or both kidneys. Other less common causes of secondary hypertension are small tumors of the adrenal glands that secrete blood pressure-raising chemicals (hormones) into the bloodstream.

The term essential hypertension is not a very specific one. It is thought that hypertension is the end result of a number of different factors that make the blood pressure go up, and it is probable that different mechanisms are important in different individuals. This may explain why a particular type of treatment may work very well in one person, but not at all in another.

Classification of hypertension by its severity is somewhat arbitrary because there's no precise level of pressure above which it suddenly becomes dangerous. For no particularly good reason, blood pressure has traditionally been classified according to the height of the diastolic pressure, although the systolic pressure is probably more important in determining the level of risk. Someone whose diastolic pressure runs between 90 and 95 mm Hg may be regarded as having borderline hypertension, and when it's between 95 and 110 mm Hg it's moderate, and at any higher levels it's severe. The most dangerous type is called malignant hypertension, which is regarded as an acute emergency requiring immediate treatment in a hospital. Whatever the underlying cause, when the blood pressure reaches a certain level for a sufficient length of time it sets off a vicious cycle of damage to the heart, brain, and kidneys, resulting in further elevation of the pressure. Not surprisingly, if untreated, malignant hypertension can be rapidly fatal. Because more people are treated nowadays than before, malignant hypertension is not common, and is mainly seen in people who have not had access to medical care.

White coat (or office) hypertension is a term used to describe people whose blood pressure is only high in a doctor's office.

Systolic hypertension is mainly seen in people over the age of 65 and is characterized by a high systolic, but normal diastolic, pressure (a reading of 170/80 mm Hg would be typical). It's caused by an age-related loss of elasticity of the major arteries.

Labile hypertension is a commonly used but inappropriate term for describing people whose pressure is unusually labile or variable. In fact, just about everyone has labile blood pressure.

As described in the previous section, in most people with hypertension there is no single curable cause such as a blocked renal artery, and they are labeled as having essential hypertension. This means hypertension that just happens, although there are a number of factors that we know can contribute to it. The important point is that there is no single factor that causes it, but a combination of several different ones that may play different roles in different people. There is a genetic or hereditary component: if your parents both had hypertension there is an increased chance that you will develop it as well. That component may account for about half of the factors that lead to hypertension. However, it is probable that no single gene is responsible and that more likely a cluster of genes that have different individual effects when acting in concert result in hypertension. There is also a big environmental component. Hypertension is, or was, relatively uncommon in the traditional tribal societies that lived in Southern Africa and elsewhere, but when the villagers moved to the big cities and adopted a more westernized lifestyle their blood pressure tended to increase. Whether this phenomenon is because of stress or changes in diet has not been resolved, but almost certainly both are involved. The typical American lifestyle, with a diet that is high in salt and fat and low in fruits and vegetables, combined with physical inactivity, certainly contributes to high blood pressure. Even more important is obesity, which may account for at least 50 percent of cases of hypertension. The good news here is that a lot can be done to treat and prevent hypertension by attending to diet and exercise.

Usually, there are no specific symptoms that indicate that someone has high blood pressure. But some population surveys have shown that a wide variety of common symptoms, such as sleep disturbance, emotional upsets, and dry mouth, are slightly commoner in people with higher pressures. The differences are small, however. Going red in the face, or feeling flushed, is not indicative of high blood pressure.

If you asked a hundred people what is the commonest symptom of high blood pressure, the chances are that the majority would say headache. In fact, not only do most people with high blood pressure not have headaches any more than the rest of us, but when they do, it's usually not from the blood pressure. Merely having a high level of blood pressure inside your head does not normally produce any symptoms; if you lift a heavy weight, your pressure may go up by 30 or 40 mm Hg, but you don't get a headache.

What can cause headache is muscle tension. Any muscle that is tensed for long enough starts to hurt, and chronic tension in the scalp or neck muscles is a very common cause of headache. A study conducted many years ago shed some very interesting light on the relationship between headache and high blood pressure. Out of 104 people who had high blood pressure but were unaware of it, only three volunteered that they had headaches, although another 14 admitted it when asked. But of 96 people who had been told that they had high blood pressure, 71 said they had headaches. The simplest explanation for this finding is that being told that you have high blood pressure makes you start to worry, and that this strain in turn causes the headaches.

There is a much smaller number of patients, mostly with very high pressures, in whom headaches are directly related to the height of the blood pressure. In such individuals treating the blood pressure will relieve the symptoms.