Updated 08 February 2018

What is hypertension?

Your blood pressure is the pressure within your blood vessels. Hypertension is a condition where the pressure within your arteries is consistently too high.

Your blood pressure is the pressure within your blood vessels. This pressure is generated by the pumping action of the heart and counteracted by the resistance of the small arteries.

Blood pressure is essential to life. It keeps the blood flowing through your body and provides oxygen and energy to your organs.

Your blood pressure is strictly regulated, as too low pressure causes dizziness, fainting and a lack of oxygen to your organs. In fact, when a doctor uses the term “shock”, it usually implies that there has been a drastic drop in blood pressure, leading to inadequate perfusion and oxygenation of vital organs like the brain and kidneys. Starved from their life-giving source of oxygen, these organs cannot function anymore, and the individual is at risk of dying unless corrective treatment is taken.

Too high blood pressure (either systolic or diastolic or both – see below) may damage blood vessels and vital organs over time. According to the World Health Organisation, hypertension is the leading global risk for death, responsible for 12,8% (7,5 million) of deaths worldwide. It is without any doubt one of the leading causes of heart attack, stroke, kidney failure, dementia, eye disease, and premature death. Together with cigarette smoking, high cholesterol, obesity and diabetes mellitus, it constitutes the so-called cardiovascular risk profile.

How does it work?

Your heart functions as a muscular pump which contracts rhythmically and pumps blood into your arteries. From your heart, through your large vessels (such as the aorta) your blood is channelled to your entire body through a circulatory system of smaller vessels. The resistance offered by these smaller arteries to prevent arteries from bursting, due to the dramatic pumping action, is very significant. Constriction of the muscle in the artery wall causes it to narrow, which increases the resistance and hence the pressure within. This can be compared to taking a garden hose and reducing the size of the nozzle. Pressure in a hose can of course also be raised by increasing the amount of water flowing from the tap. Similarly, the amount of circulating blood, and the strength of the heart muscle contractions, can also influence your blood pressure.

Your blood pressure increases and decreases according to the demands made on it. This enables you to play a vigorous game of tennis and also to get a good night’s sleep.

Blood pressure rises when you are active and falls when you are inactive. During restful sleep, your inactivity reduces the demand for oxygen and therefore blood pressure is usually lowest at night and highest when you get up in the morning.

There is also considerable variability in blood pressure. It may be transiently elevated under certain circumstances, and should not be confused with hypertension, which is a sustained elevation in blood pressure. For instance, when startled or threatened, one’s blood pressure is raised through the effects of the “fight or flight” response, when your adrenalin levels are high. Experiencing pain can also raise pressure dramatically. Alcohol, nicotine and caffeine intake can cause transient elevations of blood pressure.

Blood pressure fluctuations are not problematic, as long as blood pressure rapidly returns to a normal baseline. It is the sustained increase in blood pressure that causes havoc, especially when accompanied by risk factors like smoking, high cholesterol and obesity and co-existing disease like diabetes. Some people have high blood pressure only in a medical establishment. This is termed white coat or office hypertension and can be confirmed by showing that blood pressure is normal in a home setting. It is important to diagnose “white coating”, as treatment with antihypertensive drugs is unnecessary and can cause side effects.

Explaining blood pressure terminology

Hypertension is a condition where the pressure within your arteries is consistently too high.

If your blood pressure is too high, your heart must work much harder (and enlarges) to maintain adequate blood flow to your body. The long term effects of this pressure may cause the heart to fail and this is termed hypertensive heart disease.
A blood pressure measurement is made up of two parts: systolic and diastolic.

Systolic pressure occurs in arteries during heart contraction (which is called a systole) and diastolic pressure during the period of relaxation between beats. This is why a blood pressure measurement is expressed as one figure "over" another, for example 120/80 mm Hg Systolic pressure is obviously always higher than diastolic blood pressure.

The measurement unit, millimetre mercury (symbol Hg), is derived from the distance that a column of mercury in the measurement instrument is pressed upward. If your blood pressure is recorded as 120/80, the number on top is the systolic pressure, and the bottom number the diastolic. 120/80mm Hg also happens to be the optimal blood pressure, although many people especially young women may have blood pressures as low as 90/60 mm Hg, and this is quite normal.

Pulse pressure is the difference between the two readings. In elderly persons the pulse pressure may rise because the diastolic pressure falls as a result of the large vessels becoming stiff . This is called isolated systolic hypertension and carries an even higher risk of complications. Although the diastolic may be “normal” or even low, it is essential that this type of hypertension be treated.

When is blood pressure considered high?

Researchers are of the opinion that the average blood pressure during the day should not exceed 130/80 mm Hg. Hypertension is diagnosed when the blood pressure is consistently above 140 systolic or 90 diastolic. Between 130/80 and 140/90 it is called high normal blood pressure, and it is usual to progress to hypertension over the next 2 – 5 years unless lifestyle changes are made. In high risk patients like diabetics, blood pressure may be treated with drugs, even in the high normal range. The target blood pressure is less than 140/90, but in high risk patients (patients with more than two major risk factors, target organ damage or diabetes), this is lower at 130/80.

The following table shows the normal ranges of blood pressure and the stages of hypertension for South Africans. High normal blood pressure requires more regular monitoring. Stage 1 is less severe than Stage 3.

Blood Pressure

Systolic (mm Hg)

Diastolic (mm Hg)



120 or less

80 or less


Less than 130

Less than 80

High normal

130 -139

80 - 89


Stage 1 (Mild)

140 - 159

90 - 99

Stage 2 (Moderate)

160 - 179

100 - 109

Stage 3 (Severe)

180 or higher

110 or higher

Blood pressure above 180/110 mm Hg is dangerous, and urgent medical attention needs to be sought. If the blood pressure is greater than 200/120 it can be rapidly life threatening and cause malignant hypertension.

Hypertension represents an arbitrarily determined level of blood pressure, above which the advantages of treatment are greater than the disadvantages.

Hypertension is diagnosed when blood pressure measurements are above either of these normal levels on two or more separate occasions. In patients who are at high cardiac risk, e.g. diabetics or patients with known heart disease, treatment for blood pressure may be indicated in the high normal range.

Elevated blood pressure readings may not always indicate that a person has hypertension, as in the case of white coat hypertension – high readings caused by a natural alerting response during blood pressure measurement (the emotional stress of having a doctor take your blood pressure). At home the blood pressure readings are completely normal and this condition generally carries no risk but needs to be monitored regularly, as some people with white coat hypertension may develop true hypertension in time.

(Reviewed and updated by Prof Brian Rayner, head of the division of nephrology and hypertension, University of Cape Town and Groote Schuur Academic Hospital, November 2010)



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Hypertension expert

Dr Jacomien de Villiers qualified as a specialist physician at the University of Pretoria in 1995. She worked at various clinics at the Department of Internal Medicine, Steve Biko Hospital, these include General Internal Medicine, Hypertension, Diabetes and Cardiology. She has run a private practice since 2001, as well as a consultant post at the Endocrine Clinic of Steve Biko Hospital.

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